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THE DISTRIBUTION AND PROPERTIES OF SOILS I N  
RELATION TO EROS ION I N  A SELECTED CATCHMENT OF 
THE SOUTHERN RUAHINE RANGE , NORTH I SLAND ,  NEW ZEALAND 
A thesis presented in partial fulf ilment of  
the requirements for  the degree of 
Haster of  Philosophy in Soil Sc ienc e 
at Massey University 
Carolyn Hubbard 
1 9 78  
FRONTISPIECE 
A view o f  the south-eastern fault-controlled f ront o f  
the Ruahine Range . Car Park Creek, a subcatchment o f  
the West Tamaki River ,  is  seen in the c entre o f  photo . 
In the foreground , f ertile  floodplains  are seen .  These 
are threatened by the inundation of erosion product s  
which are carri ed out of  the mountainland by the r ivers , 
during s torm periods. 
• 
I 
• 
i .  
ABSTRACT 
The soils of  a selected subca tchment of the Southern Ruahine Range have 
been mapped at  a scale of 1 :5 , 000 . The soil mapping units  have been further 
charac t erised by measurement of a number o f  soil physical and chemical 
properties , together with an investigation of their sand and c lay minera 1-
ogies . 
The erosion history since 20 , 000 yrs B . P .  when the Aokautere Ash was 
deposi ted in the Wes t Tamaki River catchment,  has been partial ly reconstructed 
for this  ca tchment .  I t  i s  one o f  erosive periods and resulting aggradational 
gravel  deposi ts ,  al ternating with more s table  periods with soil development 
and vegetation growth. S tudies of a histoso1 (organic soil )  on the sumfuit 
pla teau of  the Southern Ruahine Range , at  the head of  the catchment .  suggest s 
tha t t his  soil i s  approximately 4600 years old ,  and prior  to this t ime the 
summit  pla teau was stripped by eros ion . 
Present erosion occurs predominantly : ( 1 )  on convex creep slopes, j ust 
below the summit plateau , and ( 2 )  on the steep valley-sides.  In the former 
zone, where Takapari hil l soils  exis t ,  deep�seated creep and mass  movements 
occur . 'In the latter zone , where Ruahine steep1and so ils  exist,  superfic ia l  
soil  and rock s lips are more common . 
An investigation of the soil-water relationships for  each so il mapping 
uni t  indicat�that a number of factors  render the Takapari  hill soils and 
Ruahine steep1and soils par ticularly susceptible to erosion . A comparison 
of soil  proper ties which affec t the erosion susceptibilit ies of each soil 
mapping unit has enabled an ordering of the units with respec t to ero sion 
r isk . Thus ,  areas of high , medium and low risk to erosion in the West 
Tamaki River ca tchment have been delineated . Many of  the deep-seated erosion 
surfaces occur in the high risk area . Thus , if stabilisation of these 
sites  is possible ,  by intensive revegetation programmes ,  the resul t wil l 
be a decrease  in the amount o f  gravels carried out of the mountain1and by 
rivers onto the surrounding fertile floodplains . 
• 
• 
I 
• 
( 
ii . 
ACKNOWLEDGEMENTS 
My course of study in New Zealand has been both benefic ial and 
enj oyable .  I would like to express my s incere apprec iation to the following , 
who have helped to make it this way : 
Drs . V. E .  Neall  and J . A .  P ollok f or supervision of my study , and f or 
many pleasurable times together . 
. Drs . D .  Sc otter and J . H .  Kirkman , Messrs  D . G .  Bowler and R . B .  S tewart 
f or their assistance  in vari ous parts of the study . 
Professor  J . K .  Syers and many other members of the Department for help 
and friendship throughout  my course . 
Mike Marden , my colleague , Keith McAuliffe and Mike Hedley f or accom­
panying me in the field . 
Messrs Rob Blakely , Dick Martin and Colin Hichie (Manawatu Catchment 
Board) and Mr Peter Stephens (Ministry of Works and Development) for helping 
to acquaint me with the Ruahine situation. In particular , I would like to 
thank Mr Rob Blakely for arranging several rec onnaissance trips , and for 
many s timulating discussions . 
Rotary International ,  f or their generous f inanc ia l assistance ;  without 
whom I would have been unable to embark on this c ourse . 
N. Z .  Forest Service f or funding my sec ond year of s tudy; and especially 
to Mr A .  Cunningham , f or his interest and assistance throughou t  the study . 
Judy f or speedy and skilful typing of the text; 
and , f inally ,  my family for their unend ing support. 
• 
, 
t 
FRONTISPIECE 
ABSTRACT . .  
ACKNOWLEDGEMENTS 
TABLE OF CONTENTS 
LIST OF FIGURES 
LIST OF TABLES . 
INTRODUCTION 
1 :1 Reasons for Study 
1 . 2  Objectives of Study 
1. 3 Choice of Study Area 
1 . 4  Methodology of Study 
TABLE OF CONTENTS 
CHAPTER I 
CHAPTER 2 
LITERATURE REVIEH ON THE SOILS AND EROSION SITUATION OF THE 
SOUTHERN RUAHINE RANGE 
2 . 1 Introduction 
2 . 2  Landscape Evolution in the Southern Ruahine Range, Through 
Geological Time . 
2 . 2 . 1  Genesis of the Mountain Range 
2 . 2 . 2  Landscape Evolution, during Plio-Pleistocene 
times 
2 . 2.3 Post-glacial Climatic Changes 
2 . 3  Present Erosion Situation of the Southern Ruahine Range 
2 . 3. 1  Erosion Situation 
2 . 3 . 2 Erosion Types 
2 . 3. 3  Causes of Erosion 
2 . 3. 4  Future Control of Erosion 
j 
iii 
page 
i 
ii 
iii 
viii 
xi 
1 
1 
1 
2 
3 
5 
6 
6 
7 
8 
1 0  
1 0  
1 0  
1 1  
1 4  
.. 
• 
2.4 Soils of the Southern Ruahine Range . . . . .  
2.5 Soil Parameters, Relevant to Erosion Studies 
2.5.1 Erosion Processes 
2.5.2 Soil Mineralogy 
2.5.3 Soil-Water Characteristics 
2.5.4 Slope Stability Studies 
2.6 Summary . . . . . . . .  . 
CHAPTER 3 
DESCRIPTION OF THE STUDY AREA 
3.1 Location 
3.2 Physiography 
3.3 Geology 
3.4 Soils 
3.5 Vegetation 
3.6 Climate 
3.7 Introduced Wildlife 
3.8 Erosion . . . . .  . 
CHAPTER 4 
HISTORY OF EROSION IN THE WEST TAMAKI RIVER CATCHMENT 
4.1 Introduction 
4.2 Method of Study 
4.3 Erosion History 
4.3.1 Deposits of the Last Stadial (Ohakean Substage) 
4.3.2 Tephrostratigraphy of the Takapari peaty loam, and 
Erosion History of the Summit Plateau 
4.3.3 Depositional Surfaces in the West Tamaki River 
Catchment 
4.3.4 Aerial Photographs (1946-1978 ) 
page 
16 
19 
19 
20 
2 2  
2 5  
27 
30 
30 
36 
38  
40  
4 2  
4 6  
4 8  
51 
52  
52  
. . 54 
5 9  
68  
iv 
CHAPTER 5 
A PEDOLOGICAL INVESTIGATION OF THE SOILS IN CAR PARK CREEK 
SUBCATCHMENT 
5. 1 Introduction 
5.2 Method of Approach 
5.3 Soils (and Soils Legend) 
5.3.1  The Ruahine steepland soils 
5.3.2 The Takapari peaty loam 
5.3.3 The Takapari hill soils 
5.3.4 The Dannevirke soils 
5.3.5 Recent soils 
5.4 General Discussion 
CHAPTER 6 
AN INVESTIGATION OF SOIL PARAMETERS RELATED TO SOIL GENESIS AND 
ERODIBILITY 
6. 1 Introduction 
6.2 Soil Physical Properties 
6.2. 1 Materials and Methods 
6.2.1. 1  Particle density 
6.2.1.2 Bulk density . 
6.2.1.3 Total Porosity and Macroporosity 
6.2.1.4 Saturated Hydraulic Conductivity 
6.2.1.5  1 5  bar water retention (A . W . C . and 
"drying effect") . . . 
6 . 2.1.6  Loss of weight on ignition 
6.2. 1.7 Soil pH in ( 1 )  water and (2 )  sodium 
flouride . 
6.2 . 2  Results and Discussion 
6.2.2. 1 Particle density and Bulk density 
page 
70 
74 
76 
7 9  
8 5  
8 8  
96  
10 1 
103 
105 
107 
107  
108 
108 
109 
1 10 
1 12 
1 1 3 
1 1 3  
1 14 
1 14 
v .  
6.2.2.2 Total Porosity and Macroporosity 
6.2.2.3 Saturated Hydraulic Conductivity 
6.2.2.4 Soil Water Retention and Available Water-
Holding Capacity . . 
6.2.2.5 15 bar Soil Water Retention, and the Effect 
of Drying 
6.2.2.6 Loss of weight on ignition 
vi . 
page 
116 
119 
121 
126 
130 
6.2.2.7 pH values in (1) water and (2) sodium flouride 130 
6.3 Soil Mineralogy . 133  
6.3.1 Sand Mineralogy 133 
6.3.1.1 Haterials and Methods 133 
6.3.1.2 Results and Discussion 134 
6.3.2 Clay Mineralogy . . . 137 
6.3.2.1 Introduction 137 
6.3.2.2 Haterials and Hethods 138 
6.3.2.3 Results and Discussion 141 
,6.4 Conclusions . 157  
CHAPTER 7 
FINAL DISCUSSION OF RESULTS AND EROSION PROCESSES, WITH CONCLUSIONS . 163 
Bibliography . . 182 
Abbreviations (used in soil profile descriptions) 191 
Appendix I :  Classification of Landslides : Abbreviated Version 
(Varnes, 1958) 192  
Appendix II : Soil Chronosequence in the West Tamaki River Catchment -
profile descriptions 193 
• Appendix III: Soil Map of Car Park Creek (in pocket 
inside rear cover) . 
Appendix IV: Nitrogen Mineralisation Data from a Laboratory 
vii . 
page 
. . (pocket) 
Experiment for the Takapari Peaty Loam . . . . 196 
Appendix V: Phosphate Retention Values for the Dannevirke 
Taxadjunct, and Dannevirke Hill Soils 197 
FIGURE 
LIST OF FIGURES 
l. Kumeti Gravel Reserve 
2. Locality Map of Study Area 
3. Longitudinal Profiles of (a) Car Park Creek; 
(b) West Tamaki River, 
(Mosley, 1977). 
4. An idealised diagram of the land surface units which 
occur on a valley-side in the West Tamaki River 
page 
15 
2 9  
31 
ca tchmen t . . . . . . . . . . . . . 3 3  
5.  Landsurface Units at the head of Car Park Creek 35  
6. Landsurface Units of a Valley-side in Car Park Creek. 35  
7. A rockslide, and deep terracette features on the 
convex creep slope of Car Park Creek . . . . 3 7  
8. Downcutting in Hut Creek, since Cyclone Alison of 
March, 1975.  . . . . . .  . . . . 41 
9. Altitudinal Distribution of four vegetational species 
in the Southern Ruahine Range, compared with their 
distribution further north 3 9  
10. The Kamahi forest in Car Park Creek 41 
11. Gully erosion in Car Park Creek . 4 3  
12. Vegetation on a Slope in Car Park Creek 4 3  
13. Rainfall Map for the Tamaki River area 4 5  
14. Illustrations and Field Description of the Aokautere 
Ash, as it occurs in the West Tamaki River catchment. 5 3  
15. Takapari Peaty Loam: Particle-size and Organic 
matter percentages. Profile Description (b) . 
16. Depositional Surfaces in the West Tamaki River 
55  
ca tchment . . . . .  . . . . . . . . . . . . .  58  
17. A reconstruction of events forming the Whiteywood 
Creek fan . . . . . . . . . . 60 
18. Whiteywood Creek fan deposit 61 
19. A soil profile developed on the Whiteywood Creek fan 
deposit . . . .  . . . . . . . . 61 
20. Old terrace, in the West Tamaki River channel 6 3  
21. A Soil Profile developed on the old terrace system 63  
viii. 
page 
2 2. An extensive gravel terrace, formed during Cyclone 
Alison. and a 98 year old fan deposit at 
Stanfield Hut . . . . . . . . . . . . . . . 65 
23. Recent soil, formed in a gravel deposit, at the mouth 
of Car Park Creek . . . . . . . . . 65 
24. Isopach Map of a Recent Gravel surface, associated 
with Car Park Creek and Dry Creek . . .  . . . .  67 
2 5 .  Car Park Creek - a subcatchment of the West Tamaki 
River . . . . . . . . . . .  . 7 1  
26 . Diagrammatic Cross-section to show the distribution 
of Soil classes, in relation to the land surface 
units, within Car Park Creek 7 3  
2 7. Ruahine steepland soil (RuS) 80 
28. Takapari peaty loam (Tp) 86 
2 9. Takapari hill soil (TpH) 89  
30. Iron and Aluminium distribution in the soil profile 
of a Takapari hill soil . . . 94  
31 . Dannevirkp. taxadjunct (D tax) 98 
32. Dannevirke hill soil (DH) . . 99  
33. Native earthworm, and native earthworm burrows 100 
34 . A Histogram to show the bulk density values for each 
Soil Class . . . .  . . . . . . . . . . ll5 
35. Total Porosity and Macroporosity of Selected Soil 
Samples . . . . . . . . . . . . . . 1 1 7 
36 . Saturated Hydraulic Conductivity Values for three 
37 . 
38 . 
39 . 
40 . 
4 1 .  
42. 
Selected Soil Profiles . . . . . . . . 1 20 
Water Retention Characteristics of Selected Soil 
Profiles . · 
The Relationship between A . W . C. and bulk density 
in selected soil samples . · 
The i\elationship between A . W . C .  and organic matter 
in selected soil samples · . . . . 
The Relationship of Organic Matter to the Effect of 
Drying on 1 5  bar Water Retention Values. for 
selected soil samples . . " . .  . . . 
+ 
X-ray diffraction patterns of NH4 saturated clay 
samples . . . . . . . . . . . . . . . 
X-ray diffraction patterns of a Dannevirke 
taxadjunct soil profile, indicating the presence of 
a small amount of pedogenic chlorite . . . . .  . 
1 2 3  
1 2 5  
1 2 5  
1 2 9  
1 4 2  
1 4 4  
ix. 
4 3. 
+ + 
page 
X-ray diffraction patterns of NH4 and K saturated clay samples from a Bw horizon of a Ruahine steepland soil 1 4 5  
44. D.T . A .  Curves of Selected Soil Clay Samples 1 4 7  
4 5. Infra-red Spectra of Selected Soil Clay Samples 1 50 
46. Electron Micrograph showing kaolinite and halloysite 1 5 3  
47. 
48. 
4 9. 
50. 
51. 
52 . 
11 11 11 the 2 forms of halloysite 
11 11 11 weathering volcanic glass 
11 11 11 amorphous gel 
11 11 
11 11 and crystalline 
1 53 
1 5 3  
1 54 
material in a Ruahine steepland soil (RuVS) . . 1 54 
Representative electron micrograph of a Ruahine 
steepland soil CRuS) . . . . . . 1 54 
Electron Micrograph showing imogolite and unidentifed 
laths . . . . . . . . . . . . . 1 54 
53. Representative electron micrograph of D tax (Ah horizon)1 5 5  
54. " 11 " 11 11 (C 11 ) 1 5 5  
55. " 11 11 11 the clay fraction 
of Aokautere Ash . . . . 1 5 5  
56. Representative electron micrograph of the clay fraction 
of a greywacke pebble . . . 1 55 
57. An illustration of the Possible Origin of Terracettes, 
Observed at the Head of Car Park Creek . . . .  1 6 7  
58. Erosion Potential Map for the West Tamaki River 
Catchment . . . . . . . . 1 78 
x. 
LIST OF TABLES 
TABLE 1. Erosion Phases in the Southern Ruahine Range (Grant, 
page 
1978) . . . . .  . . . . . . . . . . . . . .  9 
2.  Soils of the Mountain Range, in Pohangina County 
(Rijkse, 1977) . . . . . . . . 18 
3. Factors Contributing to Mass Movement in soils (Selby, 
1970) . . . . . . . . . . . . . . . . 2 6  
4. Classification of landsurface units, according to the 
NLne Unit Landscape Model, of Conacher and Dalrymple 
(1977) . . . . . . . . .  . . . . . .  . . . . . 33 
5. Percentage Eroded Area in Car Park Creek and No . 1 
Creek, from 194 6-1978 . . . . . . . . . . . . 68 
6. Variation of Properties in the Ruahine Steepland Soils 
xi . 
Mapping Unit . . . . .  . . . . . . . . . . . . . . 83-84 
7. Bulk density and Particle density values for selected 
soil profiles of the study area . . . . . . . 115 
8. Saturated Hydraulic Conductivity Data for Selected Soils. 120 
9. Soil Water Retention Values, and Available-Water Holding 
Capacity (A . W . C . )  of Selected Soil samples . . . . . 122  
10. The Effect of Drying on 15 bar Water Retention Values 
of Selected Soil samples . . . . . . . 127 
11. Loss of Weight on Ignition Data for Selected Soil 
samples 131 
12. pH values in (1) water, and (2) sodium flouride 132  
13. Sand Mineralogy of Selected Sand Fractions of Samples 
from the Study Area . .  . . . . . . . . .  l35  
14. Results of Transmission Electron Microscopy: Visual 
Identification of Mineral and Amorphous Materials . 152  
15. Average Soil and Tree Rooting Depths of each Soil Mapping 
Unit . . . . .  . . . . . . . . . . . . • 171 
16. Factors Affecting the Erosion Susceptibility of the Soils 
in Car Park Creek subcatchment . . . . . . . . 175 
CHAPTER ONE 
INTRODUCTION 
1 . 1  REASONS FOR STUDY 
CHAPTER ONE 
INTRODUCTION 
1 .  
The nature and distribution of the soils of the Southern Ruahine Range 
are largely controlled by erosion processes in an area of very high 
erosion rates. Few soil surveys have been conducted in this mountainland . 
However, soils have been mapped at a scale of 1 : 63,360 in the Dannevirke 
area by G . J .  Smith (cited in Mosley, 197 7 )  and in Woodville and Pohangina 
counties by Rijkse (1974, 19 7 7 ) . There is a paucity of detailed information 
on the soils of the Southern Ruahine Range, and knowledge of their parent 
materials, genesis and distribution pattern with reference to slope angles 
and erosion history is scarce . 
Slope stability problems centre around the failure of the soil mantle 
and underlying bedrock with subsequent formation of both shallow and deep-
seated erosion scars. The causes of this erosion in the Southern Ruahine 
Range are by no means well-established, and in the last ten years concern has 
mounted due to a commonly held belief that erosion rates have increased in 
recent decades . The apparent increased incidence of mass movements and 
slips, coupled with marked aggradation in river channels draining the 
Range has led to the possibility of flooding in adjacent lowland areas. 
These lowland areas are, in certain places, densely populated and are used 
for highly productive livestock farming . 
1 . 2 OBJECTIVES OF STUDY 
The objectives of the present study were to assess the soil resources 
of a selected study area with respect to : 
(a) their relationship to the erosion history 
(b) nature of parent materials 
(c ) their genesis and classification 
(d ) their relative erosion potential, involving measurement 
of a number of soil physical and mineralogical properties. 
2.  
It was anticipated that this information would help to explain the 
erosion processes occurring in the study area, which are similar to those 
occurring throughout the Southern Ruahine Range. 
1.3 CHOICE OF STUDY AREA 
A number of reconnaissance trips to the Southern Ruahine Range were 
made to investigate the erosion problem and the range of soils which occur 
there . Information gathered on these trips was used to choose a study 
area, in which a detailed soil survey could be carried out . 
A subcatchment of the West Tamaki River, Car Park Creek, was chosen 
as a suitable study area for the following reasons : 
(a ) the erosion problem, and range of soils, appeared to be typical 
of the erosion and soils encountered over much of the area of 
the Southern Ruahine Range. 
(b ) a considerable amount of background information is available for 
the West Tamaki River catchment, from work carried out by the 
Manawatu Catchment Board, Soil and Water Division of the 
Ministry of Works and Development and the New Zealand Forest 
Service . 
(c ) the West Tamaki River catchment provides the main water supply 
for the town of Dannevirke and is thus an important catchment in 
the Southern Ruahine Range, 
(d ) good access exists along vehicle and foot tracks at the top and 
bottom of the West Tamaki River catchment and Car Park Creek 
(the selected subcatchment). This was considered to be an 
important factor in enabling widespread ground observations to 
be made throughout the study area. 
1 . 4  METHODOLOGY OF STUDY 
(a) Fieldwork: Two major objectives of fieldwork studies were : 
an investigation of the erosion history of the West Tamaki 
River catchment, and an assessment of the soil resources 
of Car Park Creek subcatchment, at 1:5, 000 scale . 
3. 
The former involved the identification and delineation of dep­
ositional surfaces in the main channel of the West Tamaki River 
catchment. This part of the study provides a picture of past 
erosion events in the catchment, and also shows the degree of 
soil development on varying aged surfaces . The soils on the more 
stable sites of the summit plateau were also investigated to 
provide a stratigraphic control to the record of erosion events, 
over the last few thousand years, on the unstable sites within 
the study catchment. 
The latter involved a survey of the soils in Car Park Creek 
subcatchment. This entailed a detailed enquiry into the Ruahine 
steepland soils, a mapping unit used in previous surveys to 
describe the major portion of soils in the south-eastern Ruahine 
Range . Also, the relationships of soil distribution to vege-
tation pattern, slope, geomorphology and parent materials were noted, 
all of which are closely related to the erosion history . 
Aerial photographs were used as an aid to fieldwork studies . 
Photographs at a scale of 1: 20, 000, published by the Department 
of Lands and Survey were used as an aid in identifying erosional 
and depositional surfaces. A series of aerial photos of Car Park 
Creek. at an approximate scale of 1: 5, 000, were flown by 
Mr D . G .  Bowler of the Department of Soil Science, Massey University 
for use in the soil survey and for accurate determination of the 
extent of erosion scars in the subcatchment . 
4. 
(b) Laboratory Investigations: Characterisation of the soils for 
classification involved measurement of a number of soil physical 
and chemical parameters to augment information obtained in the 
field, (i.e. organic matter, bulk density, pH in NaF, P retention) .  
Mineralogy studies (sand and clay fractions) were used to 
investigate the extent of weathering in these soils, as well as 
the nature of the soil parent materials. 
Soil-water characteristics were investigated by measuring : 
the saturated hydraulic conductivity, macroporosity, total porosity, 
and 15 bar water retention (of moist and previously dried samples) 
values, in order to assess the susceptibility of the soils to 
erosion. In this way, factors involved in the erosion of soils 
within this area were defined, and these in turn revealed the units 
of the landscape which had maximum susceptibility to erosion 
processes. 
CHAPTER TWO 
LITERATURE REVIEW 
CHAPTER TWO 
LITERATURE REVIEW ON THE SOILS AND EROSION 
SITUATION OF THE SOUTHERN RUAHINE RANGE 
2 . 1 INTRODUCTION 
5 .  
In recent years, the Southern Ruahine Range has been an area of 
intensive study, chiefly by governmental and quasi-governmental agencies, 
due to concern over large amounts of gravel and other debris which are 
transported from the eroding mountain range onto the adjoining farmland . 
This detritus chokes many river beds, and is threatening between 
24,300 and 28,300 hectares of fertile, flood-free plains of the Manawatu, 
(Poole, 1 973 ) . A number of workers believe that erosion rates in this 
area have increased markedly during the last few decades (James, 1 973; 
Stephens, 1 97 5; Cunningham and Stribling, 1 977; Grant, 1 978 ) . Stephens 
has shown that between 194 6 and 1 974, a 1 20% increase in area of eroded 
slopes occurred in the No . 1 and Raparapawai catchments, of the Southern 
Ruahine Range . 
Brougham ( 1 977 ) indicates that the Tamaki and Rokaiwhanga streams 
were probably narrow ( 10-20m wide) and meandering, prior to deforestation 
of the lower reaches . Following timber removal, tree stumps began to 
rot out in the 1 920's and 1 930's, and the streams became wide, braided 
channels, scouring through previously forested areas . 
loss of productive floodplain, which continues today . 
This resulted in 
He considers that 
bed levels rose by about 0 . 5m to 1.0m between 1 910 and 1 940; and again, 
by a similar amount since 1 940 . Since 1 940, the lateral extent of these 
channels has increased by a factor of three to ten times . 
Thus the problem of gravel extending over fertile floodplains in 
this region is not a new one . It was acknowledged by Cumberland (1 944) ,  
who described "frost-bitten, windswept, golden scars of soil-stripped 
patches along the crestline of the ranges", which could be seen from a 
distance of 30 miles . Cumberland attributed the "induced" erosion to 
"the tread and grazing of animals - wild and domestic - and the use of 
6 .  
the firestick" . However, Colenso (1884 ) wrote of "very precipitous and 
broken hills and ridges", and "extensive landslips", before exotic 
wildlife or domestic animals were introduced; and when burning had only 
just commenced . 
Mosley (197 7 )  states that the precise nature, location and extent 
of the problem has been only vaguely specified . He considers soils as 
one of several factors of importance in the consideration of erosion in 
the Ruahine Range . Cunningham and Stribling (19 7 7 )  consider the soil 
resources are a key factor in the Ruahine erosion situation, deserving 
close study . 
2. 2 LANDSCAPE EVOLUTION IN THE SOUTHERN RUAHINE RANGE 
THROUGH GEOLOGICAL TIME 
2 . 2 . 1  Genesis of the Mountain Range 
Sediments that accumulated in the New Zealand Geosyncline, parallel 
to the "supercontinent" of Gondwanaland (Fleming, 19 75 ),  are mapped today 
in the Ruahine and Tararua ranges as the Torlesse Supergroup, (Stevens, 
19 74 ) .  The sediments were deformed, and raised above sea-level, during 
the Rangitata Orogeny, in early Cretaceous times (Fleming, 19 7 5; Kingma, 
1959 ) .  
Peneplanation of the Ruahine Range occurred between the Upper 
Cretaceous and Palaeocene, when a marine transgression submerged the 
southern North Island (Kingma, 1959) . Fleming (1962 )  suggests that 
this phase of peneplanation and quartoze sedimentation may be the only 
really stable phase in New Zealand's geological history since the Devonian . 
The peneplanation resul ted in a level surface now exhumed and dissected 
to form the marked summit accordance seen today in the Ruahine Range ,  
and Tararua Range (Wellman , 1 94 8 ) . 
7 .  
The sediments of  the New Zealand Geosyncline have suffered a second 
deformat ion episode , with upli f t  during the more recent Kaikoura Orogeny , 
which i s  continuing today (Bradshaw , 1975 ) . 
2. 2. 2 Landscape Evolution , During Plio-Pleistocene Times 
A c l imatic cooling during the Pl iocene was heralded by a change of  
vegetat ion in the mountain ranges of New Zealand , from Nothofagus brassi  
(Long-leafed Beech) in  Wai totaran times , to  Nothofagus fusca (Red Beech ) 
and po do carps in the Lower Pleistocene ,  ( Mildenhall , 1 973) . 
Conditions were becoming more severe , and with tectonic uplift  of  the 
main range s ,  increased ero sion rates resul ted , which are recorded by greywacke 
detritus of Nukumaruan age , on the surrounding l owlands . The Castlecl i f f ian 
S tage contains abundant fossils of warm temperature flora and fauna (Fleming , 
1 973; Mildenhall , 1 973) , indicating a milder, more stable period in the 
mountainland , before the oncoming glacial  of  the Upper Pleistocene, the 
Waimaungan S tage . Milne ( 1 973a) , suggest s  that during this and the 
succeeding cold climate  episodes , the mountainland was largely devegetated 
above the 900 metre contour l ine . He estimates  a decrease in mean annual 
temperature of SoC to 60C during these cold c l imate episodes; with 
slight ly lower rainfall  and fewer high intensity rainstorms . However,  
Soons ( 1 9 76 )  suggests  that sea-level temperatures in the central  South 
Island were lowered by not more than 4 . SoC during the most severe glacial 
of  the Ot iran S tage . Thus , the decrease in t emperature o f  SoC to 6
o
C ,  
suggested by Milne for  the southern North Island may be an over-es t imate . 
Increased erosion , during t he cold c l imate  episodes , p roduced aggrad­
ational gravel s ,  with subsequent wind removal o f  silt and f ine sand 
particles , to form extens ive terraces in some part s  of the adj oining 
lowlands (Milne , 1 973b ) and assoc iated loess deposi ts .  
Leamy e t  al . ( 1 973 )  s tudied a sequence o f  seven paleosols and 
associated loess units in the southern North Island , and concluded that 
the pal eosols indicate  interstadial periods of  relative warmth and 
increased soil profile  development compared with the s tadial periods . 
8 .  
The loess units associated with the stadial period s indicate a period o f  
higher erosion rates with a retreat o f  vegetat ion down the mountain f lanks . 
2. 2 . 3 Post-Glacial Climatic Changes 
S ince the last stadial (Ohakean Substage ,  o f  Milne , 1 973c ) , the 
Post-glacial period in New Zealand (Aranuian S tage) has been charac terised 
by a maj or warming between 14 , 700 and 6 , 300 years  B . P . , with only minor 
t emperature osc illations s ince .  
McGlone and Topping ( 1 973 ) have shown , on the basis o f  pollen s tudies , 
that podocarp forests had established themselves in the c entral North  
I s land before 1 3 , 800 years B . P . , and cons ider the Aranuian S tage to  have 
begun about 14 , 000 years ago . Molloy ( 1 969 )  considers that there was 
a general rise in temperat ure about 1 0 , 000 years ago in New Zealand . 
The magnitude of  this temperature rise is unproven and estimates are  based 
primarily on evidence from the South Island , where podocarp forests  began 
to spread over areas , formerly characterised by grassland and scrubland , 
(Moar , 1966 ;  Walker , 1 9 66) . A general r ise in temperature  since 1 0 , 000 
years ago , is substantia t ed by global evidence o f  a rapidly r ising 
eustatic sea level ( Shepard and Curray ,  1 967; B loom et al . 1 9 74 ;  Thom , 1 9 74 ) . 
Molloy ( 1 969)  details  climat ic osc illations believed to have o ccurred 
in Britain over the last  7, 000 years , indicating a c l imatic optimum 
between 3000 and 5000 B . C . , and a "Little  Ice Age" between 1 500- 1 850  A . D .  
Fleming ( 1963 )  and Wilson , Hendy and Reynolds ( 1 973 ) discuss evidence for  
9 .  
these two c l imatic oscillat ions in New Zealand . Wilson et a l .  ( 19 73) 
using the oxygen i so tope method for estima ting pal eotemperatures from 
speleo thems (cave formations ) est imate that tempera ture oscillations 
during the last  mil lenia have been + 2
o
e .  
I t  is  important to  note  that  Molloy ( 1 969 )  considers tha t  any effec t 
that these minor oscillations might have on landscape evolution would , 
in mos t  cases , be l ess s ignif icant than modifications by natural  cata-
strophes , such as f ire ,  faul t ing , natural vegetational evolution and 
man ' s  inf luences . 
Grant ( 1 965,  1 966 , 1 978 ) gives evidence for 5 ero sion phases in the 
Ruahine Range in the last 600 years . The Matawhero phase  coincides 
with the "Little Ice Age" of ca . 1 500- 1850 A . D .  These periods of 
increased erosion are a t tribu ted by Grant to periods o f  " increased 
s torminess" , and are outlined in Table  1 ,  below. 
TABLE 1 :  EROS ION PHASES IN THE SOUTHERN RUAHINE RANGE 
EROSION PHASE 
Waihirere 
Matawhero 
Wakarara 
Early modern 
Modern 
TENTATIVE DATE (A. D . ) YEARS AGO( PRIOR 
TO 1 9 70) 
closed ca .  1 400- 1 4 50 520-570 
" " 1 600 370 
ca. 1 780- 1 830 140-1 90 
1880's - 1 890 ' s  80-90 
mid 1 930 ' s  to present 0-40 
(Grant , 1 9 78 )  
2 . 3  PRESENT EROSION SITUATION OF THE SOUTHERN RUAHINE RANGE 
2 . 3 . 1 Erosion S ituation 
1 0 .  
C unningham and S tribl ing ( 1 977) outline the present erosion problem 
in t he Ruahine Range as one of mountainland erosion , and transport  of  i ts  
produc t s .  They consider the main concern to b e  the accumulation o f  large 
quantities of  gravel in the upper reaches of  the rivers . which may be 
accelerating and posing a threa t  to areas downstream . 
S chumm ( 1 977) categorises the erosion and sedimentation into 2 types : 
TYPE 1 :  
TYPE 2 :  
erosion on the steep slopes and small  tributary 
basins in the Range . This is the source of  the 
sediment that forms f loodplains. valley deposits 
and alluvial fans. A maj or contribution is from 
mass movement . 
bank ero sion and remobil isation o f  TYPE 1 sediments  
that are  stored in valley throats,  floodplains and 
fans . 
Schumm considers that TYPE  1 erosion has always occurred to varying 
degrees , and is inevitable; whilst TYPE 2 erosion has been acclerated by 
man's actions . 
2 . 3 . 2  Erosion Types 
Bedrock , so il and f luvial erosion processes are common throughout 
the Ruahine Range . Resul tan t  erosion types have been reported by a 
number of workers (James ,  1 973; S tephens , 1 975 ,  1 977; Cunningham and 
Stribling , 1 977; Mosley and B lakely ,  1 977) . The landslide  classification 
of Varnes ( 1 958 )  is used in this study for naming erosion types . The 
classification is  based on the type of  material involved and type of 
movement . I t  also considers  wat er content of  f low-type landsl ides and 
takes into account a general range o f  velocity of movement of the landslide 
1 1 .  
types . Thus , erosion types are explicitly defined (see Appendix I ).  
Some forms of soil  erosion such as  soil creep and solifluction are  not 
included in Varne's classification and these are inc luded here using the 
c lass i f ication of  Campbell ( 1 9 5 1 ) , (c ited in Land Use Capability Survey 
Handbook, Water and Soil Division , M.O . W.D., 1 971 ) . 
Examples of debris slides , debris avalanches and slumps in the 
Southern Ruahine Range have been g iven by James ( 1 973 ) , S tephens ( 1 975 ,  
1 977) , Cunningham and Stribling.(l977) and Hosley and Blakely ( 1 977) , who 
indicate  that these particular erosion types are common in the Southern 
Ruahine Range.  Hosley and Blakely ( 1 977) describe a landslide (rotational 
3 slump ) fea ture in Coppermine Creek, from which over 3 5 , 000 m o f  shattered 
rock has been supplied to the stream. They consider that a lthough this 
is one of the largest mass movement features in the south-eastern Ruahine 
Range ,  its  form and sliding or flowing type of movement (depending on 
water  content)  is ra ther common. Al so , the maj ority of  erosion events  
supplying material to  the streams occur on the walls  of s teep-sided , 
deeply-cut  inner valleys . 
There is a paucity of l iterature detail ing forms of  soil erosion in 
the Ruahine Range. Cunningham and Su-ibl ing ( 1 977) consider that soil 
sl ips ("rapid sl iding movements of soil and subsoil paral lel to the slope" , 
Campbell  ( 1 95 1 ) ) are common on riparian sites throughout the Range , some-
times developing into "debris-falls" .  An example is given in the 
Southern Ruahine Range , in the headwaters of Coal Creek . Undercutting . by 
streams , of oversteepened val ley-sides, commonly result s  in this type o f  
ero sion . They also note that  c reep and sheet erosion occur within the 
. Range .  
2 . 3 . 3  Causes of Erosion 
The frequency of erosion has increased over the last  few decades , 
as  documented by various workers ( see 2 . 1 ) .  Reasons to explain the 
• 
1 2 .  
increased erosion rates have differed . James ( 1 973 )  considers that there 
has been some infl uence of  mammal s ,  particularly opposums , on mass  
movement s  in the Upper Pohangina River . He also considers increased 
storminess , in recent  years , to be an important fac tor , but indicates that 
it  is d i f ficul t to know what sor t  of interact ion , if any , these two fac tors 
have in causing mass movements . Grant ( 1 977) , as  previously stated , also 
considers tha t  increased storminess i s  an important fac tor for periods of  
increased ero sion rates . Heavy ra ins ,  associated with Cyclone Alison , in 
March 1 975 were responsible for shif t ing a considerable amount  of  debris 
in cer tain ca tchment s of the Southern Ruahine Range , thus p roviding 
visible evidence for  this lat ter theory . 
Elder ( 1 965)  provides evidence for vegetational changes in r ecent 
years ,  on the flanks of the Ruahine Range . He considers tha t these changes 
may be explained by a climatic change . To tara (Podocarpus totara) , 
ma tai (Podocarpus spicatus)  and rimu (Dacrydiun cupressinum) are failing 
to replace themselves under forest condit ions . Also , pink pine 
(Dacrydium biforme) and cedar (Liboc edrus bidwillii) communities show 
a consistent pat tern of deter iorat ion , which is most advanced in the south , 
whilst  mountain beech (Nothofagus solandri var . clif fort io ides)  forests  
show a general deterioration , except  toward the lower end o f  its  range . 
In the Central Ruahine Range r ing counts of mountain beech give evidence 
for a retreat in alt itude and change in distribution pat t ern on slopes over 
the last  200 years ,  which suggest s  that conditions may have become wetter .  
S tephens ( 1 9 75 )  establ ishes a tentative relationship between 
ear thquakes and erosion . An increased frequency of  medium-sized earth-
quakes s ince 1 939 , may be associated with the creation of overs teepened 
slopes which are prone to erosion . Erosion subsequently occurs  af ter a 
trigger ing ac t ion , most commonly rainstorms . 
The main fac tor predispo sing steep slopes to ero sion in the 
Raparapawai and No . 1 catchments , is the instability of the densely faul ted 
1 3 .  
and shat ter ed melange-like bedrock ,  ( the "Pohang ina melange" i s  mapped 
to the north ,  by Sporli  and Bell ( 1 9 7 6 ) , and defined by Sporl i ( 1 9 7 4 )  
a s  "a body o f  t ectonically deformed rock,  charac terised by the inclusion 
of native and exotic blocks , in a pervasively sheared , commonly peli tic 
matrix" ) . Harden ( 1 9 7 7 )  considers that important fac tors partly 
responsibl e for eros ion in the West  Tamaki River catchment  bo th in the 
past  and a t  present , are the steepness. faul t ing and s truc ture of  the 
bedrock . 
Cunningham andStribling,( 1 977) a t tribute eros ion scars in the 
Northern Ruahine Range to burning and grazing. They consider that pas t  
burning and grazing may have initiated sheet erosion, which in some cases , 
develops into "debris-falls" . They consider mo st "debris-falls"  in the 
Range have developed at  the sites of  previous avalanches or  slides ,  which 
are closely assoc iated with high intensity ra infalls and so ils which for 
some period s approach sa turation . 
Hosley ( 1 97 7 )  and Schumm ( 1 97 7 )  cons ider that  significant f luctuations 
occur in eros ion rates naturally.  so that erosion in the mountainland may 
not be accelerated by any one fac tor or combinat ion of factor s ,  but is  
part  of  a natural cyclical process .  Schumm ( 1 975 )  proposes tha t  stream 
behaviour does not change progressively through geologic t ime,  but rather , 
relatively brief periods of instabil ity and inc ision are separated by 
longer periods of relat ive stabili ty . 
Thus , accord ing to Schumm ( 1 9 7 5 , 1 97 7 )  it  is po ssibl e  to envisage that 
the present  increased erosion rates  in the Ruahine Range are accounted 
for as a normal stage in the very complex denudat ional history of the 
landscape . However, this does l ittle  to aid our understanding of  how 
best  to deal with the problem on hand . 
, 
14 . 
2 . 3.4 Future Control of Erosion 
The main a im of any erosion control plan appears to be preservation 
o f  floodplains downstream of the Range ,  which are in danger o f  being 
covered with gravel during larger floods . 
Two approaches to erosion control are discussed in the l iterature .  
Cunningham and Stribling , ( 1 977) and Hathaway ( 1 977) discuss animal control 
and revegetat ion schemes as a method of  combat ing mountainland erosion . 
They emphasise  the importance of controll ing erosion in the steep 
mountainland area , and are referring to the TYPE 1 erosion of S chumm ( 1 977) , 
( see 2.3 . 1 )  . Schumm considers that  this type of  erosion has a lways occurred 
to some exten t ,  and i s  inevitable .  
Indeed , Mosley and Blakely ( 1 977) in discussing possible erosion 
control measures with respect to landsl id ing in Coppermine Creek , state 
that  no management techniques are available that could have prevented the 
landslide,  and to this extent suppor t  the concept of inevitability . 
landsliding the area has become more s table , and no ac t ion was deemed 
necessary by these authors to prevent  further erosion at this site . 
S ince 
Blakely ( 1 977) , Mosley ( 1 977) and Mosley and Blakely ( 1 97 7 ) discuss 
eros ion control in terms of the TYPE 2 erosion of Schumm, ( 1 977) ( see 2 . 3. 1 ) .  
This type o f  erosion , involving channel widening and remobilisa t ion of  gravels ,  
has been enhanced by man's actions, so that a suitabl e management pol icy ,  
aimed at  restoring the s ituation to  approximate its original s tate  is  
considered to be  the most desirable control method .  
These workers suggest that sed iment transport rates  have been 
accelerat ed by deforestation of the former , natural deposition area s ,  a t  the 
foot of the Range , where the val leys broaden and become less  steep ( the 
valley " throats"  of  Mosley ,  1 977) . Mosley and Blakely ,  ( 1 97 7 )  consider 
that the most obvious course of action is : 
FIGURE 1 :  KUMETI GRAVEL RESERVE 
The reserve acts as a constricted natural fan area . I t  has been operat ive s ince the 1 9 50 ' s ,  
when an area o f  about 20ha was p lanted in willows , poplars and pines . Previously , this natural 
deposition area had been deforested . This resulted in the scouring of  material s  further upstream 
with accelerated sediment transport rates . 
Photo : D . G . Bowler 
"to enhance the natural tendency of the streams to store 
soil and rock eroded from the valley-sides in the valley 
bottom, by judicious use of structural and vegetative 
techniques". 
This work should concentrate at valley "throat" areas. An example 
of a successful constricted natural fan area is Kumeti Gravel Reserve, 
discussed by Blakely (1977), and shown in Fig. l. 
16. 
These workers agree that the magnitude of river control work required 
at the foot of the Range can only be modified by animal control work, and 
maintenance of a vegetative cover in the mountainland. 
2.4 SOILS OF THE SOUTHERN RUAHINE Rfu"\lGE 
There is a paucity of information on the mountain soils of the Ruahine 
Range and Cunningham and Stribling (1977) have previously reviewed literature 
on this topic. 
Pohlen et al. (1947) adopted a Ruahine series to describe soils of 
the Range, south of the Ngaruroro River. They mapped eroded 
where the ernded area exceeded 10% of the to 1 area: i. e. a 
exes, 
moderately eroded phase where eroded area is 10-30% of the total area, and 
a severely eroded se, where eroded area is >30% of the total area. 
They also describe a Ruahine light silt loam, with a characteristic 
light texture and high erodibility, which occurs in patches at the foot of 
the Range, between the Ngaruroro and Haipawa rivers. The different 
properties of this soil are attributed to a change in vegetation. However 
it seems likely that its "lightness" may be explained by a contamination 
of tephras from the central North Island. 
In the General Survey of the soils of North Island, the mountain soils 
of the Southern Ruahine Range were mapped as the Ruahine stony silt loam 
soil set (N.Z. Soil Bureau, 1954). In 1 968 , they were referred to as 
17 . 
Ruahine-Rimutaka soils . Z. Soil Bureau, 1968), and classified as 
"related st soils to central yellO\v-brown earths". are 
described as shal with colour B horizons (BW using horizon des 
of FAO/UNESCO, 1974) forming over weathering greywacke bedrock, "lith some 
volcanic ash contamination. At lower elevations, at the foot of the 
mountainland, 
mapped, e.g. 
slopes of the 
mapped. 
ow-brown earth - yellmv-brown loam intergrade soils are 
and Dannevirke soils. On the western and southern 
c illuvial central yellow-brown earths are 
(1969) descr the soils in the Woodville district, uses 
litho parent material and as a basis for soil 
units. He notes that widespread occurrence of loess is a dominant feature, 
and considers that it is an indicator of largely stable sites; at least 
since the Last Stadial. Loess occurs on r crests and land of 
less than In these areas he 
which are considered to be less eros 
the soils as Ruahine hill soils, 
e than the shalla"" and stony 
Ruahine s soils which occur on steep to very steep In 
infers that the hill soils after their st counterparts, 
they are more close rela ted to the s soils than the soils of 
the rolling land (see and Pohlen, 1970 , p . 142) . 
kse has mapped the soils of Woodville County (1974) and 
County (1977 at a sea of 1 :63,360, details of which are in Table 
2. He considers that with increas elevation the soils become more 
leached and show the initiation of podzolisation. Thus, the Renata and 
Rimutaka soils are from the Ramiha and Ruahine soils on the 
basis of movement and of iron, mottles and discontinuous 
iron pans within the profile. The Takapari peaty loam, and associated 
hill soils are mapped at the highest elevations, with the hill soils 
recorded as only slight risk of slip erosion (see Table 2). 
Mosley (1977) presents the most recent soil map for the south-eastern 
Ruahine Range at a scale of 1:63 , 360, which has been compiled by C.J. Smith. 
TABLE 2: SOILS OF 
SOIL SOIL PARENT 
NAME SYMBOL MATERIAL 
Ramiha loess on 
Silt Loam Rm greywacke 
Ramiha loess and 
Hill Soils RmH greywacke 
Ruahine Steepland Soils RuS 
II " " 
Very Steep Phase RuVS greywacke 
Renata Rn loess 
Silt Loam 
Renata RnH greywacke 
Hill Soils and loess 
Rimutaka Steepland Soils RkS 
II " tt greywacke 
Very Steep Phase RkVS 
Takapari Tp peat and 
Peaty Loam greywacke 
Takapari TpH greywacke 
Hill Soils and peat 
RAINFALL ELEVATION 
(approx. mm) DRAINAGE Rt\c�GE 
1500-1800 well drained 300-500 
" " " 300-600 
" 11 " 300-1100 
1800-2300 moderately 600-900 
well drained 
It moderately " 
well drained 
tt \qell drained 600-1720 
>2300 poorly drained 1250-1300 
" " 11 1070-1370 
SOIL 
DEPTH 
40 
58 
30 
80 
30 
10 
70 
46 
EROSION 
POTENTIAL 
moderate scree 
and 
slip erosion 
severe slip 
and 
scree erosion 
moderate slip 
and scree erosion 
severe scree 
slip erosion 
slight slip 
erosion 
and 
1 9 .  
S teepland so ils  are  mapped  in areas where most  s lopes  are  greater than 300 , 
and  arp  subdivided on the ba s i s  o f  parent rock and c ima t e .  The 
units  of Rij kse ( 1 9 7 4 )  have been adop ted in this  survey . 
The so il s on the f ins , a t  the foo t o f  the have a high 
po t en t ia l  for  pa storal  use ;  whereas  the s t eepland soi l s  have  severe to  very 
sever e  so il limita t ions  for pas t oral  use . Such soil s are  best  sui t ed to  
protect ion , ( kse , 1 9 7 7 ) . 
2 . 5 S OIL PARAMETERS TO EROS ION STUDIE S  
2 . 5 . 1 s ian P rocesses  
The  natural  p ro c es s  of  erosion  c ompr ises  ( 1 )  wea and ( 2 )  
transporation o f  ma t er ial s b y  ,,;ra t er , gravity ,  i ce  and wind , (Ward , 1 9 7 5 ) . 
Transpo r ta t ion wa t er and gravity are consider ed to be  o f  maj o r  impor tance 
to eros ion s tud ies  in t he Ruahine  Range ; a lthough some  ero s ion by ice and 
wind undoub o ccurs , espec ial ly in the  northern par t  o f  the Range . 
Al l wea ther ing processes  r equire wa t er , \\Iolman and Mil l er , 
1 964)  and  involve t he 
the paren t  ma terial  for  
down o f  rocks into  smal l  f ra gmen t s , 
so il  development . Fur ther 
oc cur s \,;ri thin t he solum which is  primar chemical wea o f  the 
primary  mineral s  to  secondary minera l s .  The \-Jea thering 
ero sion wil l  be d iscus sed mor e  fully in the soil  mineralogy section o f  
this c hapter . 
o f  
Subs  to  wea • t ranspora tion o f  mat er ial s occurs  by mas s  
movement o f  rock and /o r  s o i l  and u l t imately via the r iver channe l . In 
this  way , natural eros ion processes occur within a catchment ,  
the l andscape . In  cases  where  incr ea sed o r  a c celera ted  eros ion o c cur s ,  
the  l and-surface  b ecomes d egraded and soil  i s  l os t .  
A number o f  workers have s tressed the impor tance o f  the so il factor  
20 .  
t o  eros ion processes  in  a catchment area,  together with c l imat ic ,  topographic 
and vegeta tional fac to r s  ( e . g .  Meeuwig ,  1 9 7 1 ; Baver , Gardner and Gardner , 
1 9 7 2 ; War d ,  1 97 5) . Ero sion proc es ses wi th wa ter a s  the primary eroding 
agen t  are a f fected by soil-water  charac terist ic s .  Ero sion proces ses  with  
gravity as  the  primary  eroding agen t  are a f f ec t ed by  soil  mechanical 
p roper t ie s . 
2. 5 . 2 
The initial s tage  in any eros ion process  is  wea thering . Wea thering 
occur s ,  in s i tu , when phys ical , chemical and b iological  agencies  bre.ak 
do\vn the rock surface ,  c ontributing par ticles  to the primary min eral  a s s em-
blage  o f  soil s .  Thus , in the case  o f  a greywacke bedrock, quartz  and 
f eldspar are contributed to the mineral a ss emblage ,  with minor 
amounts  o f  mica and o ther  accessory minerals . In steepland t errain,  
movement of  ma terials  downslope introduce s  ma terials  f rom ups lope ,  d epo s i t ing 
colluvial  mat erials  at the bas e  o f  a slop e .  
introdu c ed aerially a s  ( 1 ) loes s ,  from the 
Al , mat erial  may b e  
bedrock , or  ( 2 )  
vol canic  ash .  
investigated 
Thu s ,  the  pr mineral a s s emblage,  which may be 
the sand mineralogy with an micro scop e ,  
def ines  t he nature o f  the ma terial and indicat es t he extent o f  
additions from o ther sourc es . 
S tudy o f  the sand mineralogy a lso yields  informa ion on the  propo r t ions  
o f  maj o r  mineral const i tuent s which provide evidence  of  the  ext en t  o f  
weathering o f  bo th  ma terial s and so il . Thus , i f  the propo r t ions  
o f  f e ld spars  in  the sand fra c t ion  o f  the soil  i s  d i s tinc tly  less  t han in 
the parent  rock,  it is probable that t hey  have been weathered . 
The s and fra c t ion is f urther wea thered and comminut ed to  s il t  and 
then c lay-sized part ic les . The soil  c lay  mineralogy may thus be  s tudied  
in  c onjunc t ion with  the sand mineralog  to  invest  
within the soil . Arg illation , the forma tion of  c 
wea thering  p ro c es ses  
mineral s ,  i s  
described b y  Keller  ( in Rich and Kunze ,  1 9 64) as  a group o f  mul t  
2 1 .  
interre l a ted proces ses , s uch a s  sol id s t a t e  convers ion-solut ion 
transition  processes  and reac t ion with ground  wa ter , e . g . by 
ion , oxida tion r c hel a t ion . 
t z  wea ther  1 1 y  solution .  a l though t h e  sand-s i z ed 
a r e  one o f  the mas r e s i s tant  minerals  to wea thc Fieldes  
and Wea therhead ( 1 96 6 )  ind ica te tha t for  New Zealand s oils , the  ' sand 
frac t ion minus quar t z ! ,  expressed - ,  a perc entage  o f  the who l e  soil  i s  
as  
The 
decompo s it ion 
of weatherab l e  mineral s in  the so il . 
o f  f e l  
I son . 1 9 7 5 ) . 
grains  has  a marked e f f e c t  on their 
Their 
environment is no t f u l  under stood , a l  
it ion in the 
it s eems 1 that in the 
initial  i s  s con t inuous  d i s so lu t ion occur s ,  vlhich i s  pr e ferent ial  
t c er ta in weak spo t s  in  the 1 l a t t ic e ,  p roduc of  the 
gra in 1 s on , 1 9 7 5) . Fur ther ,,,ea ther involves the forma t ion o f  c l ay 
minera l s  such as  il l i t e  
Jackson , 1 95 , and  may 
a with intermixed , Mehra and 
v ia an intermediate  amo t 
1 9 69 ) . Vermicul i t e  and  mon tmorillorite  a l so  f o rm e ither d irec tly  f rom 
o r  from a s so c ia ted  mica  1 9 7 5 )  . The mica -vermicul i t e  
trans forma t ion i s  l o s s  o f  
by oxida t ion , subs t i tut ion o f  f oxygen , and  l o s s  o f  o c tahedral  
iron and magnes ium . Wi ( 1 9 7  sugge s t s  that mica-derived d i o c t ahedral 
vermiculite  is  
P edo 
groups in the 
mor e  extreme 
e common in soil  s .  
chlor it e  forms in so il s ,  when 
le vermicul i t e  i s  
cond i t ion s , mica 
of  Al 
, 1 9 69 ) . Under  
t h  b io t i t e  and mus covi 
convert  to  kao l in i t e ,  which  may be f ormed directly ,  without a 
sequen e o f  vermicul itizat ion , 
1 9 7 5 ) . 
may 
Rhyol i t ic and ande s i t ic wea ther t o  , t h e  f o rmer muc h  
less  rapidly than t h e  lat ter ,  and u 1 t imately to  (Kirkman , 1 97 7 a ) . 
2 2 .  
Differences  i n  ra te  o f  formation and sub s equent a l tera tion o f  in 
the two types  o f  i s  determined 
and poro s it o f  the re spec t ive gla s ses . 
by  the chemical  c ompos it ion 
The andes i tic tephra 
h ighly porous wea thers rap id  , and its  high A1 203 : ra t
io  introduc es  
s trains  be tween par t ic les  tend 
1 97 7 a ) , 
to  increase  i t s  wea therabil ity , 
The nature  c lay  minera l s  in a soil  influence  the s o i l  
, compressib i l i ty  and r eac t ion to  in moi s ture  ( Sowers  
and  S owers , 9 70 ) , and therefor e ,  have a n  indirect  e f fec t  on s tabil i ty 
icular  r el evance ,  s tud i e s .  The presence  o f  swel l ing  in a soil is  o f  
as  ion ,  caus • may e a la teral pres sure within the  
s o i l , increas  i t s  shear s t r ess , and S1J SC ibili  to  movement 
c i c  An 
has been r as  one  o f  the prine 
in North Wes tl and , New Zea land , ( 0 '  
Furkert ( 1 9 7 2 )  a l so refer  the  
at  a r ith-sands tone interface  
causes  of instabili  on  s 
in and Pear c e ,  1 97 6 ) . Wel l s  and 
t endency o f  under 
incr pressure .  The s truc ture o f  r esul t s  in  a 
large and capacity  for  wat er r eten t ion . The po s s ible  e f f e c t s  o f  
a1 on 1 and chemical behaviour o f  soils  d serves much  c loser 
a ttent ion ( Furkert  and  Fieldes , 1 968 ) , however , there i s  l it t l e  l itera ture 
ava ilab l e  on this ec t .  
2 . 5 . 3  Soil-Water  Charac terist ics  
Wa t er e ro s ion o f  so il is  a f f ec t ed the so il tha t c ontrol  
the rate  with  which rainfall  en ters  the  sur fac e .  Thes e  inc lude  ( i )  macro-
porosity  of  the so i l  sur face , ( moi s ture content o f  the  s oi l  a t  the t ime 
of the ra in , ( ii i )  i l  "Dd  ( 
to  and eros ion 
the res istance o f  t he so il  surface  
rainfal l  and runo f f , i . e .  the  
s truc tural  condi tion o f  the  soil  surface ,  and  soil  cohesion b ecomes  
very sma l l  as  the soil  b ec omes satura t ed ) . Soil  ero s ion by wat er involves  
raindrop splash  and sur face  runof f ,  which is  man ifest ed in  sheet ,  rill  and  
2 3 . 
u l timately  ero sion . ( Baver , Gardner and  Gardner , 1 972 ) . 
This traditional approach  to  soil  e ro s ion s tud ies is  explained by  
Horton ' s  runo f f  model , which s surface  runof f  in terms of  the  
inf iltration theory . Hor ton  ( 1 93 3 )  c onsidered tha t the inf il tration 
cap;] of so i l s  in a catchment wil l  decrea se exponentially  a 
prolonged s torm o f  constant  int ensity .  He a t t r ibutes this  to  fac tor s  
opera t ing a t  the soil  surface  such a s  c ompaction ,  s truc tural , and 
of f ine  par t ic les . Eventua l ly the inf il tration capac will 
decrease  to  a value  below rainfal l  int en s ity , and a t  this  poin t  surface  
runo f f  ( or  overland f 
of  soil . 
beg i n s  to  oc cur , c au sing ero s ion  o f  the surface  
In  recent years ,  with  a greater  amount of  in formation ava i labl e ,  
workers have suppo rted  the Hewl t t  runo f f  mode l . This  mod el ,  a s  d escribed 
by Hard  ( 1 9  a s sumes tha t inf iltration is  s eldom a l imiting f ac tor , 
and much  o f  the wa ter inf il trates  into the so i l , and moves the  soil  
as  int erflmv .  In  thi s cas e ,  in terflow may make a sub s t an t ia l  con t r ibut ion 
to sto rm runo f f . 
Thus in eros ion s tud ies  i t  is  r tan t  to c onsider  movement o f  wat er 
lat era the soi l ,  a s  wel l  as  over its  sur fac e .  y,Jar d  ( 1 9 7 5 )  
considers  that t h e  s o i l  fac tors  favouring int erf low are :  
1 )  when 1a t eral  ie conduc tivity  in the surface  soil  hori zon 
is  subs t ant  than the  overall  ver tic a l  i c  
conduc t ivi the soil  
2 )  when a thin e soil  overlies  b edro ck ,  with 
a markedly  s trat i f ied soil  p ro f i le  e . g .  a marked d i f f er ence  
in  hor i zon  t extures or  extent o f  c ementation . 
3 )  where an iron pan occurs a sho r t  d i s tance below the sur f a c e ,  
4 ) the presen c e  o f  o ld roo t hol e s  and animal burrows , and 
o ther subsur face  pipes . 
24 . 
Ob s erva t ions  in the Torl esse  • South  I sland , New Zealand , by  
( 1 9 7 6 )  show that su r face  runof f  sel dom o c curs . even a f t er an 
int ense  rainfall  o f  1 6 5mm in 3 6  hour s .  The int ensi  o f  a one-year  s to rm 
in th e Ruahine Range .  e s tima t ed f or the West  Tamaki River  catchment i s  
60mm i n  24  hours t in ,  pers . c 
considered  Thus it  is  unl ike tha t 
l ess  t han the int en s  
or  surface  runo f f  
occurs  i n  t he Ruahine , unl e s s  there  is  some o ther l imit  factor  s uch  
a s  a compac ted  soil  surface .  
is  avalanches  occur  on s s when a ma s s  o f  soil  and rock  
to  f 10,,;7. 
on f o re s t ed 
is  caused  a 
Jackson ( J  966 )  c omment s  on the oc currence  o f  debris  avalanches  
in Fiordl and , New Zealan d ; and considers  tha t  movement 
inc rea s e  in s t res s  a s  the ma ss o f  f o res t  anchored  on  
the s t eep slope  increases , i n  c ombinat on wi th c l ima tic  a bnormali t ie s  such 
as  ra infa l l s ,  when in terf lO\\T occur s .  
"va ter  accep tance and reten ion . s o il s t ruc ture and 
con s id er ed to  b e  r elated to  the se  eros ion processes . 
The soil  proper t ie s : 
format ion are  
Los s  of  1 from e levation in  the intermon 
o f  south-wes tern U . S . A .  has been stud i ed mult 
, 197 1 ) . It  was found tha t amoun t o f  c over and  s t were 
r ela t ed to  ero s ion . tant  so il  a f f ec t  mos t  c l o s  
erod ib ,,'er e  f ound to  be : organ ic  mat t er ,  anteceden t  moi sture  conten t  
o f  the sur face  soi l , bulk  d ensity and lary  
The l it erature  therefore  ind ica tes  the  e of  soil-wa ter  
chara c teris t ic s  in  ero s ion s tudies . The inter-rela o f  wa ter  and 
as ini t ia t  erosion processes i s  a l so o f  tanc e ,  
and  ha s been no t ed b y  many workers ( e . g . , 1 950 ; Jackson , 1 96 6 ;  
O ' Loughl in , 974 ; War d .  1975 ) . lnves  t ion o f  a s  an 
• involves s tabH studies and measurement o f  certa in soil  
mechanical propert ies  ( e . g .  shea r s tr ength , s tress  and cohesion) ,  which  
are  d iscussed in  the  ensuing s ec tion . 
2 5 .  
2 . 5 . 4  S tudies  
Gravity  i s  the  primary  opera ive forc e in mass  movement o f  soil  and 
rock on slopes . Nas s  movemen t occurs  when the shear s tr ength o f  a body 
of material is  exc eeded by its shear s tress ,  over a relatively c ont inuous  
surfac e ,  ( S owers and Sowers ,  1 97 0) . Parameters  contribut to change  in 
soil  shear s tr eng th or  s t ress are l is t ed Sowers and Sowers ( 1 970)  and 
fur ther d etailed by S elby ( 1 970 ) , ( s ee Table  3 ) . 
S lope s tabil ity s tud ies have shown t he importanc e o f : 
( 1 )  soil-wa ter  cond i t ions , and ( 2 )  t r ee roo t and dis tr ibution , to 
s tabil ity of so il materia l s  on a slope , (0  ' Loughlin , 1 9 74 ; 0 1  
and P earc e ,  1 97 These  \·m rkers adopt  the lnf Mode l  to 
examine movement-promo movement-resis t ing forces , opera t ing  on a 
s teep slope . This model uses  Cou Law to describe  a so il ' s  shear 
strength,  or res is tanc e to  failure a s :  
s '" C + tan ¢ P 
where  S 
C 
P 
tan ¢ 
shear s 
cohes ion 
eff ec t ive  pressure  normal 
to the shear 
coef f ic ient of f r ic t ion . 
O ' Loughlin ( 1 97 4 )  
where ¢ i s  the ang l e  o f  f ric t ion . 
ied the Infinit e-S lope Nadel  to  d a ta c o l l ec t ed 
from a , c learfe l led s lope  in sou th,iles t  British Columbia , Canada , 
and conc lude s  tha t : 
( 1 )  the main causative  fac tor in landsl  
the  tree root  sys tem ,  
i s  d e t er iorat ion o f  
and ( 2 )  pore  wat er pressures  are o f  impor tance t o  soil  stabil  and 
any changes  induc ing sa turat ion of steep should  be  avoided . 
2 6 . 
TABLE 3 :  FACTORS CONTRIBUTING TO MAS S  HOVEMENT IN  SOILS 
1. 
2 .  
3 .  
4 .  
1. 
2 .  
3 .  
4 .  
A .  FACTORS CONTRIBUTING TO  HIGH SHEll.R STRESS  
Remova 1 o f  l at era l suppo r t  
Trans i  
Removal o f  
support  
by  
stresses  
Type s  
Compos i t ion a n d  texture  
Physico-chemical  r ea c  tions 
Ef fects  of porewater  
in  structure 
Maj o r  Hechanisms 
i .  S t r eam or  wa ter eros ion . 
i i .  Subaeria l  wea ther , we t t ing , 
, and  f ro s t  ac tion . 
i i i . S lope  s teepness  increased by mas s  
movement .  
iv . Hanmade  and  p it s .  
i .  o f  rain , snow , talus . 
i i .  Fill s , was , s truc tures  
i .  Ear 
i i .  l1anmade  vibrations . 
i .  Undercut  
i i . Subaer ia l  
and f ro s t  ac tion . 
i i i . Sub terranean ero s ion  
f ines or  solution o f  
iv . a c tivi t i es . 
wa ter . 
t ion o f  
or Mechani sms 
i .  Weak mat eria l s  such as  volcanic tuf f  
i i . 
i i i . 
i .  Cation 
i i .  
i i i . 
1 .  B uoyancy 
i i .  Reduc t i on 
i i i . 
1 .  S pontaneous 
( Selby , 1 970) 
t en s ion .  
wa ter on  
tion . 
icat ion o f  the Inf ini te-S Mod el to a area ,  
the  r ea l  s ituation , p rovides  an  invaluabl e  metho d  of  
stab il  ana  
Mea surement o f  soil  mo i s ture  t en s ion s pore  pressure s  
influenc e soil  cohes ion , a r e  t hu s  r e la t ed to shear  s 
2 7 . 
h d ir ec 
of  a so il , 
as  sho"\\'1l Cou lomb ' s  ra in s torms has b e en used a s  an e s t imate  o f  
s s tab il i t y  in N ew Zealand  s it ua t ion s Jackson ( 1  Jackson 
con s ider s  t s in s may be  o f  a cyc l ica  natur e  with  a c e o f  
sl  ing ,  weather  and il evelopmen fo llowed  r enewed 5 1  
thus support  the  id ea tha t  t he  per iod  o f  inc reased ero s i on seen in the 
Southern Ruahine  , at  presen t ,  is  par t f a na tural ical proces s .  
2 . 6 
The l i t erature  ind ica t e s  that  t he ero s ion em in the S outhern 
Ruahin is one o f  
threa t o f  f lood o f  f er He 
erosion ra t es in the mountainland ; with the 
a in s .  the  ero s ion  ts  -
and t he r  d ebr i s .  which hoke r iver  downstream . 
Concern has b een sho\m number  o f  in r ec en t  
years  t o  a ac cept ed beli e f  t ha t  e ro s ion  rat e s  have increa s ed s ince  
the 1 9 3 0 ' s .  
Ero s ion rates  in the Ruahine  Rang  have varied marked wi g eo a l  
t ime . t he Pl e i s to c en e ,  l,;rer e  cons than 
are  At  this  , b esid e s  s o f  c o l d  c l imat e  
( g lac ia ls ) a l t erna t with  warmer ep isodes  ( in ial s ) , the was 
also  ec t ed to 1 f t  a s  t he Kaikoura i t s  c l imax . 
Po s ero s ion rat  have been smal l er ; however 
vegetational changes  over the las t  f ew thousand year s , (Elder , 1 
suggest  tha t smal ler c l imat ic changes have occurred . 
, causes o f  ero s ion in the Ruahine  Range are  to  be many 
and varied . The maj or o f  mas s  movemen s seem to  o c cur f rom a number of  
2 8 . 
causes s imu l taneously .  The f inal c au s e  is  a 
in mo t ion a hil l s id e  a I r  highly susc ep t ible t o  ero s ion . 
i s  un stable  due in par t  t o  s , f ault  
, s e t t ing 
The Ruah in e  
and the 
nature of bed rock I i  ies , rden ,  1 9 7 7 ) . 
Few soi l  surveys have b een c onduc t ed in the Sou thern Ruahine Range . 
A number o f  worker s have s ized t he occur r ence  o f  lo e s s  
t he basement greywacke a t  l ower l eva t ions ( e . g . , 1 96 9 ) . 
kse ( 1 9 7 7 ) , in mapp ing  he so ils  o f  
mo s t  comprehens ive s 
tha t t he so ils  are  a 
to d a te . 
ina 
and 
ha s the 
( 1 9 7 7 )  indica t e  
fac  t o r  i n  erosion s tudies  o f  the Ruahine 
A number of soil  parame ter s  a ppear  to  b e  o f  e t o  ero s ion potential  
in  the mountainland . The soil  ac t s  a s  a med ium which ac  and s tores  
rainf a l l ; and suppl e s  it  to  the  r iver  channel . The ra t e  o f  a c e  
low and s t o rage  capa c i ty  are  important  fac tors  t o  b e  considered . 
s 
Model , ( 0 '  
0 )  
and ( 2 )  
s tab il may be  c r i t ical  eva lua t ed the Inf init  
in ,  1 97 4 ) . This enabl e s  an e s t ima te  to  b e  made  o f  
ib i l  o f  s ma teria l  t o  movement ,  
rel evanc e of  cer t a in fac tor s t o  s tabili  , e . g .  roo t 
i s t r ibu t ion and so il  sa tura t ion . 
e ,  
CHAPTER THREE 
DESCRIPTION OF THE STUDY AREA 
29 . 
fIGURE 2 :  LOCALITY ��p OF S TUDY AREA . 
NEW  ZEALAND  
TASMAN 
SEA 
KEY 
, State Highways 
- - - ' �I I ndigenous forest boundary 
�-'" 
, 
" 
, 
, .­
f 
" 
" 
I 
, 
"-
, 
\ 
, 
I 
\ 
AWhaingapooa 
1400m 
SCALE l '  2 50.000 
I Ii 
- - - I 
() 
Study a rea - West 
Tamaki River catchment 
3 . 1 LOCATION 
CHAPTER THREE 
OF THE STUDY AREA 
F Leld\wrk wa s conduc ted in the \.Je s t  Tamaki River c a tchment 1 3km 
nor thwes t  o f  Dannevirke ,  on t he eas tern f lank of  t he Southern Ruahine 
, ( see  F 2 ) . The river extends  5km along  the foo t o f  the 
mountainland , in a N . N . E-S . .  \.J.  direc t ion . A d e tailed  s o i l  survey "la s  
30 . 
carri ed out in one o f  the  prine t r ibutar ies , Car Park Creek , which is 
tel  650  metres up stream of the va 
contro l  weir is  loca ted . 
3 . 2 PHYSIOGRAPHY 
S outhern Ruah ine 
The Southern Ruahine i s  cha ra c terised 
formed i f t  of this par t o f  the 
throa t where a 
a s teep eas t ern f ront , 
the wes t ern s ide  o f  the 
Mohaka Faul t .  The summit  teau , \17hich ext ends  sou thwards  a s  far  as  
ream , is  a r emnan t ero sion surfa c e ,  til ted  t o  the wes t , ( see  2 . 2 . 1 ) . 
The western s id e  o f  the away l e s s  , f rom this  surface .  
The S outhern Ruahine r i s es nor thwards f rom 9 1 4m a t  Wharite  P eak in 
t he south ,  to 1 392m a t  1 ,  a t  the head  o f  t he West  Tamaki River 
ca tchment and po int in t hi s  part of the 
A number of r ivers drain the ea s t ern side  o f  the These  
characteristically  have s teep to  very s t eep valley-s ides , f o r  • the 
mean slope e o f  s ides  i n  the Raparapawai  Stream i s  30° . 
1 9 7 5 ) . The longitudi,, ;l o f  these  r ivers , f rom their head to  the 
Manawatu River is smoothly concave ,  on a large scale ,  • 1 97 7 ;  see 
. 3b ) .  I t  is  this  form of  pro f il e  which i s  assoc iat ed with  s treams in 
equilibrium with the ir geologic and hydrologic environmen t ,  so that on a 
-E 
-
c o 
F IGURE 3 :  LONGITUDINAL PROFILES OF : (A) CAR PARK CREEK; 
(B)  WEST TANAKI RIVER ( from HOSLEY , 1 9 7 7 ) . 
(vertical  exaggeration approximat XIO) 
Distance source m )  
B 
500 
3 1 .  
Tamaki  
R 
O �O------�5------�1�O------�15------�------�2-5 ---
source ( ) 
broad sca l e  the rivers  have lis t ed t o  carry t heir load . 
Hest  Tamaki River  c a  hment 
32 . 
The course  o f  the  West  Tamaki River runs leI  to the s eas t ern  
front o f  the  wi t h  Ho lmes Rid ge  ris  s on its  eas t  
to a o 6 5 Om .  Hit hin i t s  catchment , it i s  
inc i sed ; el or  suhca tchment s  i t , from the wes t  
and north . The se  subcatchmt'nts  are  s t eep to  very s eep-s ided . The 
of t he valley- s id e s  in Park Creek subcatchment s b etween 
and 4 A i tudinal  pro  i1e  o f  Car  Pa rk Creek b e  with  
tha t o f  the t Tamaki  River ( see  I t s  b road pa t t ern i s  s een to  
l ess  conc ave , icat  t ha t  the channel is  no t so  1 
usted  within i t s  c a tchment . Sl  s in  the  curve ind ica te  the  
presence o f  \"laves . 
The uni t s  presen t  in Car Park Creek s eem represen ta t ive  
o f  the subcatchmen t s  f \\fest  Tamaki  River  cat chmen t . . 4 . f '  l�  1.e s  
a id e in the  catchmen t  t o  which  the n ine-uni l andsurface  mod e l  
o f  Conacher and e ( 1 9 7 7 )  has b een ied  ( se e  Tab le 4 ) . 
Hie f rom Car Park  Creek a re i llustra ted in , 5 6 .  
In 5 , uni t  i s  the  interf luve and o f  l imit ed exten t .  Unit  2 
i s  t seepage s lope , charac ter ised  a small  f whils t  the 
convex creep  ( unit  o c curs be low i t  on o f  up  to Thi s  
unit  i s  d e f ined a s  one in which  soil  c reep and terrac e t t es 
occur . s c a l e  d eep t errace t t e s  been not  on uni t  3 in Car  Park 
Creek,  which in  es  ext en d  for  about 1 00m across  the hi l l  ( se e  , 7 ) .  
suggest ing tha t a c t ive  soi l  creep ,  with  creep  o f  colluvial 
materials  below the solum , is a s so c ia ted  with this l andsurface  unit .  
The f a l l-fa c e ,  uni t  4 ,  i s  charac terised by exposed rock , and i s  d e f ined  
response  t o  the  pro c e sses  of  fall  and  rockslid e ,  with pressure r el ease  
an  tant und process " ,  . 5 shows this unit  in the  sub-
FIGURE 4 :  
on 
TABLE 4 
Idea l ised  Dia gram a the  Land sur face 
e i n  the \\cs t ki  River  Catchment . 
1 2 3 5 
i t s  1 7hich Occur 
5 
CLASSIFICATION OF LANDSURFACE UN ITS , ACCOFpING TO THE N INE  
UNIT HODEL OF CONACHER AND 1 97 7  
3 3 .  
LA .. �DSURFACE UNIT  DI STINGUISHING 
NUMBER 
1 
2 
3 
4 
5 
6 
7 
8 
9 
NAl'1E 
Interfluve 
S 
Convex c reep 
Fall  face  
RELEVANT TO THE PRESENT STUDY 
soil  in  s itu . modal 
above Fe pans ; 
soi l  water  movement 
t eral  sllbsur fa e 
soil  creep ; t erracette  forma t ion ; processes  
resul  from subsurfac e  soil  wa ter movemen t 
soil formation res icted  
fall  and s fea tures ' 
isol  
tational  contrast  areas o f  and s ha low soil s in  
Colluvial  foot-
Al luvial  toe­
sl o pe  
Channel wal l  
Channel bed 
an area o f  mas s  movement s  
heterogeneous soil  mantle  addi tions  
from upslope ; o ccurrence  of  eosol  
horizons 
a lluvial redeposit ion ; 
eosol hor i zons 
intermit tent regosol  
corrasion , slumping,  fall  
o ccurrenc e  of  
) f orma t ion ; 
no  soil  format ion : ta tien o f  mat er ial  
downvalley by s tr ", am ac t ion ; t ion 
34 . 
catchment , with  a rockslide ( to the right of  c entre) and two smal ler d ebri s  
slides contributing t o  gully ero sion ( in t he lef t  o f  photo ) . Uni t  5 ,  
the 
eros ion .  
rta t ional midslope  is  characteris tically very susceptible t o  
Several erosion scars  may be  seen in this  zone on Fig . 6 .  
Below this  unit , a colluvia l  footslope (unit 6 )  somet imes occur s  
�vhere the dominant process  i s  r edepo s i t ion  o f  col luvial ma teria l  f r om 
6 ) , 
up 
unit . 
, a lthough some mat er ial  i s  undoubt edly  transpor t ed acro s s  this  
Unit  6 is  charac t e  i sed b y  d eeper col l uvial  so il s .  
4 indicates  tha t  the l uvial  toeslope  7 )  i s  o f  limi ted 
extent  within the i>Jest  Tamaki River ca tchment , in  which Car Park Creek is  
located . However , this uni t  may be  s een in 6 ,  on the ins a t  
the foot  o f  t he Uni t  8 ,  the channel \.;ral l  o ccurs in the ca  hment ,  
but only over shor t ,  d iscont inuous sec t ions of  the r iver c hannel  where  
some downcut occurs . Uni t  8 ,  t ogether with uni t s  7 and 9 ( the c hannel 
b ed ) are  mos t  extensive on t he a c ent  l owland area a s  shovm in . 6 .  
In Car Park Creek ,  uni t  9 is  choked with to of be tween 
2 . 5-4 . Sm c i ted  in 1 97 7 ) . and to greater  
in c er ta in local i t i e s , where waves occur . This  unit  a e  s as  
a s zone ,  due to  t ion ; with intermit ten t  ta t ion 
of mat erial do��stream , and a f te r  intense  rainstorm even t s .  In 
the case of Hut ano ther subcatehment of the Tamaki River ( 
a amount o f  debris  ,.;ra s  i e d  to  the channel bed (unit  9 )  dur 
Cycl one  Ali son 40 year s t orm event ; R .  Martin ,  per s . c orum . ) . Al 
some material was undoub fur ther downstream .  the l arge 
bulk of  the d e tr i tus  a c cumul ated in the c hannel bed with the net  r esul t 
o f  
has  taken 
subcatchment .  
S ince  Mar c h ,  1 9 7 5 ,  when t he 
, s o  that a c hannel wall  
lone o c curred , do��cut t  
) i s  seen in  this  
. 8 ) ,  
The erosion p ro�es ses  in t he West  Tamaki River c a t chment de terluine the  
and extent  o f  land forms , which a r e  adequately  desc r ibed by the N . U . L . M .  
35 
Fig . 5 :  Landsurface Units a t  the Head o f  Car Park Creek 
Fig . 6: Landsurface Units  o f  a Valley-side in Car Park Creek 
cone o f  Conacher and Dal 
3 . 3 GEOLOGY 
Southern Ruahine 
36 . 
e ,  ( 1 9 7 7 ) . 
The Ruahine ha s b een d e s cribed  a s  a hor s t  faul t s , with 
ver t ical and d ex tra l t ranscu rren t movement . on both  sid e s ,  , 1 9 5 9 ) . 
The if t ed mas s  is int d e fo rmed and cons i s t s  o f  indura ted  basement 
ro cks o f  t he Tor l e s se  C S t  , 1 9 7 4 ) . 
The l i  o f  the eas tern Ruahine consi s t s  of 3 tec ton ical  
concordant struc  ra l b el t s . t north-eas t  i and Bell , 1 97 
The three  b e l t s  cons i s t  o f  coherent  and d s equence s  o f  gr  
( 101,.1 e s e d  sands with argil l i t e  band s , cher t s  and 
i tes . 
o f  the bedrock geo l o gy the ea s tern f ront o f  the S ou thern 
Ruahine indica tes  2 l itho , ( Hubbard  e t  a1.  1 9 78 ) . The ea s t ern-
mos t  con s i s t s  o f  massive  t ernat s equences  of  sands tone , s i l t st one  and 
llit e ;  and the we s t ernmos t  is an a ssortmen t  of s and vol can ic 
rocks , " f loat  in  a rna ix  o f  black  laceous  t er ial . 
S icant a c cumu l a t ions o f  gr  loess  occur in  s ome i t ie s , 
dur the pc ial cond i t ions of the  Pleistoc en e  
(Milne , 1 9 7 3b '  Rhea , 1 9 68 ) . Thi s  l oess  c ontain s  i c  in 
many c a se s .  
Thus , the Ruahines  ar e a r ela t young mountain rang e ,  
with  int ense  fault  and sha t  o f  aIr  f o lded  s trata . Their 
r a t e  upl i f t  i s  rapid , 1 . 14mm per  y ear f o r  the  west 
f l ank,  over the las t  0 . 8  million years .  (Boel l s torf  and Te , 1 9 7 7) . 
These  e f fec ts  on the l andscape  evolution o f  the  S outhern Ruahine 
are discussed in the L i t er a ture Review ( 2 . 2 ) . suggest  tha t  the  
geological s i tuation in the  Southern Ruahine Range is one  
prone to  eros ion . 
FIGURE 7 :  A ROCKSLIDE AND DEEP TERRACETTE FEATURES (dashed line) ON 
THE CONVEX CREEP SLOPE OF CAR PARK CREEK . 
Photo : D . G . Bowler 
38 . 
Tamaki River c a t  hment 
he  \.Je s t  Tamaki  River c8 tchmen t , the main litho ies  
are  indura t ed s ands e ,  I t s tone  and a rgill i t e .  No i t es , 
t es or  mic robrec c ia s  have found , • 1 9 7 7 ) . 
S truc tural l  • the s �  ta  a re s t  and over turned  to  the  
t rock s r 1, leI  to  the The s t ruc ture  
with  the frac  and  faul t ed ture 0 t he bedrock rend ers  the greywa cke 
to ion et al . 1 9 7 8 ) . - --l itho type 
s er o f  c rush  zan al ong t he g enera l l ine the tchment  
fo r a I . Skm \\Tide  fau l t  zone ,  1 97 7 )  . 
J ea s t  Z zone s  occur  Car  Park  Creek . 1 t 
scarp s in s t  Tamaki sugges t  tha t  these  faul t s  have b t ive  
dur the la t e  erna ry ,  en . 1 97 7 ) . 
3 . 4 
Southern Ruahine  
st  so il surveys described  the soils  of the  thern 
Ruahine a t  a reconnaissanc survey l evel , with l i t t l e  invest  t ion 
o f  so il  it  the broad s soil  unit s .  Prev ious 
survey  r t s , which are  d etail ed the L i teratur Review ( 2 . 4 ) , 
that the  s pa t t ern s 
and age , in this area . 
Wes t  Tamaki River  
The detail ed 
G . J .  Smit h  cited  in 
y r e l a ted to  par en ma terials , c l imate  
t chmen t  
i l  s1lrvey o f  this  ca tchment has b een  ed  
, 1 9 7 7 ) , at  a scale of  1 :  6 3 , 3 60 .  Thi s  survey 
map s  84%  o f  t he ca tchment a s  Ruahine soils  vli th 1 2% 
a s  Rimutaka soils  a t  eleva t ions , and 4%  a s  
Kopua s s i l t  10ams a t  the mouth  of  the  catchment .  A 
invest  t ion o f  a s e lec t ed subca tchment in  the area , car r ied  out a 
par t  o f  t he presen t  , i s  d escribed in 5 .  
E 
FICURE 9 :  A LT 1Tl'nI :�j\ L  DISTRl  BUT roN OF r(lUR VEGETA.TION/\L 
SDEC  FS  T THE SOJTTHrp\l 1<P/, ] l  
THE IR  DISnnBPT 10I\ FUETHER :WWrH . 
thern  
-- - - L  
c---c 
\---K 
R uah 
t henmod  
Ced r 
.- - b 
( 01 ria c o l enso 
I 
I 
I 
( 
Libocedrus b idwil i i  
A rac emosa)  
ted  f rom Elder , 1 9 6 5 )  
n 
39 . 
3 . 5 VEGETATION 
Sou thern 
4 0 .  
The vege ta t ion  o f  the Southern Ruahine o ccurs in thre e  ma in 
a lti tud inal r dwoocl  f ores t , i kamahi f o r es t , and 
i l ea t herwood  sc rub , wi th  vest  of  c eda r forest  and  snowgrass 
tussock ( Elde r ,  1 9 6 5 ) . A t a t ion map ,  t a s cale  o f  1 :  2 50 . 000  a l so 
summarise s  the  p r inc vege ta t ion o f  the I s ,  9 7  
9 5hm"s tha t c t a in vegeta  ion species  show a 
in their  t in Sou thern 
( i . e .  r imu , eclar nd Olea ia in c ompar i son with t he veg e t a t ion 
o f  t he fur ther  nor t , (E  , 1 9 6  This  s tha t  condit ions  
are more  a dver s e  in t he s outh  for  the  upper  l im i t  of  in ea ch  
zon e .  Rea s f o r  thi s ar e mo s t  ikel : 1 )  lima t ic s tr e s s e s  ( se e  3 . 6 ) .  and 
2 )  uns table  s it e s  for  te  tree  roo t e stabl i shmen t . 
Thus , c c d gi  tures  of  t he Sou thern Ruahine 
hinder  the  normal d evel opment of  vege ta t ional zon es . 
In  r ec en t  year s  d e t er iora t ion i n  t he f o r e s t  stand has been t ed 
s everal  s ( Cunn an d S tribl  . 1 97 7 ) . Fores t  deteriora tion 
resul ts  in gr e a t e r  eros  ep t ib i l i t y  o f  the land fac e ,  due t o :  
) l e s s  t e r  in take t ra t s ,  so  that soil  
i s  mor e  , and  
( 2 )  d ec r ea s ed root  d i s t r ibut ion , which  p rovides  a icant  decrease  in 
s bil  ( e  . .  0 '  in , 7 6 )  . 
( 1 97 7 )  considers  tha t the  f o re s t  deteriora t ion in 
Southern Ruahine pr imar a t t r ibuted  to e f f ec t s  o f  introduced  
wildlif e .  s ( 1 97 5 )  c onsiders  tha t  kamahi death due 
to climatir but S t rand ( 1 9 7 7 )  s tha t a number tors 
are  , and  a t  presen t  the ead tha t i s  visible  i s ,  
s ,  p a r  o f  a normal eye 
Fig . 8 :  
4 1  
Downcutt ing in Hut Creek , s ince Cyclone Ali son , of  
March,  1 9 75  
Fig . 1 0 .  
The Kamahi forest in 
Car Park Creek . 
(no te  many dead kamahi 
t rees in foreground) 
4 2 .  
Hes t Tamaki River  catchment 
The b road  a l t i tud inal vege t a t ion zones  o f  the , all occur in thi s  
c a tchment . S tr and ( 1 9 7 7 ) ha s a c omprehensive l i st  o f  t ional 
species  o c curr  wi thin t h e  ca tchmen t .  H e  no tes  tha t the 
hardwood  f o r e s t  occur s t e l eva t ions  up to 600m al  i tude , 
a l  there  i s  considerable  overla p be tween the three  broad zones . 
In l o ca l it ies where  c l imax exist s .  such  as the  
hardwood  f o r e s t  or kamahi fores t .  the  t ion  is  in a s t a t e  
o f  ra t . 1 However , the  vege tat ion i s  a t  many s i t e s  con-
tro l l  ed  e s ion , and  i s  in  various  era1  s tages  of  t . 1 1 ) . 
ta tion  on the  s vall ey-s ides  i s  a f f  t ed a dver s e  natural  
cond it ions s . 1 SOL. and un stab l e  s tra ta ,  a s  s hown in 2 .  
This  f e s hows tha t v!here  the e d ecrea se s  a t  the  f o o t  o f  the  
there  i s  a r e l a t  den s e  vegeta t ion  H ere  i t ioDs are  
mor e  stab l e ,  l l uvium tend s t o  accumula t e  and  s o i l s  a r e  d 
Na tural  co loni sa t ion 0 bare  sur f e s  na t ive  i s  Inlt  
dominated  to  toe , ra , nett l e s ,  and f e rn s .  I n  1 9 6 9 , 
the New Zea land  For e s t  S ervic e a r evege ta tion  programme with  exo t i c  
a t  s tab iU. s  s l  fac es  and conso lidat  banks and channel 
wat er s e s . Co  isa t ion of  thes e s i t e s ,  in  t h e  
former c as e ,  and s and wil l ows in  t he l a t t er s e ,  has  b e en 
suc ce s s fu l . 
3 . 6  CLU1ATE 
The Ruahine mountainland a cool  c l ima te  and annual 
rainfal l s .  Wa ter i s  the mos t  impor tant o f  eros ion 
in this mountain range ;  the rela t ionship between mas s  movement and 
rainfall s  New Z ea land well  e s tablished ( e . g .  1 9 6 6 ;  
Gr an t , 1 9 66 ) . Hater , therefore ,  plays an important  rol e  in soil  erosion 
and f luvial  erosion processes . format ion on the d i s tr ibution and 
43  
Fig.  1 1 :  Gully erosion 
in Car Park Creek 
( the vegetation is  in 
a range of seral s tages , 
being largely controlled 
by the erosion proces ses ) . 
Fig .  12 :  Vegetation on a Slope in  Car Park Creek 
(No t e  ( i )  the relatively dense vegetation at base o f  
s lope ,  where slope angle decreases , ( ii )  the presence 
of podocarps on the very s teep slopes adj acent to an 
erosion surface ,  suggesting tha t  stable  s it es do exis t  
on  this landsurface unit ) . 
o f  ra infall  in the  is  consequently of  much tance to 
erosion s tu d i es . 
The Ruahine ree  ives int ens e  rains tor1l' s at t imes , pr C:UVlll-l-l'CH 
f rom the south  or south-eas t ,  probab a ffec t in g  t he southern part of the 
range  mos t  o f  a l l . 
Southern Ruahine 
Other clima t ic s tresses  on the 
inc lude : 
W ind - Wharite  Peak is one o f  the wind ies t 
reco rd exc ept ional  h i g h  ,,,find speed s ; 
t ion  o f  the 
c e s  in  New Zealand , 
c loud  cover - sun s in the sou thern par t  of the range  are  
30%  less  than in  the nor thern par t ,  and  2 0% 
l e s s  than in the c entral  pa r t o f  the range ; 
i i i )  
snows , 
low humid i t ie s ,  and ; 
f reeze- thaw proce s se s . 
The la t ter  four fac tors  probably only have a minor ef f e c t  on ero sion in 
in mos t  year s . the Sou thern  Ruahine 
Effects  of  l ima t ic on  ero s ion rates  have  b een 
discus s ed  in the  Lit era ture  Review.  Gran t ( 1 9 6 5 , 1 , d i scuss  
c l imi' ic data for  the Tukituki River ca tchmen t  and eastern Hawke ' s  Ba  , 
sugges t s  tha t there have been s var ia tions in ra infall  over the 
past  f ew c enturies . He  s tates that " the c l ima t e  is  s eldom constant  for  
long" , and  a t tributes  much o f  the  vegeta tion 
to high rainfall  intensi t ies  a s socia ted with s to rmy 
Tamaki River ca tchment ( se e  Fig . 1 3 ) 
This c a tchment es a high annual prec 
and eros ion  
ion , which 
inc reases  s igni ficant ly ,"i th elevat ion from 2 000mm to  280Omm, as does  the 
amount of  c loud cover . 
The ca tchment ies  the effec t s  0 ;em int ense  r a instorm ,  
Cyclone  Ali son , a 4 0  year even t , which occurred in March,  1 9 7 5  and caused 
44 . 
FIGURE 1 3 :  RAINFALL HAP FOR THE TM1AKI RIVER AREA , 
SOUTHERN RUAHINE RANGE . 
I SOHYETS 
1 200 
1 600 
2 000 
2400 
2800 
3200 
4000 
f i e ia Zea land  MetereD 
O f f ic e d a ta , 1 94 1 - 1 9 7 
NOTE : In mount ainous  areas  where  
there are  very  f ew s ta t ions  and  
o f t en ra inf a l l  ient s ,  
the  i indica t e  the  
gene r a l  pa t tern o f  the rainfall  
d i s tr ibut ion . In  such a r ea s  
cannot  b e  used t o  obtain  an  
e st imat e  o f  t he r a infa l l . 
ura t e  
4 5 .  
4 6 .  
a amoun t o f  ero s ion throughou t  the catchment .  Today ,  terraces  
r esul  f rom this  event are  s t i l l  upstream o f  S tanf i el d  Hut 
in the ma in channel and in Hut Creek .  
o f  the eroded  a rea s i n  the upper r eaches o f  the subcatchments  
a r e  a ssocia t ed '.vith permanent or  per iodic  wa ter  channel s indica  that  
f luvial eros ion and  inc i s ion . c lear ly r a infall  int en ty ,  may 
an rtant role  in rej uvenat ion of 
Need le  ic e has been ob s erved a t  the 
e ero sion pro cesses . 
eleva tions in the cat chmen t ,  
ind icat  tha t  freeze-tha1d proc e s ses contr ibute to minor erosion  
the  winter mon ths , on  the sma l l  a r ea s  of  bare  soil  in the catchment .  
wind -thro\ro trees  have been no ted in the catchment , ind ica  
wind velo c i t ies have o c cu r red on  many o ccasions  in  the 
Af t er a long ,  1 during the Summer o f  1 9 7 7 - 1 9 7 8 ,  s o i l s  a t  
elevation in the  c a t chment vJere s t il l  very moi s t . A ificant  
o rganic  accumulat ion on  the soil  surfa c e ,  i n  t he  Leatherwood  
zOhe enabl e s  so i l s  t o  r etain  much o f  their mo is ture . I t  s eems 
tha t ,  in mos t  cases , ines s will  a f f ec t  t ion a t  
l ower e l evations , where there i s  l e ss  accumulation  o f  
the  s urfac e ,  and  rainf a l l  i s  decreased . 
l i t t er on  
3 .  7 INTRODUCED WILDLIFE 
The N ew Zea land evolved lvi thout any na t ive 
mammals .  On arrival , introduced  a number o f  mammal s  into the 
mounta inland . and were introduced  over 2 00 y ea r s  a go . Red 
d eer were l iberat ed in  the Northern Ruahine Range  in  t he la t e  1800 ' s ;  
and by the  1 9 00 ' s  a l l  three  wer e  present  in  the Southern Ruahine 
Range , , 1 9 78 ) . Opossums were probably in the Southern 
Ruahine Range by 1 940  (Cunningham and S tr ibling , 1 9 7 7 ) , and o ther introduced 
4 7 .  
,vUdl i f e  ind s s ika deer . rabb it s .  ra t s  and mice  ferret s  and  s toat s .  
There i I i  e d t e  in t he I i t era ture  t ha t  the in t ro du c ed d l i f e  
i n  t he Ruahin e  ha s h ad  an  advers e  e f f e c t  on the forest  canopy _ 
to brows ing and  gra z o f  t unders torey . a s  a s  many o ther 
e f f e c  • for  e .  ba rk rubb . so i l compac t ion and  f mic ro -
c l ima t e  due t o  o p  
on  t h e  r e la t i  e fee t s  
mechanism o f  ion . 
up f t h e  anopv . However . there  is  
roduced  wil d l i e ha s had s a t r  
( 1 9 7 7 )  ons i d  f o r e s t  d e t er iora t ion in r e c ent  decades  i s  
pr imar i  f f  t 0 d • goa t a n d  opossum ,  whic  s in turn  l ed to  
s in stabi  i t y .  Gran t ( 1 9 6 5 )  and  ( 1 9 7 3 ) onsider  tha t  the 
f wi] d an 1 s on ta t ion damage  and  e r o s ion may have been  cons  
less  t e f f ec t s  o f  in s ed  s tormine s s in recen t  decades . Mos l ey 
( 1 9 7 7 )  ha s evid enc o f  canopy e in the S tream 
1 92 0 ' s  to 1 D ' s  whi c h  i . he c on s i d er s . before  introduced wildl i fe could  
have p l  a s t role  in t ion modif ica t ion . 
However , conc ern about the  e f f ec t s  o f  introduced wild l i f e  on 
, and resu l t  s lope  ins tabil  has  b een suf f i c  t o  
p romp t many imal c on t rol  opera tions  in the Southern Ruahine  ince 
t 
the 
9 38 ;  with  i ture  e l a s t  year s  ( 1 9 74 -7 7 )  be  about $ 7 00 , 000 , 
• 1 9 7 8 ) . 
\'ies t  Tamaki River  c a t chment  
The c a tchment was in 1 9 7 5-7 6 the New Zea land Forest  Servi c e ,  
t o  a s se s s  c ond ition  o f  and r ela t ive  e f f e c t s  o f  animal 
t ion d ensit ie s . I t  was c onc luded f rom thi s  survey that deer contro l 
operat ions should c oncentrate  on  the  cedar t ion  b el t ,  and tha t special  
a t t ent ion should b e  t o  opo s sum control , with to  kamahi  
( S trand ,  1 9 7 7 ) . However , an exc lo sure plo t  set  up within the catchment 
indica t es that the unheal nature o f  kamahi  can be  a t t r ibut ed to exposur e 
a s  wel l  a s  to  opossum browsing , ( S t rand , 1 9 7 7 ) . I t  i s  evident in the  
lower reaches of the ca tchment ,  that trampling by cattle and has 
also had a marked effec t on  deterioration of the fores t 
The presence of fuschia . normal a highly species to  
opossum . has been no ted at  elevations bet\\Teen 9 0 0m and 1 00 0m on the 
wes tern o f  the catchmen t . I t  commonly appears at  the heads of  
ies . and is in  a heal condition .  I t s  survival may be due to  
i t s  occurrence at  al titudes above which opossums browse , or pos  
that  these ts are an 
3 . 8  EROS ION 
Southern Ruahine Ran 
atable  variety . 
48 . 
The erosion situation in the Sou thern Ruahine has been reviewed 
in ter 2 . 3 . Much of the e ro s ion  oc curs in the form of  mas s  movement 
and there are many examples of debris slides and avalanches , and 
rockslides  the t of Varnes , 1 9 58 )  in thi s  area . Creep 
ero s ion of soil and screes al so o ccurs . as does i cial movement s  
of soil ,  i . e .  soil sl and Soil Division , MOwn , 1 97 1 ) . These 
so il slips may int o  more ted eros ion , espec if  
ion of  the erosion scar is  by  animals ,  s o  tha t  
the eroded surface does no t become suffic s tabilised before the next 
int ense rainstorm .  
Wes t  Tamaki River catchment 
The ty of eated eros ion sites o ccur in a zone along the 
upper reaches of the subca t chmen ts of the v,Jes t  Tamaki River . This  zone , 
to  an alt itude of between about 7 00m and 1 000m is  characterised 
by debris  slides , debris avalanches ,  rock slides and ero s ion . 
In Car Park Creek , deep-seated eros ion types  are largely gully eros ion 
(Fig . 1 1 ) ,  debris  slides and rock slides (Fig . 7 ) . Fig . 1 1  shows gully  
eros ion a deeply incised  V-shaped valley . With undercutt o f  
the valley-sides debris slides are produced which become integrally 
4 9 .  
a s sociated  wi th the gul ly ero s ion . When t he gully becomes a wat ercourse , 
our ,',le t  win ter p er iods , removal o f  detr itus  and f luvia l under cut o f  
thes e  s lides  occur s , dur ac t ive eros ion .  Fig . 7 i s  a ver t ical  aer ial 
photo graph o f  a rocks lid e ,  n amed Ca t s  Paw S car , in Car Pa rk Creek . This  
rocks l i d e  i s  a l so evident in  aerial  pho tos  taken in  1 94 6 ,  where i t  i s  
vi sible  as a p rominant ero s ion f eatur e . Here ,  ero s ion has with 
shea r f a ilure  a zone of  'tJeakness , ei  her in  the solum o r  a a 
bed d in g  
1 00m in 
in the Large  s ca l e  t errace t te s  ( a pproxima tely  
c an a l so b e  s een in . 7 ,  to the r ight o f  Ca t s  Paw 
S ca r ,  on the convex c reep It  s eems likely tha t no t only soil  creep ,  
but a s cale  c reep o f  a l l  sur f ic ia l  ma terials  and wea thered bed rock 
may be o c c urring in  this  r eg ion . 
At  l ower e leva t ions in the  c a t chment , on s teep to very steep slopes , 
surf i c i a l  eros ion scars  a re  d eveloped , g enera in f orm o f  s oi l  and 
earth  s l  the  termino l o gy o f  \{a t er and So il  Divis ion , MOlATD , 1 9 7 1 ) . 
The se  o c cur a t  po int s  on the overs t  inherent ly weak basement rock ,  
where  i s  unabl e  to s tabil i s e  the  soil  ( e . g .  Fig . 6 ,  1 2 )  • 
Field  observa t ions , and s tudy o f  pas t  aerial  pho tographs that these 
f ea tur e s  occur r epea t edly  at the same vulnerabl e s i t es , with int ermediat e  
s tages  o f  r egrowth a t tained before  the n ext  s l ip occur s .  These  s l ips  t end 
to o ccur  predominant ly on the nor th-eas t  fac ing s ides  of subcatchments  in 
the Wes t  Tamaki River catchmen t ,  indica t  
causal fac to r .  
tha t  a spec t may b e  an important 
The Wes t Tamaki River c a tchment has many of f luvial eros ion 
depo sits  preserved in i t s  c hannel bed . Fluvial  eros ion processes have 
gradually  developed the channel  pro f il e , widening its  banks , and sometimes 
forming terrac e ,  fan and gravel  wave  deposits . Bank ero s ion  is  part icularly 
evi d ent  where it  is undermining gravel t erraces and fan s , and only 
occurs in periods  of  high water flow,  when high ero sion veloc i t ie s  are  reached . 
ity  is  def  as  
of  a s i z e  t o  move 
or a r e  l i f ted  into suspen sion  " , 
" the  c r i t ical  v e loc  
r o l l  o r  s l  
S trahl e r ,  1 9 7 1 )  . 
50 .  
when pa r t  l e s  
on  the  s tr eam bed , 
tes  o f  bank eros ion 
are h var iable  no t only  over t ime but  a l so f rom one  reacb to another . 
However , an avera ge  annual r a t e  o f  channe l  
o f  mounta  
tin ,  p 
ahout t,'JO-fo  d 
s .  comm . ) .  
i e ima ted  to he o f  the  0 
. ) . ren t ra t es  of  channe l  
t he and rea . a t  he foo t o f  
t h e  hush-cla  areas 
- 1  
r o f  0 t o  O . 5m yr  
inc ea s e  
r t in ,  
CHAPTER FOUR 
HISTORY OF EROS ION THE hIEST TAMAKI RIVER CATCHMENT 
4 .  1 
and 
CHAPTER FOUR 
H I STORY OF EROSION I N  THE IvEST  TA..MAKI RIVER CATCHHENT 
INTRODUCTION 
The ero n o f  the  s t  Tamaki River c a t chment i s  
par number  o f  d i s continuous 
sur fa c es  oc cur in the r iv er channe l  each r epresen t  
5 1 .  
i t-
a t  least  
one  ero even t .  The degree  o f  i 1  pro f i le  d eve  and vegetation 
r;ca tur  aD  each of  these  sur fac e s  enables a t entat ive  order o f  the 
s i t s  and thus  the prev ero s ion t s .  Da o f  ero sion events  
in  the  c a t chmen t , ha s b een ided  the  pre s enc e o f  3 
'V7a imihia i ,  Pum 1 a s  radiocarbon da and d en ro-
( Grant , c i t ed in S s ,  1 9 7 7 ) . 
I t  is  rent  that  or  f a  tors  have  cont r ibuted  t o  erosion events  
in  the  c a tch['\ent ,  in the  pa s t . are :  
4 . 2  
a )  t h e  c r ea t ion o f  the  s t e ep eas tern fron t o f  t h e  Southern  Ruahine  
b)  
ver t i c a  faul t movemen t s .  which have the  
gradi  t s ,  and � 
ra  ins t o rms . e .  . 
events  a s  a ta 
METHOD STUDY 
lone Al i son o f  Har ch ,  1 9 7 5 .  and  ear l ier  
Gran t ( 978 ) . 
A number o f  method s were 1Jsed  to  recon s t ruc t the ero sion history o f  
this catchment : 
a )  mapping o f  depos i tional surfaces within t he ca  
b)  o f  soil  pro f iles on  these  surfac e s ;  and observa t ion 
of the maturity of  vegetation s tands , 
c )  e s timation o f  the  organic  mat t er content  of  the  soils  describe d . 
( I t  i s  considered tha t ,  in the ini t ial s tages  o f  soil  format  
5 2 .  
mat t er ac cumula t ion increa s e s  with t ime , and i s  
therefore  a n  aid  to a chrono 1 of  even t s . 
S tevens ( 1 9 68 )  also  u s ed th1. s  t echnique w'hen 
a chronosequence o f  so 1. 1 s ,  in the Franz Josef  Glacier  
d u se  o f  da t e s  ed  by radiocarbon 
and  t chronology , 
mapp the thickness  of one f an and terrae  it  to  e 
an i map , 
f )  u se  o f  a er ial  , spann the  l a s t  34  year s ,  es t imate  
reI  t ive propor tions f eroded  a r ea s .  
4 . 3  EROSION 
-- - --- --� 
ter  2 reviews li t era ture ,  concerned with recon s truc t the  
land s c ape  evo lut ion  o f  the Ruah ine geo ime . As the  
has b een if ted  it has been ec t ed to  ero s ive 
int errup ed more  s table  period s ,  which have been con t roll ed  
maj or  c limat 1.c f luc tua t ions . IHth1.n the Wes t Tamaki River  catchment , dur 
each ero s ive event , t ional deposits  have formed in the r iver 
channel . During a more  stable  , so il  and 
vegeta t io n  occurs  on  t he se  s u r f  a e s  a s  the r iver b ecomes inc ised  
wi thin them to  f orm a channel . A par t ia l  reconstruc t ion o f  e rosion events  
is outl ined below . 
4 . 3 . 1 s o f  the Last  S tadia l  
) 
A t ephra bed , 2 5-40cm in thickness  was discovered  on the main channel 
val l ey - side ,  at  T23/68 2 1 6 3 .  A field d escrip t ion i s  with  14 . 
The t ephra occurs a s  a silty-sandy l ens , containing proportions o f  
rhyo l i t ic gla s s , as  wel l a s  minor amounts  o f  quartz , hyper sthene .  
augite  and rare  hornblende . Using transmiss ion elec tron microscopy , the 
FIGURE 14 : 
moderately 
cemented 
s tones 
f ine 
ash 
coarse 
ash 
� 
�  � 
___ 
f
_
l
_
· n
_
e_a_
s
_
h 
___ 
_ 
'" "'" coarse ash 
f ine ash 
large sub­
rounded stones 
and boulders 
t> � � � 'iT<\] (> � V �"{ � 'V"l t> 
"1 �  'l � \I t> � ''l � � t> 
V \7 � c> '\l 'V "" 
t:.. 'J � 'J \J f> � t>-q -
I--
Q 0 C)\) 
°O(]OO 
Illustrations and Field Description o f  the Aokautere Ash a s  i t  occurs 
in the West  Tamaki River catchment . 
- - --------------- -
VI 
W . 
54 . 
< fra c t ion i s  s een to  b e  a s i l iceou s 
( see . 5 5 ,  6 . 3 . 2 ) . 
, with  a f ibrous mat r ix 
The Aokautere  Ash i s  believed to  have b een erup ted from the 
1 9 7 3 ) . I t  ext ends  a s  Vo l canic Cen tre  abou t  2 0 , 5 00  years  B . P .  
fa r  south a s  Amberley  in Nor th Can • South I s land (Nea l l , p er s . comm . , )  
and on  the Tararua and Ruahine it  is  conf ined to  f l a t t ish  o r  
gently  r o l l in g  s l ope s , up t o  an a l t i tude o f  about 400m ( Cowie , 1 964 ) . 
Rhea  ( 1 968 ) d e scribes  the o c  urence  o f  the Aokautere  Ash the  
Dannevirke s t r ic t . The field  an labor atory da ta indi ca t e  tha t the 
loca  t ed in t hE' ea i s  the Aokautere  Ash. 
on  
The Aokaut ere Ash o c curs  on the side  o f  the main vle s t  Tamaki r iver 
channel , la rge bou lders , with  a t  l ea s t  1 0m o f  s tones  above i t . 
On the face  o f  these  s tones , 1 9 0cm o f  l oess  has accumula ted which 
a friabl e ,  so il  has 
The presenc e o f  the Aokautere Ash a t  this  locali  , indicate s  t ha t 
about 2 0 , 5 0 0  y ears  ago , i . e . the l a s t  s tad ial  (which ext ended f rom 
approxima te 2 5 , 000  year s  to  1 5 , 000  years  B . P . )  s were no  than 
this  point  in t he r iver b ed . The ra is  found 1 0 . 1 metres  above 
the l evel o f  the r iver c hannel . The l oess  accumt:1a ted  
upon t he s t oward  the end o f  the la s t  s t a d ial  Subs , and 
Po s t  i a 1  
4 . 3 . 2  
This s o i l  o ccur s  t he wes 
summit of  the Ruahine 
) t ime s . 
A 
of  the c a tchment on the  
• thicker than the 
mod al profile ,  was s tudied to provide  a s tratigraphic control  to  the r ecord 
o f  events  involved in the genesis of  this  soil . A pro f il e  
par t icle-size  and organic mat ter analyses  are  given i n  Fig .  1 5 .  This  soil 
samp l i ng 
units 
em 
10 
.. 
20 
.. 
<I> 
30 
'" 
Q Q 
40 ., " 
" 
t' 
sand 
SO 
" 
57 .. 
flO 
.. 
.. 
70 
" 
" II> 
10  
.. 
" 
'" O\J 
" 
G 
% (by weight) of total sample 
:w 30 40 50 
coarse fnedi u rn 
si l t  s i lt  
(5 - 20�) 
IiIO 70 
SO II> 
.. 
1 1 0  
" 
.. 
fiGURE 15  TAKAPARI  PEATY LOAM : PARTICLE-SIZE 
AND ORGANIC MATTER PERCENTAGES· 
PROFI LE DESCR IPT ION ( b) 
organic  
matter 
100 
TAUPO 
PUMICE 
1E1 19  ± 1 7  
Yr&. S .P  . 
¥,S. B.P. 
(pocketing.! 
Le�therwood scrub 
dark reddish brown (5YR  2.5/2); 
peaty loam; slightly sticky, slightly 
He1 plastic; moderately developed fine 
crumbs; abundant fine · coarse 
roots . 
dark brown (7,5YR 2/2);  slightly 
Ha2 peaty loam; sticky; slightly plastic, 
moderately developed medium 
blocky; many fine · medium roots . 
55 
dark brown (7.5YR 3/2) ;  slightly 
Ha3 peaty loam; sticky; plastic with 
greasy feel; moderately developed 
coarse blocky. 
1'17 
Ha4 
1 t>1 
R 
dark brown (7.5YR 3/2); peaty 
loam; sticky; plastic with greasy 
feel; weakly developed medium 
blocky. 
on weathering greywacke 
5 6 .  
has 
leathenvood  
Wi thin a 
as  organic mat t er has  slowly accumulated under  a dense  
vege ta t ion . 
i le  stud ied o f  the Ta pea ty loam , a thin o f  
cont inuous coar se  a sh ( O . 06 3-2mm) and f ine  lap i l l i  ( 2-Smm) wa s found  a t  
approximately  60cm depth ,  and a thin isc ont inuou s  
8 7 cm depth, 
Hac ro 
1 5 ) . 
ly , the upper band appears as  ovate  
o f  coarse  a sh  a t  
elonga te  vesic l e s .  This  band shows u p  c in many c u t t ings  a 
with 
the 
Delaware  track.  which runs  t he  c r e s t  o f  t he I t  i s  
pa rt i cul no t ic eable as a hand o f  sma l l , whi te  
has d r i ed out  s ome',.That dur t he summer months . 
, when the pro f il e  
The l ower band a l so 
c on s i s t s  of pumiceous grains , general  sma ller  and less  d i s t inc t .  The 
o f  both  bands  s ho'l,.TS l arge  a mount s  o f  l i t ic glas s .  wi th 
embedded tes  and hyperstheo es . Minor amoun t s  o f  hornbl ende and fel  
wer e  a l so no  ed . 
Hoar ( 1 9 6 1 )  describes  the  presence  o f  the Pumice and Waimihia 
in the ,·!es ternRuahine Range  and Elder ( 1 9 65 )  mentions the presence  
o f  7 -Scm of  \\'a imihia Lapi l l i  in the  nor thern Ruahine Pullar  and 
Birrell  ( 1 9 7 3 )  discuss  the presence  of Taupo Pumice in the nor thern ha l f  
o f  t he Ruahine  
On  the bas i s  o f  macroscopic evidence ,  mineralogy and  i sopach maps 
(Pullar  and B irrel l , 1 9 7 3 ) , the  upper 
pea ty loam, i s  ident if ied  as the  
band , found wi thin the 
B . P .  , 
Pumic e .  This  was 
a s equence  o f  erup tions , entred on Lake 
1 8 1 9±1 7 year s  
I t  i s  
considered  that the  l ower band is  par t o f  the  Waimihia Formation , erup ted 
3420±70 years  B . P .  from a c entre  eas t  o f  Taupo . 
Interpretation 
These  t ephras indicate  t hat  the so il above 8 7cm has accumulated in the 
l a s t  c a .  3440 years , with 2 7 cm accumulating  in ca . 1 62 1  years between the 
5 7 . 
two erup tions  and  60cm ac cumulating  in the ca . 1 8 1 0  years  s ince the  
eruption . The presence  o f  the Waimihia il l i  toward the  base  of the 
pro f il e  indica t es t ha t  there was l it t l e  s oi l  deve t to  c a .  3440 
y ears  ago , or  a l t erna t tha t if  s o i l  d d i d  o c cur a 
relativ s table  s ince end  o f  the  l a s t  s tadia l , the  so il  ha d 
been s t r  ro s ion p rocesses  t o  this  date . Ero s ion , 
to the cur r en t  so il f orma t ion d e ,  could  be a ttributed to a) a 
o f  c l imat i c  d eterio ra t ion wi th  inc reased  ero s ion rates  o r  b )  a natural 
catas  
tat ion  
s uch as  f ire . 
this a r ea t o  
, there no  evidence o f  burn t 
the  l a t t er thes is . 
Par t ic le-size  a nd organic ma t t er es o f  the i l e  ( s ee  Fi g .  
indic a t e  that  the r e  are  two s o f  maximum organic ma t ter  accumu la t ion , 
t ed 
and 60cm d ep 
from the  zone  
l i t ic 
a zone  o f  s il t  c on tent  between 
Examinat ion o f  the  sand  f rac t ion ( 0 .  
o f  s il t content , r eveal s  a i f i can t  vo 
e ,  thene , hornblende , 
2 5cm 
d e r ived 
with 
z and , derived rom a p rovenance .  
ic l oes s . 
Thi s  a s s emb 
has the al  charac teris t i c s  of  a 
Thu s , i t  i s  tha t the  present  o f  s o i l  f ormat ion w'a s  
init iat e d  pea t a ccumulation over greywa cke bedrock a s ho r t  t prior  
to  c a . 3440  years  a go ( ca . 4 64 0  y ears  B . P .  i f  the r a te  o f  pea t a ccumula t ion 
befo r e  t he Waimihia erup 
between the Ivaimihia and 
was the  as  t he rate  of  accumulat ion 
erup tion s ,  i . e . about l cm in 60 yea 
Then , a per iod o f  increased 10es s ia l  a ddit ion o c curred , 'iv-hen 
soil  i l e .  become a d ominant component i n  t h e  a c cumulat  
t ime , l oe s s  add i tions have  d imin ished , and  accumulation  o f  
materials  is  again the maj or a d d i t ion to  the i1e .  
l o e s s  
S ince  t h i s  
FIGURE 1 6 :  DEPOSITIONAL SURFACES IN THE WEST TAMAKI 
RIVER CATCHMENT . 
A P PRO X .  
A GE ( years )  
SOIL 
D EVELOPME N T  
I' 
I 
'. 
3 
� 98 
770 
� 770 
I 
.I 
J 
/' 
",,, 
. ./. 
, 
'. \ 
'
. ... . .... 
. .... 
n one 
E n t i so l s  
I n ceptisols 
I n c l?pt isals 
! 
/ 
,­
(---
\. 
J 
( 
\ 
.I 
I 
i 
A()I(AUTEitE ...,...�_ 
, ASH " I 
\ -._./ 
\. " .... ". 
./ 
/ 
.' .- ' 
1 
- '-'/ 
/' 
./ 
. ., .... , 
" . 
'. , 
" " ', 
/-_ . 
I 
o ." 
,-
.1  i � i  � . 
� /  4.". � 
� .I 
� . ./ 
J 
I 
I SC A L E  
I 
I 
I 
I 
metres 
K E Y  
" 
A - It d�pos itlona l 
s urfaces 
J 
, ,. 
.�. 
c a tc hment 
bo und ary 
I 
58 . 
./ 
./ 
59 . 
4 . 3 . 3  s i t i a n a l  Sur f a c e s  in t h e  Wes t Tamaki Riv e r  Ca t chmen t 
ion . 
A numbe r  o f  d i s c on t in u o u s  d e p o s i t ional sur f ac e s  a r e  p r e s erved 
s id e  t he main c hann el of the \,;re s t  Tamaki River c a tc hmen t . On e o f  t he s e  ha s 
been dated ( Gr an t , p er s .  c omm . ) ,  and two f ur t he r  a g gr a da t io n a l  
p er i o d s  hav e  b e e n  r a d i o c a r b on d a t e d . Fu r ther inv e s t  , a s  
o f  t h e  p r e s en t  s 
t he c a t c P.men t ( s e e  
, ind ca t e  s ev e r a l  mo r e  t ional sur fa c e s  w it h in 
. 1 Each s i t  rep r e s en s a t  l ea s t o n e  e r o s ion 
event . 
The s o i l s  on t h e s e  sur f a c e s  m2 y b e  c o n s i d e r e d  a s  a 
c hron o s equenc e ( " a  s equence o f  r e l a t ed s o i l s  tha t d i f f er , o n e  f rom t h e  
o ther , in c er t a in i e s , a s  a r es u l t  o f  t ime a s  a s o  
f a c t o r " , S o il Sc i .  S o c .  Am . •  1 9 7 5 ) . The 4 s o i l - f o rmin f a c t o r s  
( J enny , 1 94 1 ) may b e  c on s i d er e d t o  have b e en 
so far a s  t h e s e  s u r f a c e s  a r e  c o nc e rn ed : 
p a r en t  ma t er i a l  
v e g e t a t ion 
g r av e l s 
p o d o c a rp -har dwo o d  f o r e s t 
c o ns i s t en t  
t io n  
c l ima t e  
r e l i e f  
c on s t a n t  a l o n g  the m a i n  c hanne l  
s imilar su r fa c e s ,  down s t r eam . 
o f  s imilar a l t i t ud e .  
( There i s  o n e  e xc e p t i on t o  t h i s , in the c a s e  o f  t he t erra c e  s ys t em o n  which 
s o i l s  A and hav e Th e s e  c o n t a in a c on s i d e ra b l e  c ompo n en t  o f  
tephr i c  l o e s s , i . e . 
in t h i s  
ma t er i a l  i s  n o t c ons tant a s  a 
Pro f il e  d e s c r ip t ion s , and l o s s  o n  i o n  da ta ( in d ic a t in g  
ma t t e r  con ten t )  a r e  p r e s en t e d  in App en d ix I I .  
I n t e r p r e t a t ion 
f a c t o r , 
The Whi teywoo d  Cr e ek f an d epo s i t  ( Fi g . 1 8 ) shows evi d en c e  f o r  a t  l ea s t  
thr e e  p e r io d s  o f  a g g ra d a t i o n . A po s s ib l e  r e c o n s truc t io n  o f  even t s  
t o  i t s  p r e s en t  f o rm i s  g iv en in . 1 7 .  At t he b a s e  i s  a s ma l l  bur i e d  s o i l , 
FIG . 1 7 :  A Recons truc t ion o f  Even t s  Fo 
N 
i 
N 
t 
N 
t 
6 0 .  
the Whi Cre ek Fan . 
1 2 , 500 yrs . B . P .  -
aggrada t ion s e  
a n  d eposi t  
ive  
A-D - B  r iver chann el  
( b ) 7 7 0  yrs . B . P .  - s econd  
t ion s e  
second f an  d epo s i t  
A-D -B bl ocked r iver channe l  
( fo rmer r iver channel becomes 
inunda t ed s ,  and s ide  
c r eeks a r e  b locked to  form a 
swampy a r ea a t  fan  t rminus ) . 
( c ) p resen 
p er iod vli th  s o i l  
vegeta  ion h 
t and  
A-C -8 present  r iver  cha nnel 
( river ha s c u t  a new cour s e  
fan d t ,  and p ea t 
bog has formed in blocked 
tributary at D) . 
6 1  
Fig.  1 8 : Whi teywood Creek 
fan deposit  -
( 1 )  vegetated with a rimu stand , 
( 2 )  records a t  leas t 3 aggradation 
periods . 
Fig .  1 9 :  A Soil Profile  
developed on the  Whiteywood Creek 
fan deposit . 
62 . 
seldom exposed at  r iver level , ",hieh overl ies the earliest , a s  yet undated , 
gravel deposits  o f  the fan . The bulk o f  the fan i s  a middle  unit  that 
forms the lower two-third o f  the c l if f  fac e ,  a t  T2 3 / 6951 8 5 . A woo d  speeiman 
of Grisel inia l it toralis  (broadleaf)  found within this unit , about 3m above 
river level ,  has b een radiocarbon dated at  1 2 , 1 502:.15 0  years B . P .  (NZ4 3 14B)  
suggesting that active  deposition was  occurring  on the fan ca . 1 2 , 000 years  
ago (Fig .  1 7a ) . 
A third period o f  aggradation is  indicated by the upper gravel  unit  
of  the fan , on  which a mature rimu s tand has  developed .  Tree-ring dating 
of  one old  rimu a t  4 5 0  years  old  (Grant , c ited  in Stephen s , 1 9 7 7 ) , g ives 
a minimum age for  this last  p er iod of  aggradation . 
in which the present-day soil  is formed ( Fig . 1 9 ) . 
It  is  this deposit  
At the t erminus o f  the 
fan ,  three metres of carbonac eous silts overl ie medium to  coarse  greywacke 
gravels .  A wood  sample obtained  from the base o f  these carbonaceous s il t s  
has been radiocarbon dated a t  7 7 0±60 years B . P .  (NZ4547C ) . This provides  
a more accurate date  for  the  third aggradational period . Field  observat ions 
suggest  that the former r iver channel r an further to  the eas t  of  i t s  present  
course ,  ( Fig . 1 7 a ) , and that during this  third aggradational perio d ,  the 
f an deposit  f il l ed this river channel , and formed the swampy area at  i t s  
t erminal margin (Fig . 1 7b ) . 
S ince this t ime , the r iver has cut a new course through the fan , s o  
that the s equence o f  fan d eposits  a r e  expos ed o n  bo th s ides  o f  the main 
river channel ,  ( Figs . 1 7c ,  1 8 ) . 
An ancient  t errace system (Fig . 2 0) i s  preserved downstream o f  Whiteywood  
Creek fan a t  two localities  marked A and B ,  in Fig . 1 6 . The soils  
developed on t hese  sur faces , c ontain a s ignificant accumulation  o f  loes s ,  
mixed with a large  number o f  boulders (Fig . 2 1 ) . A dark, well 
s tructured Ah hor izon overl ies  friable , weak to  moderately s tructured AB 
and Bw horizon s . The soil  prof ile  indicates  tha t  a c onsiderable amount 
o f  time has elapsed s ince  so il  development bagan . By soil  
63  
Fig .  20 : Old t errace  in 
the West Tamaki River 
channel-vegeta ted with 
a podocarp-hardwood stand . 
Fig . 21 : A Soil Pro f il e  
developed on the o ld 
terrace sys tem .  
64 . 
profile developmen t,  and organ ic matter contents  it  is suggested tha t 
these surfaces predate the upper surface o f  the Whiteywood fan (Fig's 18 , 
1 9 ) . The vegetation on bo th surfaces is a ma ture podocarp-hardwood forest 
(see Appendix I I ) . 
At the mouth of  No . 2 Creek. another extensive fan deposit  is  vegetated 
with a mature podoca rp-hardwood stand ( see Appendix I I ) . The soil devel-
oped on this  surface ( soil C)  is about O . 5m thick , and is extremely 
bouldery ,  wi th little or no loessial component . I t s  morphology is  
similar to  soil D on  White�vood fan (Fig . 1 9 ) . and it  is suggested that these 
two so ils  are of a comparat ive age . A buried so il , at  the mouth of  No . 1 
Creek ( soil  E) al so has a s imilar appearance , and it is considered that 
it may a l so be of a similar age . Al l three soils are c lassed as  Incept isols 
(Soil  Survey Staf f .  USDA, 1 975 ) or  clini-fluvic soils (NZ genetic classi-
fica tion) . The surfaces on which they have developed may represent lateral  
downvalley continuations of a single depo si tional episode .  
A weakly developed soil occurs on  the fan surface , on which Stanfield 
Hut is loca ted . The red beech stand (No thofa� f usca )  ha s been tree-ring 
dated a t  98 years old (Gran t ,  pers .  comm . ) ,  providing a minimum age for 
this surface (see Fig . 2 2 , arrowed) . The soil . which has developed in the 
surface gravels of No . 1 Creek fan is at a similar stage of development ; 
and it  seems possible that it  is o f  similar age . Both are AC so il s ,  and 
are thus c lassified as En tisol s ( Soil Survey Sta f f , USDA ,  1 975 ) , or c linic 
soils  (N . Z. genetic classificat ion) . 
Younger terraces , vegeta ted predominantly with mahoe (Melicytus 
ramif lorus ) , occur downstream o f  S tanf ield Hut and No . 1 Creek ( soils G and 
H) , and a t  a number of other localities in the s tream channel . These are ,  
in general ,  no more than O . 5m above the present river bed . The soils 
developed on these terraces have weakly s truc tured Ah horizons overlying 
unweathered gravels .  These so ils  are a l so Entisols, ( c linic soils,  
New Zealand genetic classifica t ion ) ,  and appear to be � 98 yea rs old .  
65  
Fig . 2 2 :  An extensive Gravel terrace ,  formed during Cyclone 
Alison ( foreground) ,  and a 98 year old fan deposit  at Stanfield 
Hut (arrowed) , vegetated with a Red Beech stand . 
Fig . 23 : Recent  soil ,  formed 
in a gravel deposit , at the 
mouth of Car Park Creek . 
6 6 .  
In addit ion , a further  fan and terra c e  syst em is  
f rom the mouth o f  Car  Park Creek , for about 1 . 2km downs t r eam . 
The gravel s  are  and  very fresh-looking , and there  has been l i t t l e  
t ime f c' s o i l  d evelopment ,  ( so il K ,  Fig ,  2 3 ) . A t  the mouth o f  
and i n  r iver bank exposures  downs t ream o f  Dry Creek ,  these 
C r eek ,  
s c an b e  
s een an older  \ve l l-deve loped , ye11mv-brown soil . This  soil  i s  
s and o ccurs on low,  f l at t erraces  fur ther downs tream . I t  has been 
as the s tony sil t loam G . J .  Smith  !ted  in  Hosley ,  1 9 7 7 ) , 
and c la s s i f ied  as  a lo"l-bro"m low-brown earth 
( s  enleached a lvi-fu1vic soil )  b y  Rijkse  ( 1 97 7 ) . 
The thicknes s o f  gravel  above the buri ed s o i l  was mea sured a t  a 
number  o f  po int s from the Car Park Creek fan to  i ts downstream on 
the in of the  main West  Tamaki River channel . An i map 
( Fig . 2 4 )  a l l ows an e s t imate  to b e  made of the vo lume of d e tritus  in this  
fan and  t errace  d epo s i t . A minimum o f  1 9 0 ,  o f  ( 1 2 5 ,  o f  
bedrock ,  the factor  used  by  Mo , 1 9 7 7 )  was eroded  and 
this ero s ion  event . S o i l  development on thi s sur fac e ind i ca t e s  
t hat  the ero s ion event occurred a t  some t ime 
may wel l co incide  wi th the period  of scour 
braided channels , a f t er the  1 9 20 ' s- 1 9 30 ' s , which 
( Chapter  2 . 1 ) . 
the l a s t  98  year s , and 
ion o f  wide , 
( 1 9 7 7 )  d iscus s es ,  
A s e t  o f  very l ow t e rrae the West  Tamaki main c hannel , 
were dur the mo s t  recen t  s i can t  ero s ion  even t , Cyc lone  
of  March,  1 9 7 5 . Thes e  terraces  R in Fig . 1 are  
bare surfac es  2 2  in well 
between Head Creek and Hut  Creek (Fig . 1 6 )  in t he main West  Tamaki  channel , 
and in Hut Creek ( Fi g . 8 ) . 
Ck .  
FIr;URE 24 : [ SUPl\CH :: i.AP OF RECENT GRAVEL SURFACE ,  
ASSOC IATED (.lITH CA� CREEK MW DRY CREEK 
SUBCATCI I;'IENTS I�  THE TAHAKI  RIVER CATCBl1ENT .  
o f  
ALE  1 :  0 
N 
s above buried so il 
f urthe s t  ext ent  o f  s 
6 7 .  
4 . 3 . 4  Aerial  Pho 
DATE OF PHOTO % OF SUBCATCHMENT ERODED 
( i . e . bare ground 
9 . 10 . 4 6 3 . 4 
1 . 1 1 . 6 6 1 1 . 2  
28 . J . 74 5 . 6  
2 0 . J 78  4 . 9  
---""----
TABLE 5 :  P ERCENTAGE ERODED AREA IN CAR PARK CREEK AND No . 1 C REEK 
FROM 1 94 6- 1 9 7 8  
The earl i e s t  a er i a l  o f  the s tudy area t aken the  
r tment of  a nd Survey .  date  back  to  1 946 . At this  t ime , the  
ca tchment appear s to  have  b een in a r e l a t ive stable  s i tuat ion , wi th 
68 . 
a f ew ero sion s c ars  evident . The stream beds  o f  the  subcatchmen t , 
a t  this  t to  be  narrower than t a re  toda y .  HOTweve r , the  
vegeta  s imma ture  in a c e s , e pa appearance on  the  a erial  
togra Thi s i s  pa r t  cul a r l y  evi d ent  on  the north-eas t  fac  s teep  
o f  the  subca tchment s , and a the ma in  channel  o f  the  West  Tamaki 
Riv er . 
1 9 66 . the  subca tchment s  had a number  o f  e ro s ion 
scar s .  Tab l e  5 ind ica  e s  t ha t  the  area o f  bare  had increa sed  mor e  
than three-fo l d  s inc e 1 94 6 .  The o r  o f  these  eroded  a reas  o c curred  
on north-ea s t  f ac and in the  heads  o f  subcatchmen t s .  
the  f o l lowing 1 2  yea r s , a c er tain  amount  of t ion o f  
these  area s , bo th n a  and b y  the  N ew Zealand  Forest  Serv ic e ,  has taken 
place . A comparison o f  aerial photographs taken in 1974  and 1 97 8  indicates 
that although the same eroded areas are evident , both natural and exotic 
tion growth on these  scars has o ccurred in the last  4 y ears . 
the  Ca t s  Paw Scar  is  s een to be  vege ta ted over a t  least  50% o f  i t s  area , 
exo tic  species . plan te d  the  New Zealand  Fores t  S ervi e .  
A f ew o ther e r o s i o n  s c a r s  i n  the s e  two s ub c a t chmen t s  hav e b e en 
wi t h  exo t ic s p ec i e s ; and many o f  t he s e ,  t o g et h e r  w i t h  t h e  o th er e r o d e d  
a r e a s  a r e  s b e ing c o l on i s ed by na t iv e  s p e c i e s . 
69 . 
CHAPTER F1VE 
A PEDOLOGICAL INVESTIGATION OF THE SOILS  
SUB  CATCHMENT 
5 . 1 
CHAPTER FIVE 
A PEDOLOGICAL INVESTIGATION OF THE SOILS 
IN CAR PARK CREEK SUB CATCHMENT 
INTRODUCTION 
7 0 .  
A pedological invest iga ti on o f  t he so il  r esourc e s  within this 
subca tchment was carried out p r imar ily to investiga t e  the r el a t ionship of 
soil c lasses  t o  eros ion forms . The method o f  inves t iga t ion was : 
a )  the def in i t ion , map p ing and naming o f  soils  within the survey area;  
and b )  no t ing the rela t ionship o f  s o i l s  d is t r ibut io n  to  vegetation 
slope ,  geomorphology,  paren t  materials and eros ion forms within the surv ey 
area,  with po s s ib l e  ext r apol a tion o f  f ind ings to  s imilar a r ea s .  
Thus , in t he process  o f  this  survey some detailed inf o rma tion has 
b e en o btained about so ils  o c c urr ing in the area prev iously as 
l lRuahine s t eepland soil s " , ( e . g . Rij k s e ,  1 9 74 ; G . J .  Smi t h ,  c i t ed in 
1 9 7 7 )  . 
The g eneral charac teri s t ic s  o f  the survey area , Car Park Creek, 
are d i scus sed in Chapter 3 .  I t  i s  a subcatchment c harac t er is ed by s t eep 
to very s teep slop es , and i t  has b een cons iderably mod if ie d  by ero s ion 
proces ses ( F ig . 25 ) . The s u rvey area i s  approxima t ely 1 8 0  hectares in 
and in this s tudy it  ha s b een mapped a t  a sca l e  o f  1 : 5 , 000  ( see 
S o i l  Map , App endix III ) . I t  was previously mapped a t  a s c a l e  o f  
1 : 63 , 360  G . J .  Smith ( cited  in Mo sley , 1 9 7 7 ) . 
Gibbs ( 1962 )  desc r ib e s  s t eeplands a s  land forms formed , o r  forming , 
under the influence o f  e ro s ion . He  a t t r ibutes the main e f f e c t  to stream 
cut t ing but a l so acknowledges the S ignif icanc e of fault  movemen t s  with 
subs equent s tr eam ero s ion , b e fore so i l s  can ac cumula t e .  The material 
f ound on s lopes,  both r ego l ith and soil , is  not p ermanent l y  f ixed , but 
moves downslope eithe r  imperceptibly by soil c reep , or by mas s ive sl id 
7 1  
Fig : 2 5 : Car Park Creek - a subcatchment of t he West 
Tamaki River . 
(no te  very steep valley-sides in a subcatchment which 
slopes s teeply upto its  head , due to upthrow by the 
Mohaka Faul t  which runs along t he main Wes t  Tamaki River 
Channel) .  
Photo : R .  Blakely . 
72 . 
movements . Consequently , a s tudy of soils in such terrain is  necessarily 
related to  current  erosion processes . A soil mapping unit  o f  common 
usage in New Zealand to cover the intricacies of the soil pat tern in such 
terrain is that of "steepland soils" . This was initially used by Gibbs 
( 1 954 ) to  cover a complex and diverse range of soil s ,  formed on steep 
terrain . He describes these so ils as  being "formed and maintained by 
erosion" , ( 1 96 2 ) . 
Other workers , such as Campbell ( 1 9 73 ) , recognise a ca tena·-like pattern 
of soils on valley-s ides . Campbell ( 1 9 73 )  describes four dist inct so il 
unit s occurring on a) a ridge,  b) an intermediate steep slope ,  c) an eroded 
slope and d) an accumulation s lope . He considers that this approach provides 
a basis for better definition of  mapping unit s ,  and separation of the 
taxonomic  units  within them .  
Campbell ( 1 975 )  and Laf fan and Cutler ( 1 9 7 7 )  discuss landscape 
periodicity on rolling and s teep land . They show how the catena-like 
pat tern is further complicat ed by rej uvenation processes . The resul t ing 
slope deposits  may represent p eriods of accumulation , for example of loes s ,  
or eros ional and depositional events  (Laffan and Cutler , 1 9 7 7 ) . The soils  
developed on each surface mark a period of landscape s tability , (Butler , 
1 95 9 ) . The resul ting mapping unit  is of increased complexity and variation , 
with  i t s  range of  properties widened . 
A similar situation appears to exis t in the south-eastern Ruahine Range .  
Here ,  a number of  geomorphic units can b e  recognised on the valley-sides , 
each  characterised by a cer tain range of soils . Superimposed on these units 
are deposits  result ing from 1 )  accumulation of  loess and volcanic ash,  and 
2 )  rejuvenat ion p rocesses . The resul ting soils pa t tern is  very complex ,  
al though certain common characteristics  will b e  shown to apply to many o f  
t he so ils .  
The soil mapping units  employed in the survey include  two soil types 
and their related hill soil s , and steepland so il s .  Some broad mapping units  
· 26 : Di ic -sec t ion to  show the Dis tribut ion o f  
1 
SOIL  
NAME 
Dannevirke 
i 
hill  so i l s , 
Dannevirke 
hill soi ls ,  
Ruahine 
soils , 
i l  Classes , ela t ion to the landsur e un i t s , 
thin Car Park Creek . 
or D tax 
SOIL LANDSURFACE DESCRIPTION OF 
SYl'1BOL UNIT -k LANDSURFACE UNIT  
interf luve and 
& 2 seepage 
D t ax 
3 convex creep 
DR 
no 
4 f all- f ace 
Recent  soils  R 
* to  Conacher and 1 97 7 )  
7 3 .  
74 . 
have been used because ,  in certain areas , soils  are  grea modified  
erosion , so  that la rge variat ions c an be  found over  s hort  
The soils  have d from a number o f  parent mat er ials .  These  
range f rom p ea t  with loess  to  basement 
d er ived from , c o l luvium , alluv ium , s cree  
and  include  loess  
and  sha t t er ed 
faul t zone ma t er ial s .  
The r i  and Dannevirke so il s o c cur  on flat  to  easy roll  
S' urfaces  1 and 2 ,  Conacher and  Dal , 1 9 7 7 ) . Their  related hill 
soi l s  occur in a reas  where the land sur fac e becomes more  s 
tha t i s  on more s t eeply sloping interfluves and on  the  convex c reep  
3 ,  Conacher and Dalrympl e ,  1 9 7 7 ) . Ruahine soils  o c cur on  
the s t eep to  very  s t eep val ides (uni t s  4 ,  5 and 6 o f  Conacher and 
Dalrympl e ,  1 97 7 ) . A tic  c ross-s ec t ion o f  a - sid e  in  Car 
Park Creek , to show the relat ionship of so il  c la sses  to  landsurfa c e  units ,  
i s  g iven in Fig .  2 6 .  
5 . 2  METHOD OF �,� �----�"�--��-
Ives ( 1 9 7 0 ,  1 97 2 )  discusses  the problem o f  mapping in  the  New Zealand 
h igh 
Can t  
Having mapped so i l s  o f  t he  Mowbray catchment  in  South 
he found d if f icul  in  rela t ing taxonomic and mapping uni t s  
iden t i f ied , wi th tho s e  o f  o ther surveys .  This was att ribu t ed  to : 1 )  
unit ,  non-definit ion  o f  the range o f  propert i.es  within a taxonomic o r  
and 2 )  insuffic ien t  awareness and use  o f  da ta  f rom o ther surveys . Thus 
any soil  survey should  adequa tely  define , accura map and uniformly name 
soil s ,  in order to b e  able  t o  c o rrela t e  them and en sure tha t  one so il  has 
the same name wherever it o c cur s .  Mo s t  o f  t he soil  uni t s  adop ted  
in this  study have been previously described ( e . g .  Rijkse , 1 9 7 7 ;  Pohlen 
e t  al . 1 94 7 ) . I t  was ,  thus , cons idered o f  i.mportance  to  1 )  
the d i f f  and overall  s imilar ities  within and 
and to 2 )  es tablish  correla t ion with taxonomic unit s  used  in 
surveys .  
This  soil  survey was carri. ed out  in three 
7 5 .  
uni t s  
1 )  - enabling the surveyor t o  become with 
th ero s lon in t he Southern Ruahine , and b) the range of 
so il s which exist  ther e .  These  trip s  were also  used a s  an to 
choose  a suitable in which a detailed  soil survey could b e  
carried out . 
) o f  the catc hment were  
s tudied to  the nature of  the land  surfac e ,  b) 
t.o 
, and c ) extent  and  l o cal i t ies  o f  ero sion . 
In fo rmation from 1 and 2 was u sed to  
3 .  
a suitable  
Fiel d invest  tion - This  involved a number traverses  across  
the  catchmen t , and description o f  soil i1 e s  enco unter ed on  the total  
range of  landforms . Subsequently ,  a number o f  s oil units  were  
identified , and their ext en t  mapped on a b a se  map o f  1 : 5 , 00 scale  
I I I ) . The range in pro f ile characteri s t ic s  within each uni t  was 
s tudied . Eros ion forms within the area were no t ed and their 
d istribution wi th  respec t to the soil  units  was 
ThE t erminology o f  and Fohlen ( 1 9 70 )  was used a s  a bas i s  fa  r 
soil  ile  descriptions .  Horizons were des  to  the 
FAO-Unesco  sys t em ( 1 9 74 ) . 
Soil  c olours were recorded moist . Roo t  s ize  and abundance  were  
describ ed according to the  Soil  Survey o f  and Wales  • 1 ) 
as Taylor and Pohlen ( 1 9 70)  do not s e t  down a format for root  
Ives  ( 1 970 )  present s  some addi tional t erms for pro f i l e  
• which h e  d eveloped whilst  surveying t h e  Mowbray catchment , 
in South I sland . One o f  these has been adop t ed in the 
"extremely stony" , as  an additional c lass  for s toniness 
indicating the presence of > 60% 
stones (or  gravel ) . The very s tony 
clas s ,  of Taylor and Pohlen ( 1 970 )  i s  
modified accordingly . 
The textural qualifying terms "peaty" and "stoniness" are used in 
this s tudy .  S toniness is  inc luded a t  the beginning of  the horizon 
descriptions together with the textural clas s ,  as it is  considered to be  
of  major  importance to  the soil  characteris t ics  in  soils  which are  very 
7 6 .  
- extremely stony . A f ield assessment o f  the feel o f  many o f  these soils  
indicated a gri t t iness . This suggests  that colluvial greywacke material 
present in the soil  has had insufficient  t ime for breakdown to  f iner particles . 
Thus , a "gritty feel" is added to the horizon descriptions where relevan t ,  
a s  it  i s  considered t o  be an important fac tor characterising soils  in which  
rejuvenat ion is  an impor tant process . 
Also , some soil  prof iles  had a "greasy feel" , which was considered to 
be indicative of a l arge allophanic component within the soil clay component . 
5 . 3  SOILS 
The soils encountered in the s tudy area are listed below according to  
topography . They are classified according to : 
( 1 )  the New Zealand genetic c lassif ication ( common and t echnical names) and 
(2) Soil Taxonomy (Soil  Survey S taf f , USDA ,  1 9 7 5 ) . 
SOILS LEGEND ( to accompany soil map )  
Soils of the Flat to Rolling Land of  the Mountain Range 
( 1 )  very strongly l eached organic soil (very s trongly enleached 
eldelodic soil)  
( 2) Lithic Borosapris t ,  with inclusions o f  Typic Borosaprists  
TAKAPARI PEATY LOAM (Tp) 
7 .  
0 )  Taxadj unc t t o  s t rongly l eached int  b etween 
brmvn loams and 10w-bro\'Illl ( 8  enleacbed 
alvi-fulvic soil ) 
( 2 )  cp t ,  (or  , in G .  Smiths 
provisional c la s s if ica t ion of Andisol , 1 
DANNEVIRKE S ILT LOAM TAXADJUNCT 
So il s o f  the Hod erate  to Hod 
( l )  S t  soils , relat ed t o  low-brown ear ths  ( 
en leached c o-fl l 1 vic c l in ic ,  
( ) ic Dys trochrep t  
RUAHINE S SOILS MODERATELY 
-cl in ic so ils )  
( 1 )  Hill  i l s ,  rela ted  t o  very s t rongly l eached 
en leached el delod ic soil ) 
( 2 )  wi th inc lus ions  o f  Hist ic 
and Humic t s  
TAKAPARI H ILL  SOILS 
( 1 )  Hill soil s ,  rela t ed to l eached 
yellow-brown loams and earths ( s  
a lvifulvic so il ) 
( 2 ) Entic  Dys trandept  , wi th inclus ions  of  Lithic 
soil  
enleached 
Typic  Haplu dand , in G .  Smiths  c lass ificat ion 
of Andi so l s )  
DANNEVIRKE HILL 
Taxad j unc t - a t erm used by  the  U . S . D . A . (USDA. 1 967 ) for  the  c lassi­
f ication o f  a soil , who s e  d e f in i t ive characteris t ic s  t least two )  are  
outside  but n ear  tbe  l imits  of  an already defined s eries . So s marginal 
to defined  s eries  may al so  be handled a s  unc t s .  Thi 
concep t is s imilar to t he  concept of the so il variant a s  i t  is used in 
N ew Zealand l it erature ( and Pohlen , 1 97 0 ) . 
Soils  o f  the S t  s 
( l )  s soil s ,  related  to ear ths  ( 
S oi l s  o f  the  
enleached c o-fulvic c linic , fulvic-cl inic so i l s )  
s with inc lus ions o f  L ithic Udor thent s  
(RuS )  and vi ce  versa ( RuVS ) 
RUAHINE ) 
S OILS VERY 
( 1 )  Recent s o il , on sur faces  so ils )  
Ud 
RECENT SOILS 
7 8 .  
y 
N . B . The mapping symbol s wr i tt en in bracke t s  a f t er the soil  names o f  each 
mapping unit are  u sed herea f ter as an abbreviation for the so il name , for 
, \vhere used  in the t ext , refers  to the TAKAPARI PEATY LOAM s o il s .  
Mos t  o f  these  mapping uni t s  have b een previously  d escribed  
1 9 7 7 ;  Pohlen et  . 1 94 7 ) . However , t he D tax and DR soi l s  are  d escr ibed 
�--� 
for  the f ir s t  t ime in thi s survey . Al so , the RuMS has been added a s  an 
addi t ional to t he "Ruahine s t eepland s o il s " . This occurs a t  
local i t i es i n  the catchmen t  where a soil  has d eveloped o n  a colluvial  faa 
TpH i s  a mapping uni t  adopted by Rij kse ( 1 9 7 7 )  in Pohangina County .  
t eau . I t  is  described  I t  occurs  just  b elow Tp , which exis t s  on  the  summit  
as  b eing about  0 . 5  metres  d eep .  with  a pea ty A I t  has been 
Rijkse  
In the 
1 0 7 0-1 3 7 0  metres  eleva t ion on  the wes tern sid e of the 
survey . these  soils  occur on r idge-sites a t  eleva t ions as l ow 
a s  7 5 0  metres , and in many cases  are  consid erably  thicker than 0 . 5 metres . 
Their representa t ive pro f il e  description shows t hem to  b e  
to their r elated counterpart ,  Tp , which occurs on  l e s s  s teeply 
t errain . 
d i fferen t  
The Ruahin e  s teepland so il s are  loca t ed on the s t eep to  very 
val l ey-sides o f  the subcatcl�en t  (uni t s  4 ,  5 and 6 o f  Conacher and 
• s ee Fig . 26) . They may b e  subdivided into three 
7 9 .  
• o n  t he 
bas i s  o f  o f  s lope  and mean soil depth.  • 26 shows shallow 
RuS and very sl� llow RuVS s o il s  occur r ing on the 
wherea s the RuMS s o il s o c cur on t he col luvial foo t slopes . A wide 
range of p ro f il e s  o ccur within each phas e  and s ome s how no s o il 
( s e e  Tab l e  6 ) . The ex t ent o f  each wi thin the Ruah ine 
soils mapping uni t  i s  s hown on t he soil map , where t he del in ea t ed areas a r e  
domina t ed by t h e  s o il phas e  to which they a r e  a s s  • btl may contain 
inclus ion s  o f  the o ther two soil  pha se s .  Thi s  i s  par t  
in a r ea s  mapped a s  RuS in which a sub s tant ial number o f  RuVS inclusions may 
occur and vice ver sa . A f ew s tabl e s i t e s  o ccur within each whe r e  
s o i l s  a r e  r ela t ively well d eveloped ; however , these are  consid ered to  be 
minor inclus ion s wi thin any o f  the three so il phases . From the t o tal number 
o f  > average s o il d ep ths have b een calculat ed a s : 
Soil  total  depth o f  A and B horizons 
RuMS 60 cm 
RuS 4 5  em 
RuVS 3 5  cm 
Roo t  dis t r ibut ion is commonly throughout  t he ful l ext ent  o f  the which 
undoub t edly 
soil  
an impor tant role in  so il s tabil isation . 
ion i s  giv en b elow f o r  each soil 
Tab l e  6 which ind ic a t e s  t he range of  proper t ie s  within 
so il s .  
A 
with 
Ruahine 
so .  
(a)  Ruahine steepland soil CRuS) Fig.  27 
s lope :  30
0 
- 3S
o 
elevation : approx . 500- 1 000m 
topography : s teep slopes 
drainage : well drained 
vegetation : severely damaged podocarp-hardwood forest  
(Lack of  canopy trees . sub-canopy dominated by  kamahi ,  wi th various 
small trees, broadlea f ,  ferns and scrub . Open understory) 
classifica tion : Typic Dystrochrep t 
profile : c m  
o +1-0 
Ah 0-9 
org .  l i tter of leaves , twigs ; 
root mat .  
dk brown ( l OYR 3/ 3) ; V .  stony 
sil t loam; friable  with gritty 
feel ; wk . dev .  f-mdm. nut ty 
struc ture ; many f-cse roots .  
Indistinc t ,  irreg . boundary .  
9-4 5 
dk greyish brown ( l OYR 4 / 2 ) ; 
V .  stony silt  loam ; friable 
with gritty feel ; wk . dev . mdm 
blocky struc ture , breaking to 
f .  nut s ; many f-cse roo t s .  
C on very shat tered weathering 
greywacke . 
8 1 .  
Ruahine 
a � 4 0 ;  el eva t ion : a pprox .  SOO- I OOOm 
very 
dra well  drained 
slopes 
t ion : podocarp-hardwood  
(mainly scrubby broadleaf s ,  p eWerwood , f erns , mahoe  and  s tunt ed 
kamahi , with  a f ew podocarps ) . 
c lassifica t ion : Lithic  Udor thent 
pro f il e  Ah 0-20cm dark brown ( l OYR 3 /  
s il t  l oam; f irm with  f eel ; 
weakly d f ine  to  medium 
s truc ture ; many f ine-coarse  root s ;  
d is t inc t ,  wavy boundary 
C 20-4 Scm sha t t ered , wea 
R on solid , bedrock 
( c )  Ruahine st  
: 1 
o �30 ; eleva t ion : approx . SOO-80Om 
topography : modera tely s teep to  s 
well drained  
t ion : damaged podocarp-hardwood forest  d ense  
o f  broadleaves , pepperwood , l ianes , f erns wit h  I<amahi and a f ew 
podocarps ) . 
c lassif icat ion : 
pro f il e :  
�pic  Dystrochrept  
o +1 -0cm 
Ah O-1 2cm 
l i t t er o f  l eaves  and twigs ; 
roo t mat .  
dark b rown ( l OYR 3 / 3 ) ; very s tony 
sil t loam ;  f riable ,  with  t feel ; 
madera f ine to  medium 
nut ty s t ruc ture ;  abundant f ine-coarse  
root s . Indi s t inc t ,  wavy boundary . 
B,.r 12-33cm brown/dark brown ( l OYR 4 / 3 ) ; extmly 
s tony s i l t  loam ; f r iable  with  gri t ty 
f eel ; 
s truc ture ;  many 
Indi s t inc t ,  wavy 
med . blocky 
roo t s .  
Bw2 33-52cm brown ( l OYR 5 / 3 ) ; 
loam; f r iable ;  very 
developed f ine to  medium blocky 
8 2 .  
common f ine  t o  coarse  roo t s .  
c 52-l 00cm gr s ,  brown 
( l OYR 5 / 2 ) matrix ; s tructureless  
R on greywacke bedrock .  
Ero s ion is  undoubtedly the  overriding process  affec ting 
d evelopment wit hin these s oil s , with t he occurrence  o f  a number 
of ero sion s lips  and scree  deposit s .  Other processes which o ccur include 
a l imited amount of  accumula t ion  and incorpora tion of  o rganic  mat ter ,  with 
moderate  to  strong l eaching . 
Eros ion processes  wi thin these s t eepland soil s comp l icate  the three 
pha se so il  patt ern ( shown in F ig . 2 6 ) by prodUCing wit hin each pha se : 
bare rock and  scree  d eposi t s ,  buried . trunca ted  and skeletal  soil . As a 
consequence ,  each soil  phase  encompasses  a wide  range o f  
All three  soil  phases  do  n o t  occur  in any one slope  uni t . RuMS 
soils  occur over a l imited area  where the slope i s  suf f ic iently 
co lluvium to  a ccumula t e  and where l i t t l e  undercu t t ing  by  the stream channel 
occur s .  The distribution of  RuS and RuVS soils  is  c on trol led primarily by 
the e of s lope  and  ero s ion pat tern . 
ion 
A considerable  variabili ty  within each soil  pha se results  in the 
presence of Dystrochrepts  and Udorthents within this mapping unit . RuS 
soils  are  charac teris tically Dys trochrept s ,  which meet the colour and depth 
requirements  o f  the typic subgroup , and RuMS soils  being s lightly  deeper a l so 
TABLE 6 
HORIZON 
Ah 
Bw 
c 
VARIATION OF PROPERTIES IN THE RUAHINE STEEPLAND SOILS MAPPING UNIT 
OF 
COLOUR TEXTURE CONS I STENCY STRUCTURE THICKNESS 
dark s i l t  very f r iable 
dark brown ; f r iabl e  
very dark sand ; f irm 
brown loam 
, 
dark brown/brown ;  
brown ;  dark loam 
dark 
brown 
" 
I I  
. .  
brown ; 
bedrock t en 
fault breccia 
I f  
s ( e . g .  scree  
with loess  
colluvium 
grit  f eel ) 
f irm 
tl f eel ) 
sit)  
with some 
nut s ' 
crumb 
f ine,  medium . coarse 
o r  very coars e  
to 
f ine nut s ,  in many 
with f iner matrix ,  with or without 10 
5-44 
STONINESS 
very s tony 
ext reme s 
NOTES : 
1 A and B hor izons may or  may 
ero sion  pro c es ses  
t b e  present  in  any so il  or  r emoval 
2 )  Several  so il s have been desc rib ed with buried horizons  e . g .  
o +l -Ocm 
C 0-3 5cm 
Ahb 3 5-60cm 
BT",b 60- 1 1 5cm 
Cb 1 1 5+ 
l eaves , 
fresh .  s 
dark b rovm ( l OYR 3 ) ; very s s i lt  loam ; very 
f riab l e ;  weakl y develo  very f ine  to  f ine  nut ty 
s t ruc ture ; many f ine to coarse  roo t s ; nat ive earthworms 
present . 
b rown 
friable ;  
brOlVl1 ( l OYR 4 / 3 ) ; very stony s i l t  loam ; very 
medium s truc ture ; 
common f ine  to  coarse  roo t s .  
wea 
8 5 .  
f i t  in to  this subgroup . 'ivithin the RuS il  a number o f  
Udorthen t s  occur a s  inclus ion s .  The modal RuVS soil  i s  classified  a s  
Lithic Udo r thent , with  inclus ions o f  Dys 
to the  New Zealand c la s s i f ic a t ion the Ruahine 
soils  are  s t  l eached  c l ini  -fu lvic , fulvi-cl inic  or  c l inic  soils  
in  order  of  d e crea so i l  development ,  ( common name : 
so il s ,  rela ted to low-brown earths ) . 
The peaty  loam ( Tp)  is loca t ed on t he l evel  t o  
leached 
summit  t eau  o f  the  crest  o f  the  Ruahine s 1 and 2 ,  o f  
Conacher and Dalrympl e ,  1 97 7 , see  . 2 6 ) . Here , i t  occurs under a dense 
lea therwood ion , and is  commonly about 0 . 5 metres  In local 
s ions , i t  may be  c onsiderably t hicker than 1 metre  and in very 
drained  a r eas  the  l eatherwood  succombs to a snowgras s  t ion . 
Two He  t ions in the  t ex t  are  o f  a modal 
e ,  made  toward the  end o f  a dry summer ; and (b)  of  a ver s i on  
of  the soil , made toward the end  o f  the winter  sea son (Fig . 1 5 ) . I t  i s  
tha t d i f ferences  i n  soil colour and 
tions is due , in par t , to  a d i f ference in dra  
b e tween the two 
charac teris ti c s  
between the  two soi l s ,  but a l so to  a d i f f erenc e in s o i l  moisture  due  t o  
the t ime  o f  y ear a t  whj ch  the re spec t ive pro f i l es were described . 
in i le  ( a ) : the Ha 1 horizon has a l e s s  red  hue t han tha t  o f  
(b ) , and i n  Ha2 there are  s o f  mo t sh1: 
Thus 
cracks which  have f o rmed a f t er a long , 
Al so . 
perio d ,  and c harac t eris t ic 
o f  a p ea t ,  occur in e (a ) where the soil ile i s  exposed , i . e . at 
a roadside cutting , and on  the bare ground surfaces . 
86 . 
(a)  Takapari peaty loam (Tp) Fig . 28 
slope : o - 60 . , eleva tion : approx . 1 1 00-1 30Om 
topography : summit pla teau 
drainage : poorly drained 
vegetation : dense leatherwood scrub (Olearia colensoi) , with snow grass 
(Chionochloa pal lens) in very poorly drained areas . 
* 
classif ica t ion : Lithic Borosaprist 
profile : ( cm) 
e m  
o +2 -0 
fresh to well decomp . org .  l i tter 
0- 1 2  
d k  brown ( 7 . 5YR 3 / 2 )  peaty loam ; 
sl  s t icky ;  sl  plastic , mod dev 
mdm - c se nutty  structure ; no 
stones ; abundant f-cse roots . 
2 1 2-48 
dk brown ( 7 . 5YR 3/2 )  pea ty loam ; 
sl  st icky , sl plastic , mod dev .  
cse-v c s e  blocky structure ;  a 
few gwk chips (weather ing to 
brownish yellow,  1 0YR 6 /8 )  
toward base ;  cmn mdm-cse  roots ; 
mo t tl ing increasing toward 
profile base , par ticularly assoc . 
with roots  and worm channel s .  
( 2 )  C on 
weathering , shat tered greywacke 
bedrock .  
8 7 .  
Genesis  
The i p ea ty l oam occur s in a stab l e  area o f  low 
erosion po ten t ial . Due to a cold , wet c l imate , soil  development on the 
of the summit plateau i s  slow with accumulation o f  
matter  the  dominant proces s .  Thus a peat  has a t  this 
elevation ,  with the proc e s s  palud ization (ac cumulation of a thick mas s  
o f  ma t eria l s  i n  a poorly drained s i t e )  being  In  the  
wet  winter months it  i s  probab l e  that the  soil  is  water-
and r educ ed . In the dr ier  summer months , s ome mo t t ling may o ccur 
in the pro f il e ,  part icularly associated  with c racks and roots  chann el s .  
A labo ratory lnves o f  nitro gen within the soil  
s that nitrification i s  inhib i t ed , with a o f  the 
produced b eing in the ammonium form ( s ee ix IV) . 
Clas 
Th ese  s o i l s  are c l a s s if ied as  ( So il Survey • USDA, 
1 9 7 5 )  on a basis  of  ma tter  determina t ions , the  pyrophospha t e  colour 
test  and soil  temperature Pro fi l e  (a ) , a modal pro f i l e ,  is conside r ed 
to b e  a Lithic Borosap r i s t � ( see  foo tno te )  whi l s t  (b )  approaches a 
* In order to s lot  pro f il e  ( a )  into the l ithic subgroup , this 
i s  r equired by S o i l  Taxonomy to  have a l i thic contac t within the control 
s ec tion . Clearly the organic soil mat er ial r es t s  upon rock a t  
a depth o f  about O . Sm .  which i s  however shatt ered , s o  tha t  c racks 
occur a t  int erval s l e s s  than I Oem,  a condition which does not  
defini tion o f  a l i t hic  contac t .  However , no roo t s  Due 
to i t s  tured na ture ,  chunks o f  gr aywacke may be dug out 
with a spade - a c r iteria for the para lithic contac t . However . it 
improbab l e  that greywacke f ragments  of U . S .  gravel s iz e  ( 2 -7 Scm) would 
af t er end-over-end shaking in wat er o r sodium hexametaphosphat e  solution , for 
1 S-hours .  Henc e ,  the b e s t  availab l e  subgroup category is c onsidered t o  be 
Thus ,  for  the purposes  o f  this survey l i thic  conta c t  i s  c on sidered 
to o ccur where :  " the s hat tered greywac ke (with many c racks a t  interva l s  c 10rer 
than 1 0cm,  but withou t or  only the bar e s t  minimum of roo t t ion  down the 
c racks . and with gravel fra gment s  ( 2 -7 Scm) that can be  b roken out  but d o  not 
d i sperse  in sod ium h exametaphospha t e  s o lution) o c curs within the prescribed 
l im it s  estab l i shed f or soil  profiles  c on si s t ing o f  mineral o r  o rganic  soil 
mat erials" . 
In p ro f il e  (b)  (Fig . 1 5 )  weathering greywacke o ccurs a t  l05cm. 
One c r i t er ia for  the definition of a Typic Borosaprist  is t ha t it  doe s  no t have 
a l i thic  conta c t  wi thin the c on trol  s ec t ion . The d ep th o f  the c on t ro l  s e c t ion 
for  a Hls t o so l  of sapric organic s o il materials  i s  1 30cm. However , if  a l i thic  
or  paralithic contac t o c curs a t  a shallower depth , this c on tac t i s  taken 
as  o the  control  sec tion (p . 68 , S oi l  
In 
for Histoso l s  are • and 
pro f i l e  ) is  c onsidered to 
t ion . 
to  t he. 
o f  the Bora t great  group . 
and represen an inclusion , where  er 
ha cumula t ed in a d epression in the und bedrock .  
o f  
Accord to the N . Z .  classification , the loam is a 
very enlea ched e l d el odic soil , ( c ommon name : v ery  
l eached or  ic  s o il ) . 
i hill  sol  
Descr  
The hill  so11s  have b elow this 
area,  and occur on of up to  30° , i . e . int erf luves and on  
stable 
convex creep a r e  charac t erised a pea  loam Ah horizon or  
a p ea ty Ha horizon , over a mineral soil . 
At t eleva t ions and in local  sions , a Ha  horizon 
occurs , o f ten wi th an Ah below i t . In o ther  a Ah horizon 
may b e  found . Below this is  grey , horizon ,  which i s  in  mo s t  c a s es 
mot tled . This can b e  a s  thick  a s  40cm to  S Ocm , a t  lower 
elevat ions this  reduces  to the order of a f ew c entimeters thick .  In  the 
f ormer ca se  a wel l  d iron p an ( or  i c  occurs  a t  the 
base  o f  the  dra ined B horizon . This  i s  c emen ted by black metallic 
manganese  i n  the  top ha l f , and a brO\;Ju irou accumulation  in the 
lower hal f . I t  i s  no  grea t er than one to  tvvo c entimetres  thick and i s  
very wavy and d i scontinuous . 1;vith d ecreas  e levat ion , the 
depth  of  the B horizon decrea ses , and the thin iron pan i s  seen 
to be  l e s s  wel l  developed , and no t as c emen t ed .  The two feature s  appear  
to  be  r elated . Below the iron pan i s  
hori zon , der ived f rom colluvial mat eria l . This  overlies  the 
89 . 
basement greywacke which in places appears very weathered , and can be dug 
into wi th a spade .  The total soil profile is commonly between 0 . 5  and 
1 . 0  metres deep . 
Two representat ive profile descriptions are given below : 
(a)  Takapari hill soil , (TpH) Fig . 29 
o 0 slope : 12  -30 ; elevation : approx . 1 000-1 1 0Om 
topography : convex creep slope , below summit plateau 
drainage : poorly drained 
vegetation : leatherwood scrub dominant ,  with some broadleaves ,  
pepperwood , ferns , coprosmas ,  pink pine , Dracophyllum spps .  
classification : Typic Placaquep t .  
profile : cm 
AhO- 1 8  
l i t ter of  l eaves , twigs ; well  
decomp . organic mat .  
dk brown ( 7 . 5YR 3 / 2 )  peaty loam ; 
friable ;  mod .  dev cse  nut ty 
structure ; abundant f-cse 
roots (+ assoc inc ipient 
mottling) ; sl  s tony . 
Indistinct ,  wavy boundary.  
1 8-40/56  
dk  greyish brown ( l OYR 4 / 2 ) ; 
v .  stony s il t  loam ; yellowish 
brown ( 1 0YR 5 / 6 )  mot tles ; mod .  
dev . c se blocky s tructure ; firm ;  
a few mdm-cse roots ; mixing by 
native earthworms of overlying 
horizon materials .  
Di stinct , wavy boundary.  
40/ 56-4 1 / 5 7  
discontinuous cemented pan , black 
at top , s t . brown at  base . 
4 1 / 5 7-1 1 0  
yellowish brown ( lOYR 5 / 6 ) ; 
extmy s tony s ilt loam ; gritty 
feel ; no  roots ; wk . dev . f -mdm 
blocky struc ture to s tructureless 
R on greywacke bedrock . 
(b )  
90 .  
o 
-30 ; elevation : approx . 8 00m 
sloping interfluve , between two subcatchments  
dra poorly drained 
ion : Kamahi-broadleaf dominan t ,  with some l ea therwood  scrub 
classification : Typic Placaquept 
profile : 0 +2-0cm 
Ahg 0-1 2cm 
Bg 1 2- 1 7cm 
B s  1 7 - 1 8cm 
Bwg 18-4 0cm 
Cw 4 0-68cm 
organic l it ter and wel l  humus .  
dk brown ( 7 . 5YR 3 /  dark 
yellowish brown ( lOYR 4 /4 )  mottles ;  
s lightly s tony ; friab l e ;  modera 
developed coarse nut ty  s tructur e ;  
abundant f ine  t o  coarse  roo t s  
associated oxidat ion roo t 
Dis t inc t ,  smo o th boundary 
greyish brown ( l OYR ) s ilt  
sl ight ly  f r iable ;  devel-
oped medium blocky s truc tur e ;  a few 
roo t s ;  
Dist inc t ,  wavy boundary 
accumulation iron and manganese  
oxides , weakly c emented  
yellowish brown ( l OYR ) s i l t  
with dark yellowish bro�l ( l OYR 3 /4 )  
mo ttles ; stony ;  weakly developed 
coarse blocky s tructur e ;  a few medium 
to coarse  roots , 
yellowish brown ( lOYR 5 / 6) ; very 
silt  loam ;  friable with  f eel ; 
weakly 
d eveloped c o a r s e  blocky s t ruc ture to 
s truc tur e l e s s  
2 C  68+ gr eywacke b ed r o c k  
The morpho logy of t h i s  s o il ind icat e s  tha t g l  
are occur r ing . Ro s s  and Hew ( 1 9 7 5 )  d i scus s some podzo l s  f rom Wes tland . 
in South I sland , N ew Zealand , which have c er t a in morpho logical 
to TpH . However , in the Wes tland so i l s ,  iron pans a r e  mor e  
and a r e  o f t en a s s o c ia t ed with a zone o f  humi c  a ccumula t ion above them. 
Thes e  so il s have f o rmed on glacial outwash s ur f a c e s  and show a p ea ty 
topsoil , over l y ing a gr ey-o l iv e  impervious sub s o il ( so me t imes with mot t l in g )  
over l y ing a n  iron pan . They c on s ider t ha t  thes e  o f  the s o i l , 
together wi th i t s  wat er logged na tu r e  are f ea tur e s  o f  • r ather than 
podzo l i sa t ion . They s uggest tha t pod zol i s a t ion was once and may 
s t i l l  be today to some ex t ent , but gl eying i s  now the d ominan t proc e s s .  
Cromp ton ( 1 9 5 2 )  d e s c r ibed the morpholog ic a l  f ea tures o f  some 
d rained s o i l s  in Gr ea t  B r i t a in ,  N ew Zealand and Aus t r al ia . He d e sc ribes 
a thin iron-pan s o il occurr ing a t  a l t i tudes above 305 metre s  in 
where gl o c c u r s  in a pea t -forming c l ima t e . The morpho logy o f  the thin 
iron-pan so i l  i s  d es c r ib ed a s : 
! fA p ea t  surf a c e ,  from a f ew inches t o  s ev er a l  f ee t  
thick , over l i e s  a dull browni sh-grey hor izon which 
merg e s  int o  p a l e  blue-grey g l ey ed l o am or l o am 
r e s t ing s harp l y  on a thin c o n t inuous hard iron-pan 
rarely mor e  than !t;" thic k ,  which in turn r e s t s  upon 
yel low-brown freely d r a ined , or a t  the mo s t ,  s l ightly 
imp er f ec t ly d r a ined , C hor izon " .  
Thi s  d escr i p t ion b ea r s  a s t r ong r es embl an c e  t o  that o f  TpH . crompt on 
( 1 95 2 )  sugge s t s  that this s o il has d eveloped a s  a r e su l t  o f  r e du c t i on o f  
iron i n  a n  hor izon o f  g l eying imme d i a t ely b en ea th the peaty due t o  
t h e  p ea t  a c t ing a s  a sponge a n d  mainta ining satur a t ed c ond i t ions immed ia t e l y  
below i t .  Subsequen t l y ,  the reduc ed i ron i s  oxidi s ed a t  a lower b e tt e r  
92 . 
dra ined part  of  the pro 11  , and an iron pan d s .  on 
sugges t s  tha t the iron pan may with age . I f  r educ tion and 
oxida tion ar the ma in processes  involved in the pan genesis  then the mos t  
int ense  and t be  expec t ed above the sha llow 
ess ions 
sites l ead s 
the inc t pan . With  t ime , t he intense 
fur ther removal o f  iron to  grea t er in the 
at t he s e  
He 
means o f  a d if fusion mechanism . Thus ,  the  resul  is  great er contort ions 
in the form the iron pan ,  a s  thes e  d epression s  b ecome 
ton ( 1 952 ) and S t evens  ( 1  d i scus s  and eview l it era ture 
conc the gen es  of  versus  l is ed so il s .  S t even s  ( 1  
suggests  pos s ible  mechanisms o f  pod zo li sa t ion and 
tha t  t he forma tion of a pod zo l ,  o r  spodosol  ( Soil 
He  cons id er s  
S taf f , USDA , 1 9 7 5 ) , 
mo s t  1 involves the mob ilisat ion chela t ion) and translocation 
es down the pro f ile , with immobil isat ion , micro-
des truc t ion of  the I in the spod ic horizon . 
In  TpH horizon o f  uncertain occurs  b eneath the 
plac ic horizon . To conf irm whether this  horizon beneath  the ie hor izon 
( 1.  e.  b elow is  a Ic horizon or not , extrac table iron and aluminium 
were obtained from s taken a t  J Oem intervals down the entire  
Two extraction were  used : 
1 )  ext rac tion with  O . lH sodium pyropho to ext rac t 
or iron an d alumin ium , the f orm expec t ed in a 
horizon , and 
ile . 
2) ext rac tion with sodium d ithionite  and citrate  to  extrac t pr 
inorgan ic amorphous iron and aluminium c ompounds ,  such as may be f ound  in a 
cambic  hor i zon o f  an Andept .  
One o f  the criter ia laid  down by Soil  Taxonomy ( 1 97 far a 
horizon is : 
0 . 5 
The r esell ob t a ined from the above  are  shown in . 30 . 
ind ica t  tha t  there has  b een l i t t l e  removal  o f  iron and a lum inium 
from the horizon into the r eg ion below the  ic  hor izon . There i s  
a sma l l  amount o f  iron and a luminium accumu la t ed a t th ; but  t he rat  
o f  pyro to  d i thioni t e-c itra te  extrac table  Fe  and Al is O .  
therefore  not  sa t isf  the  c r i t er ia for  a horizon . Los  on 
data  ind ica t e s  a minor accumulat ion o f  mat t er i n  the  lower 
hal f o f  the il e .  
The rat io o f  t e  to d ithioni t e-c itrat  Fe  and Al in the 
hor i zon  ranges f rom 0 . 55 to 0 . 64 .  I t  i s  t hus no t 
o f  spha t e  extrac table  iron and a luminium ,  which  woul d  b e  
expec t ed f o r  the albic  horizon charac teris a soL This  c an 
probably b e  accoun t ed for  the  fac t that  wat er movemen t  this  soil , 
o which occur s  on slopes  o f  up  to 30  • i s  la teral  ra ther than ver t ical , so  t ha t  
the (or Br  horizon never b ecomes eluvia t ed ;  with i lluvia t ion 
in t he hor i zon insuf f i c i en t  to  produce a horizon . The 
colour of this  underlying horizon i . e .  the one beneath the ic hor izon 
indica t es that  iron i s  in i t s  oxid ised form , and no mot t l e s  are  
apparent . The  mor e  nature o f  this  hor izon may b e  
e o f  s o i l  wa t er movement a t  the ic horizon .  with 
above this  po int . Thus , the horizon b enea th the p lacic  horizon 
r emains r elat ively wel l a erated  and weather ed ,  and i s  a 
Cw hod.zon f il e  ion a ) .  At  l ow'er elevat ions in the  catchment 
on the s te eply sloping interfluves the iron pan is  l es s  well , and 
in p laces  is l argely  uncemen t ed . In  this  c a se ,  wat er i s  not  to 
such an ext ent  and the horizon i s  mor e  weathered . 
Some root penetra tion  is  pos s ible  with  a sl ight inc r ea s e  in s truc tural 
and co lour This horizon is ted a 
Bw hor izon • i f  mo t occurs ) , and may or  may no t be  underlain by  
HOR I ZON  0 
h 1  
Ah 2 
Cw 
10 
,--.. 
S 20 -......--
I 
t-
0... 
� 30 
LL 
40 
0... 
50 
60 
FIG � 30 IRON 
Loss on 
Fe AND CONCEN AT I  N ( p p m ) ion 
400 600 (%dry 
1::.-1::. N04 P207 extracted o .  39  A-A Na4 P207  A t  
0-- - 0  c i t .  d 
111---III c i t . d ith . Ai  
0 . 9 0  28  
'-'--- ---0". 
0 . 64 2 0  I I I I I I I 
/ I 
I / 
6 / .. f 0 . 55 1 5  I 
I 
I 
0 . 62 1 6  
-��- � - -- - - - --- ___ �..M" ____ -___ _ _ _ _ _ _ _ - 0 . 58 
0 . 48 1 7  
and AL"UMHHUl:l DISTRIBUTION THE SOIL PROFILE OF A TAKAPARI HILL SOIL . 
9 5 .  
a Cw hor f  ile  description b) . 
Thus , i t  i s  considered  tha t  i s  the dominant so il  
process  in and pod zo l i sa t ion .  i f  it  does  occur , i s  a minor 
soU proc es s .  Ac c umula t ion and of mat t er 
take , with decompos i t ion being s low so  that  a 
soil  Pedo turbat ion by native  earthworms is  a 
proce s s ,  with evidence  o f  a c tive burrowing in the of  iles . 
a lso oC,c ur s ,  with eluvia t ion  and i t ion  o f  iron and a luminium 
in t he ile .  Water movement the is  considered to b e  
predominantly  i n  a lateral  direc t ion , which may to some ext en t  
the Br)  hor i zon  does no t fully d evelop into an eluvia l  (or  a lbic ) 
horizon . 
Classif icat  
The Br ) hor izon satisfies  t he c r i ter ia a cambic hor izon . 
On a bas is  o f  i t s  colour  ( a  hroma o f  2 or  less , i f  mot t l ing i s  
or 1 or  less  i f  there is  n o  mot t l ing)  
epipedon and t he presence of  a thin 
with t he nature of the 
ic  horizon within the modal 
the soil  i s  c la s s if ied  a s  a Typic Placaquep t ( S oil Survey S taf f ,  USDA ,  1 97 5) . 
In places  on t he convex creep s lopes , where the has sufficient 
thickness  and mat ter  content  to sa t he c riter ia for a hist ic 
epipedon ,  inclus ions of  H i s t ic Pla c aquept s  may b e  found . At 
elevat ion s ,  on the  sloping  int erf luves , the iron pan is  less  well 
and largely uncemented . 
Haplaquepts  will o c cur . 
Her e ,  i t  i s  l ikely tha t  inc lusions o f  Humic 
The Takapari  hill  so il s have  previousl..'Jb een classif ied  a s  soils  
related to  very  s trongly enleached eldelodic soils  (common name : 
soils)  in the New Zealand c lassifica t ion . However . on  a bas i s  o f  
pre s en t  s tudy , t he prof il e  d evelopmen t  o f  these  so ils  tha t  
are  very s enl eached maden t ic so ils  (gley  so , and as  such 
l it t l e  similarity  to  the i loam s oils .  
5 . 3 . 4  The s ��----�--�----�--.� 
On the l evel to  s loping near  the 
throats  o f  ( 1 )  the subcatchment ,  and ( 2 )  the  West  Tamaki River catchment , 
a considerable accumula t ion f loess (0 . 5-2 . 0  metr with a 
contamination o f  minerals , i s  found .  
The s oH d eveloped in  this  mat erial  has a 
s truc tur ed Ah horizon , over lying a f r iab le ,  yellow-brown Bw horizon . 
nut ty 
Where the  soil  has on l evel  to  roll  (units  1 and 2 
o f  Conacher and Dalrymp l e ,  see  . 2 6 ) , i . e . on  ( a )  
surfaces  a t  the val l ey throa t ,  and  ( b )  on s (unit I ) . it i s  
v ir tual ly s toneles s ,  . 3 1 ) . On t he 
sides  o f  the r s (unit 3 )  the soil  i s  s tony and admixed with 
lo essia l  c o lluvium , espec ial l y  toward  the base  o f  the solum 
t he hill  soil  s of  the  mor e  wel l  s 
Thes e  are  
soils , 
. 3 2 ) . They have l i t tle  o r  no 0 horizons and thinner  A hor izons . 
The so ils  d escr ibed in this  survey have been c ompared  with  so ils  
described  in  o thers surveys of  adj ac en t  areas . The s tone-free soil  
resembles  the Dannevirke s il t  loam,  o f  the  Dannevirke s eries .  d escribed by  
Rijkse ( 19 7 7 )  a s  occurring on high t erraces  on the wes t  s id e  o f  
Pohlen --,-_c.:. ( 1 94 7 ) , when init ially describ ing the s eries , commented on  
the o ccurrence  o f  a Dannevirke heavy silt  l oam , in  a f ew smal l  areaS  a t  
the foot  o f  the Ruahine ( about 30km north  of the present  survey 
It was not ed t ha t  this soil  type  was stony in 
A c ompari son of  pro f il e  descriptions o f  the Dannevirke soils  kse .  
1 9 7 7 )  with t ho s e  of  so ils found within the survey area (no t the 
hill  soils ) , indicate  tha t  t exture ,  consis tence , stoniness and hor izon 
d epths  are similar  in all hori zons , and colours  are identical  f or B 
and C horizons . However , t he so ils o f  the present  survey have a darker 
9 7 . 
coloured A horizon and a dif f erenc e in s truc tural  I t  
are  c los  rela ted  to the Dannevirke silt  loam ,  appears tha t  
described b y  kse ;  and have thus been mapped a s  a ( s imilar 
to  the New Zealand mapp uni t ,  the var iant ,  Taylor and Pohlen . 1 9 7 0 .  
s ee  5 . 3 ) o f  the Dannevirke s ilt  loam ( D  
a s soc ia t ed hill so il s (DH) . 
with their 
The Dannev irke hil l soil s (DH)  are  def ined in this  survey a s  tho s e  
soils  occurr on the of the 3 o f  
and Dalrymple , 1 97 7 ) . Unl ike the soils  formed on the more  s table  sites . 
they have l ittle  or no  0 horizon , thinner A horizons , are  
and are  f ormed in col luvial mat erial ,  int imat ely mixed with the t 
loess  material . 
ta t ive pro f il e  descriptions o f  D tax and DH are  over : 
The Dannevirke soils  exist  in a r elatively stable  zon e .  Some  bank 
ero sion occur s ,  where these  soils  ext end  down to the main chann el . However , 
the friable ,  
suscept ibili ty . 
incorporat ion o f  a 
dra ining D tax so il s are  in a zone o f  low e ro s ion 
or  80 i1 forming p rocesses  are  the accumulat ion and 
mat t er ,  with moderate  amount o f  
Pedoturbation b y  nat ive earthworms i s  o f ten suf f icient  to  
as  wel l  a s  numerous tovinas of A horizon mat erial in the B 
vic e  versa . 33 a,  b ) . The D tax soils  maintain a good 
an AB hor izon ; 
, and 
ive 
cover , with s everal mature podocarps .  The average  soil  depth  i s  calculated 
to be  l OOcm . A f ew smal l  eros ion features  are  a s sociated  with the DR 
so ils . Small  t erracettes  may be  seen on the ground surface ,  and the  soil  
morphology  indicat es tha t some so il creep is  occurring . However , t he 
friable ,  f reely-draining nature o f  these  soils  mean s  that root  d istribution  
i s  extensive , incr easing their s tability . 
98 . 
(a)  Dannevirke taxadjunc t (D tax) Fig . 3 1  
slope : 
o 
0 - 1 2 ; elevation : approx . 4 1 0m 
topography : flat depositional surfaces , ridge-crests and foo tslopes 
at  valley throa ts 
drainage : well drained 
vegetation : mature podocarp-hardwood forest 
classification : Typic Dystrandep t (or Typic Hapludand ) 
profile : em 
o +5-0 fresh l it ter , well  decom. 
humus . 
0-24 v dk.  brown ( 1 0YR 2 / 2 )  sil t 
loam ; friable with greasy 
feel ; mod . dev .  mdm to c se  
nut ty struc ture ; abundant f-cse 
roots . 
Indistinc t ,  irreg . boundary . 
24-34 
dk.  yel lowish brown ( 1 0YR 4 / 4 ) ; 
sil t loam ; friable with greasy 
feel , mod . dev c se blocky 
structure ,  breaking to mdm nut s ;  
many f-cse roots ; a v few small  
gwk chips ;  interfingering of  
A horizon due to  mixing by 
native earthworms . 
Distinct ,  wavy boundary.  
34-90 
yel lowish brown ( 1 0YR 5 / 6 ) ; 
sil t loam ; friable with greasy 
feel ; mod dev . cse  blocky 
st ruc ture ; a v .  few small  gwk 
chips ; a few cse  roots  with some 
dead and burnt tree roots  evid­
ent .  Indistinc t ,  smooth boundary . 
90- 1 90 
Lt . olive brown ( 2 . 5Y 5 /4 ) ; sil t 
loam ;  firm with sl . grit ty 
greasy feel ; massive ; no roots . 
R 1 9 0+ 
greywacke bedrock 
99 . 
(b)  Dannevirke hill soil , (DH)  Fig . 3 2  
slope : 1 20_300 ; elevation : approx . 500rn 
topography : modera tely steep-steeply sloping flanks of  ridge tops . 
dra inage : well drained 
vegeta tion : ma ture podocarp-hardwood forest 
c lassif ica tion : Entic Dystrandept (or Typic Hapludand )  
profile : cm 
o +1-0 
fresh l it ter o f  leaves and 
twigs . 
Ah 0-1 7  
v .  dk . brown ( 1 0YR 2 /2 ) ; s tony 
silt  loam ; friable  with greasy 
feel ; mod dev . mdm . -cse nut ty 
structure , abundant mdm-cse 
roots . 
Distinc t ,  wavy boundary 
Bw1 1 7-57 
dk . yellowish brown ( 1 0YR 4 / 4 ) ; 
v .  s tony silt  loam ; friable 
with greasy feel ; mod . dev . 
ese .  blocky s tructure ; many 
mdm . -cse roo ts . 
Indistinc t ,  wavy boundary 
Bw2 5 7-99 
yellowish brown ( 1 0YR 5 /4 ) ; 
v .  stony s il t  loam ; v .  friable  
with greasy feel ; wk . - mod . 
dev . cse .-v . c se . blocky struc t­
ure ;  a few roots . 
C on shat tered greywacke bedrock.  
MASSE'!' Ul' IVERSlrt 
LI IV. Y 
1 00 
Fig . 33 Nat ive Earthworm and Nat ive Ear thworm Burrows 
(b)  Nat ive ear thworm 
krotovinas of A 
horizon material 
in the underlying 
B hor izon . 
APPROXIMATE 
SCALE 
1 cm 
(a)  The presence of nat ive 
earthworms in many soil 
profiles undoubtedly 
has an important ef fect 
on soil forming processes 
and soil prof ile develop­
ment . 
SCALE 
1 0  cm 
Classificat ion 
The D tax soils are classified as Typic  Dystrandepts  (or Typic 
Hapludands ,  using G .  Smiths provisional c lassificat ion of Andisol s ,  
1 9 78 ) ; whilst the modal DH soil i s  classified as  an Entic Dystrandept 
(or Typic Haplu dand ) on a bas is of  f ield and laboratory data . Bulk 
1 0 1 .  
densities , pH in NaF ,  1 5  bar water retention (of  dried and undried samples)  
values , sand and clay mineralogy studies and P retention values (Appendix V) 
all suggest  that  the exchange complex of these Dannevirke soil s i s  dominated 
by amorphous materials , ( see also Chapter 6 ) . 
Thus , according to the New Zealand c la ssification these soil s are 
classified as  alvic soils (yellow-brown loams ) . This  is  in contrast to 
previous  New Zealand c lassificaitons of  the Dannevirke series ( e . g .  Rijkse ,  
1 9 7 7 )  where it has been classified as an alvi-fulvic soil ( common name : 
intergrade between yellow-brown loam and yellow-brown earth) . 
5 . 3 . 5  Recent soils  (R) 
Description These soils occur on recent depositional gravel surfaces 
i . e .  terraces and fans , on the stream channel bed , where alluvium has 
accumulated ( i . e . unit  7 ,  of  Conacher and Dalrymple ,  1 9 7 7 ) . The alluvium 
forming the parent material for these soils is characteristically coarse ,  
angular greywacke gravels ,  sometimes interbedded with f iner s il ty and/or 
sandy beds . The so ils formed within these alluvial  materials ,  in the 
Car Park Creek area ,  have an accumulat ion of organic lit ter on their 
surfaces to a thickness of several centimetres ,  and a poorly expressed Ah 
horizon overlying the greywacke gravels .  They charac teristically contain 
no B horizon, al though many f ine roo ts penetrate down through the C horizon , 
indicating the initiation o f  pedogenesis below the Ah horizon. A 
represen tative soil profile  description is  given below ( see also Fig.  23 ) . 
Recent soil , (R) 
slope : level - gently sloping, 0-60 
topography : terrace surface in main channel 
drainage : well  drained 
vegetation : toetoe , grasses , few small shrubs 
classification : Typic Udifluvent 
profile : o 
Ah 
+1 0-0 
0- 1 2 
root mat , l itter , humus , f ine gravels 
very dark greyish brown ( 10 R 3 / 2 ) ; 
1 02 . 
extremely stony loamy sand ; sof t  consistency; 
weakly developed f ine crumbs around roots ; 
abundant very f ine-medium roo t s .  
Genesis 
C 1 2-1 00+ stratified greywacke gravels ,  with s ilts  
and sands . 
The extent o f  so il development on depositional gravel surfaces depends 
on the age of  the surface . Thu s ,  an investigation o f  soil development on 
a number of  deposit ional surfaces can be used as an aid in the chronological 
ordering of  these surfaces , as  discussed in Chapter 4 .  Soil formation is  
initiated by accumulation of  organic matter on  the surface o f  the gravel 
deposit , and an Ah horizon develops as roots  and their associated soil fauna 
penetrate the so il . Soil development appears to progress more rapidly 
in a parent material with a greater proportion of f iner-textured materials 
within the gravel s .  Fig . 2 2  indicates the limited extent of  colonisation 
of a terrace surface formed , three years prior to the date when this photo 
\\'as taken, during Cyclone Al ison . Evidence suggests · that complete 
colonisat ion o f  the surface o f  such gravel deposits  will  take at  least  1 0-
20  years . After 40-50 years ,  the surface is colonised with mahoe trees , 
grasses , ferns , l ianes , thistles and foxgloves ,  and af ter 90- 100 years 
there has been suffic ient t ime for a young stand of trees to develop (e . g .  
1 0 3 .  
red beech, in the case of  the S tanfield Hut terrace surface ) . 
Classification 
These soils which have little  evidence o f  pedogenic horizons are 
classified as Udifluvents ,  being Entisols  which have formed on depos it ional 
surfaces on the stream channel bed.  They are free-draining , with colour 
values which satisfy criteria for the typic subgroup of Udifluvents . 
, 
In the New Zealand genetic classification they are luvic soils , i . e . 
recent so ils rej uvenating by water-borne accumulat ion . 
5 . 4  GENERAL DISCUSSION 
Within the survey area the distribution of soil classes i s  associated 
with specif ic geomorphic surfaces (classified according to the nine-unit  
landscape model , of  Conacher and Dalrymple ,  1 9 7 7 ) . At highest  elevations 
on level to gently sloping surfaces (landsurface units 1 and 2 ) , the 
Takapari peaty loam (Tp)  occurs . Below this on sloping interfluves , and the 
convex creep slopes (l andsurface unit 3 ) , the Takapari hill soils (TpH) 
occur . These are a distinct grouping of  soils , and unlike the Tp soil s ,  
their peaty topso ils are underlain by 0 . 5  to 1 . 0  metres o f  mineral soil . 
The Ruahine steepland soils (RuMS , RuS , RuVS ) occur on moderately steep 
to very steep valley-sides ( units  4 , 5 , 6 ) of the subcatchment . On the inter-
fluves at lower elevations , and their gently sloping footslopes ( units 1 
and 2 ) , the Dannevirke taxadj unct  soils (D tax) have developed , and on the 
more s teeply sloping f lanks of the interfluves (unit 3 ) , the Dannevirke hill 
soils (DR)  occur . Soil morphology of  DR closely resembles D tax , al though 
i t  is slightly less  well developed , and contains a large number o f  stones . 
On the young discontinuous depositional surfaces ( unit 7 ) , on the s tream 
channel bed , Recent soils occur . These are extremely stony with l ittle  or no 
B horizons . At many s ites these soils bury older soils which have formed 
during a prior s table period .  
1 04 .  
Erosion is a particularly important process affecting the development 
o f  Ruahine steepland and Takapari hill so ils .  Virtually all  o f  the 
erosion in Car Park Creek occurs within the areas mapped as  these two soil s .  
Relat ively deep soils form on the crests of  the interfluves . At 
lower elevat ions the Dannevirke soils occur , but with increasing elevation 
these soils become more strongly leached , and the A horizons become more 
peaty . The soils become imperfectly drained , and iron and aluminium 
becomes reduced and mobilised within the profil e .  An inc ipient iron pan 
is seen a t  approx imately 750 metres elevation . With increasing elevation , 
the peaty nature of the Ah horizon increases , the mottling o f  the Bh horizon 
becomes more intense (ul t imately developing into a Br horizon )  and cementa t ion 
and thickness of the iron pan increases . Thus the Dannevirke soils  grade 
upwards into the Takapari  hill so il s with increasing elevation . 
The average soil depth of the TpH soils i s  equivalent to the DH soils 
i . e . 70cm. However , root  distribution is impeded by the imperfectly 
drained nature of  the B horizon and the underlying iron pan . No roots  
penetrate below this iron pan , and as the Bg horizon becomes increasingly more 
developed , conditions for root growth become increasingly adverse . On the 
convex creep slope a t  the head of  the subcatchmen t ,  root abundance decreases 
markedly between the Ha (or Ah) and underlying B horizons . Vegetation on 
this soil unit  is typically scrubby with only a few large podocarps a t  
lower elevations .  Much o f  the roo t growth is lateral rather than vertical .  
The TpH so ils occur in a zone characterised by deep-seated erosion 
scar s .  Examples o f  rock and debris slides , debris avalanches and gully 
erosion may be seen in Car Park Creek, within this mapping unit .  I t  
i s  l ikely that soil creep together with creep of underlying colluvial 
materials  is  a dominant process in this zone , responsible for large-scale  
terracette features , seen on the landscape in this area (Fig . 7 ) . 
CHAPTER SIX 
AN INVESTIGATION OF SOIL PARAMETERS RELATED 
TO SOIL GENESIS AND ERODIBILITY 
1 05 .  
CHAPTER SIX 
AN INVESTIGATION OF SOIL PARAMETERS RELATED 
TO SOIL GENESIS AND ERODIBILITY 
6 . 1 INTRODUCTION 
Soil-water relat ionships are an important fac tor in understanding 
erosion processes within a catchmen t ( see Chapter 2 . 5 ) ,  and a number of  
measurements have been made to  characterise such relationships in the 
present study . 
Saturat ed hydraulic conduc tivity measurements were made  on cores 
taken from both surface and subsurface horizons of  sel ected profiles . 
To tal poros ity and macroporosity of  the different  soil units  were 
measured . In this study , total  porosity is defined as  the fraction of 
the total volume of  a piece of  soil that is occupied by a ir and water,  
(Gradwell , 1 97 1 ) . Macroporosity ,  the amount of  large pores (New Zealand 
Soil Bureau , 1 968 ) , is defined as the frac tion o f  soil volume drained by 
a matric potential * of -4 9mb (-50cm) of water (Gradwell , 1 9 68 ) . 
* The t erm "matric potential" is used throughout the text to describe  
the soil water energy s tatus . I t  is defined as " the amount of work 
that mus t  be done per unit quantity of pure water in order to transport  
reversibily and isother mally an infinitesimal quantity o f  water from a 
pool containing a solution identical in composition to the soil wat er at  
the elevation and the external gas pressure o f  the point under consid­
eration to  the soil water" (Commission , 1 ,  I . S . S . S . , 1 963) . The matric 
potential is  commonly measured by applying a suc tion to the soil , and 
relating the suc tion force to moisture conten t .  For an unsaturated soil , 
the matric potential is  a negative quantity,  as  water from the reference 
pool would flow readily into the soi l ,  with a release  of  energy . Below 
a free water table ( i . e .  when the soil becomes saturated ) , the matric 
po tential  becomes zero . 
The term macroporosity is  synonymous  with "aeration porosity" as used 
by , for example ,  Townsend ( 1 97 3 )  and Kohnke ( 1 9 68 ) , and refers to pores 
greater than O . 06mm in d iameter (Townsend , 1 9 7 3 ) . Macroporosity is  an 
important fac tor affec t ing rainfall acceptance  and transient storage of  
water within a soil , as discussed by Periera , ( 1955 ) . I t  is  l ikely to 
be  related to the ul timate infil tration capacity of a soil with the 
topso il macroporosity being a particularly variable property at any one 
site  (New Zealand So il Bureau ,  1 968) . 
1 06 .  
Loss  on ignition (approximate organic matter content)  and bulk density 
data were also obtained . I t  is considered that these fac tors are 
related to struc tural stability ,  aeration and water movement within a 
soil , as d iscussed by, for example ,  Meeuwig ( 1 9 7 1 )  and Gradwell and Arl idge 
( 1 97 1 )  . Meeuwig ( 1 97 1 )  found organic matter to be  the most  important 
soil parameter affec t ing erodibility in high elevation rangeland in U . S . A . ; 
and Gradwell and Arl idge ( 1 9 7 1 )  found that  s igns of  progressive deterioration 
of soil s tructure , where so ils had been used for market gardening over a 
period o f  twenty or more years , were related to decreases in organic  carbon 
content . The la t ter workers also discuss the soil dry bulk density,  and 
its  indirect relationship with root growth and aeration . 
Estimates of available water-holding capacity, (AWC) were made .  This  
is  defined as the amount of  water held by  a soil between Field Capacity and 
Permanent Wilting Point . Field capacity i s  approximated by an estimate  
o f  soil wat er content after init ial  rapid drainage,  and lies in the range 
-4 9mb to -350mb (Russell ,  1 9 7 3 ) . In this study it  was taken to be  the 
water content at  -4 9mb . Permanent Wil t ing Point is  approximated as the 
soil water content at  a matric po tential  of  - 1 5  bars , below which water 
becomes unavai lable for plant use . 
The I S-bar water retentions of  ( i )  field moist  soils and ( ii )  soil s ,  
air-dried a s  a pretreatment , were measured and the decrease in value of  
( ii )  compared with (i) was noted . The magnitude of  this decrease in 
I S-bar soil water retent ion , between so ils ,  previously undried and 
previously air-dried (hereafter referred to as the "drying ef fec t")  is 
considered to be  a measure of amorphous c lay content ,  (G.  Smith,  1 9 78 ) . 
The importance of soil water retention values to erosion studies 
107 . 
has been discussed by workers such as  Jackson ( 1 966)  and Ward ( 1 9 75 ) . 
When so il is saturated , a positive soil-water pressure develop s ,  the 
shear strength of the soil is reduce� and in turn , the likelihood of soil 
movement downslope is  increased . Consequently a soil , in which the 
soil-water characteristics promo te its tendency to become saturated , 
under t,he prevailing environmental condit ions ( e . g .  low macroporosity,  low 
hydraulic conductivity and small  A .W . C . ) has a grea ter susceptibil ity to 
erosion than one in which saturation is not l ikely (e . g . high hydraulic 
conductivity , large macroporosity,  large A .W . C . ) .  
The sand and clay mineralogies of selec ted soil s were also investigated . 
These provide informa t ion about the na ture and provenance of  soil parent 
material s ,  and also indicate the amount of wea thering that has occurred 
in the respec tive so il profiles , ( Chapter 2 . 5 ) .  
6 . 2  SOIL PHYSICAL PROPERTIES 
6 . 2 . 1 Materials  and Methods 
Soil samples were collec ted from : 1 )  modal profiles of  the soil 
mapping units ,  and 2) a number of other selec ted s it es for investigation . 
Undisturbed cores and c lods were collec ted in the f ield for hydraul ic 
conductivity , bulk density and porosity measurements . Field moist , f inely 
aggregated soil was used to estimate I S-bar water retention values . 
soil sample s ,  collec ted in the f ield , were used for particle  density , 
pH and loss on ignition analyses . 
Bulk 
1 08 .  
6 . 2 . 1 . 1 Particle Density,  (P . D . ) 
The me thod described by Gradwell ( 1 9 7 1 )  was used . I t  was 
considered that some of the samples may have an allophanic component .  
I t  has been found that  P . D ' s  calculated for oven-dried allophanic soils 
are smaller than those  calculated on the moist  sample .  Thus , moist  soils 
were used in this study , and the dry weight of soil was not obtained 
until  after the other relevant measurements were made . 
6 . 2 . 1 . 2  Bulk Density, (B . D . ) 
Bulk density o f  samples was measured , us ing 2 techniques .  Triplicate 
measurements were made  for each soil horizon of each modal soil s it e .  
Method 1 The dry B . D .  of  soils was estimated , using the procedure of  
Gr  adwell ( 1 97 1 ) . Thin-walled metal cylinders , of  internal dimensions 
SOmm long by 4 8mm wide ,  were used to collect intact cores from the f ield .  
"Ie 
The area ratio of these cores was 0 . 14 . 
a The soil cores were dried in a ventilated oven at  1 05 C ,  for 24 hour s ,  
before being weighed . Then : 
dry bulk density dry weight of soil (k�) volume of  cylinder (m ) 
Method 2 In the case of weakly structured , stony soils , dif ficul ties were 
encountered in taking intact  cores , as described above . Invariably , 
stones were pushed by the corer into or away from the soil cores ,  causing 
considerable disturbance of the soil struc ture . In these cases an 
alternative method for calculat ion of B . D .  was used . 
*Loveday , 1 9 74 ,  discusses so il sampling techniques for physical measurements ; 
and considers that an important charac teristic o f  a core sampler is its  area 
ratio : 
Area ratio = area of  the annulus of  d isplaced soil It It I t  sample  at  the cut t ing edge .  
It  is  considered that  for  minimum , or negl igible  disturbance o f  the 
struc ture in the sample , the sampler should have as Iow an area ratio as 
possible ,  certainly no greater than 0 . 1 5 .  
1 09 .  
Al though , this method is generally considered les s  preferable than 
Method 1 (Loveday , 1 974 )  it was considered that  in the existing situation 
it  would provide more accurate resul ts . This method was also used for 
comparison with results  obtained by Method 1 .  
Intac t c lods of  soil were collected in the f ield for porosity 
measurements as described below. Particle densities were also measured 
for these so ils .  Thus , B . D .  was estimated for c lods using the formula : 
B . D .  = ( 1 - porosity) X particle  density -3 (kg . m  ) .  
6 . 2 . 1 . 3  Total Porosity and Macroporosity 
Porosity measurement s were made ,  using the procedure detailed in 
Methods of Soil Analys is ,  Part  1 (American Socie ty of  Agronomy, 1 965 ) . 
The apparatus consists of  a burette connec ted by means o f  plastic tubing 
to a Buchner funnel , the burette and funnel forming the two arms o f  a 
U-tube . The porous plate of  the Buchner funnel consists of  s intered glass 
which can withstand a pressure difference of  about 1 00mb , without air entry . 
Samples were undisturbed clods of  soil , col lec ted in the fiel d ,  and 
sealed in a plastic bag to retain their mo isture .  The samples were o f  
a large enough s ize  t o  approximate the f ield conditions of porosity;  but 
small  enough to fit into the Buchner funnel without touching the sides . 
To ensure good contact between the sintered glass plate and soil clod ,  loose 
soil par ticles were sprinkled onto the plate ,  before placement of  the c lod . 
A petri d ish was placed over the top o f  the Buchner funnel , and a plug 
of cotton wool  in the top of the buret te  to minimise loss of water by 
evaporation . 
To tal poro sity was calculated by measuring the amount o f  water held by 
the soil at  saturation , i . e .  zero pressure potential . In prac tic e  the 
soil  was considered to be s til l  saturated at -3mb ( -3Omm) matric potentia l ,  
so that there was 'no free water s tanding above the s intered glass plate .  
All measurements  were calculated as the difference in height between the 
meniscus in the burette , and the centre of  the clod . 
The amounts of  water , which drained out of the sample at matric 
po tent ial s of -20mb (-200mb) and -49 mb ( -50Omm) were also noted . In 
most  ca ses , the soils were left  for several hours to equilibrate at a 
1 10 .  
certain water po tential . Soil samples were not allowed to dry out before 
placement on the plate ,  so that equil ibration could be reached relatively 
quickly . 
Total porosity was calculated using the formula : 
and macroporosity was calcula ted using the formula :  
mf v m 
tf + Vs 
where f total poro sity 
mf macroporosity 
tf total volume of  wa ter in clod , at  sa turation , 
v volume of  so il solids s 
v volume of wa ter held between saturation and m 
-49mb matric potent ial in c lod . 
Between 3 and 10  repl icates were used for each soil horizon . 
6 . 2 . 1 . 4 Saturated Hydraulic Conduc tivity ( Ksat )  
A method developed by  Corker ( 1 9 7 7 )  was adopted for  the measurement 
of Ksat of three soil profiles  in the labora tory . Cores ( four replicates)  
were taken from two depths in  two localities for the Takapari hill  soils . 
Cores ( four replicates)  were taken from three depths at  one locality for 
the Dannevirke taxadjunct  soil . 
Soil cores , 7 5mm long and 7 5mm wide ,  were obtained using standard 
soil core sampl ing equipment (Richards ,  1 954 ) . The cores were carried back 
to the laboratory , and processed the next day .  This .involved gently pushing 
1 1 1 .  
the core out o f  its aluminium liner and coating i t ,  by painting on hot ,  
melted paraffin wax . This method prevents  any seepage down the sides 
of  the so il core whilst measurements are being made . The l iner was 
attached by wax to the top of the soil core to 'hold  a constant head o f  
water . A funnel was at tached to the base of  the soil core to d ir ec t  the 
effluent into a measuring cylinder .  Across  the top o f  the funnel a layer 
of tissue paper and fine mesh net ting was waxed to the base of the soil 
core to prevent soil aggregates and loose par t icles  from breaking off . 
The processed core was clamped into position and a constant head of  water 
applied to its  sur fac e .  Any leaks in the wax coating were detected at 
this  time , and coated over with some addit ional layers o f  melted wax . 
The rate of  movement of water through the core was estimated by 
collec ting the eff luent in a measuring cylinder over a c ertain time period . 
Hydraulic conduc tivity was then calcelated using Darcy ' s  Law ( see e . g .  
11arshall , 1 96 7 ) : 
where Q 
A 
1 
Ksat Q rom . day ���---,�------A ( 1 ) 
l+h 
3 - 1  flow rate (rom . day ) 
2 surface area of  soil core (mm ) 
length of  soil core (mm) 
h = head of water above soil core (rom) . 
-1 
Measurements  were made as soon as possible after the so il was 
sampled because it was considered that with t ime , microbial ac tivity might 
change the channel system within the core altering the value for Ksat . 
Also , water was added to the surface of  the soil core through a paper 
tissue ,  minimising compaction effects at the surface ,  which might decrease 
inf iltration rates . 
6 . 2 . 1 . 5  1 5  bar Water Retention (Permanent Wil t ing 
Point , P .W . P . ) 
The 1 5  bar water re tention of  so il s was measured on : 
( 1 )  undried samples , in order to estimate  the ava ilable water-holding 
capacity (A. W . C . )  of soils ( in conj unc t ion with estimates of field 
capacity,  F . C . ) ,  where 
1 1 2 .  
- 1  -3 A. W . C .  (vol . vol .  %) = (F . C .  - P . W . P )X  dry bulk density (km. m  ) .  
In the presen t  study , it  was considered that  gravimetric soil water content , 
at  a matric po tent ial of  -4 9mb is an approximate measure of f ield capacity;  
and gravimetric soil  water conten t at a matric potential of  - 1 5  bars 
is an approximate measure of  permanent wil ting point . 
( 2 )  undried and air-dried (overnight ,  at  40oC)  sample s ,  in order t o  estimate 
the "drying effec t "  ( i .  e .  the decrease in 1 5  bar water retention of  an air-
dried soil , compared with values given by an undried soil , being calculated 
as a percentage of  the original undried values) which is used as a crude 
measure of amount of amorphous material  presen t .  This i s  necessarily a 
rough estimate , the amorphous material existing in many dif ferent  forms and 
hydration states in the soil . Thus the "drying effec t" will depend on the 
initial hydration state of the amorphous material . 
A pressure plate  apparatus , as described by Gradwell ( 1 9 7 1 )  was used 
to determine the water content  of  soil samples , equilibrated at  a pressure 
of 1 5  bar s ,  for one week . Soil samples ,  which consisted of  f ine aggregates 
with some loose material , were lightly packed into rubber rings , IOmm long 
and 52mm wide , and placed on a ceramic plate .  These were thoroughly wet ted , 
and left  for a few hours (undried samples)  or overnight (dried samples)  
to ensure that  the whole soil  was saturated before a pressure of  1 5· bars 
was applied .  The 1 5  bar water r etention is then calculated a s  the 
gravimetric water content  of soil s ,  which a ttained an equilibration water 
content under a pressure of 1 5  bars , after one week . 
1 5  bar water retention 
weight of  water * 
weight of dry soil 
6 . 2 . 1 . 6  Loss of Weight on Ignition 
1 00 �% x --
1
-
1 13 .  
Over-dry soil samples ( 1 0S
oe )  were ignited for 0 . 5  hour at  7000e in 
a muffle furnac e ,  to give an approximate figure for organic matter conten t ,  
(Bear , 1 9 69 ) . A number of  test runs , in which samples were heated for 0 . 5  
1 . 0 , 2 . 0 , 4 . 0  and 6 . 0  hours ,  indicated that there was no further loss o f  
weight after the f irst  0 . 5 hour . This time was thus  considered adequate 
to give complete combustion of  the sample  at this temperature .  This 
method only gives an approximate estimate of organic matter because during 
ignit ion some s tructural  water is  also lost from the c lay fract ion , so that 
the loss of weight value may be an overest imate for organic mat ter . However , 
the accuracy of  this method was considered adequate because organic matter 
data was required primarily for comparative rather than absolute values . 
6 . 2 . 1 . 7  So il pH in (a ) Water and (b ) Sodium Flouride, (NaF) 
( 1 )  The pH of  soil samples was measured in a 1 : 2 . 5  soil : water suspension . 
The suspension was lef t overnight , and the pH was then r ead with electrodes 
in the supernatant l iquid , (N . Z . Soil Bureau , 1 968) . 
( 2 )  To investigate amorphous material content , the pH of  a suspension of  
1 . 0g of  soil  in SOml o f  IN  NaF was recorded af ter two minutes (using the 
method of Soil Taxonomy , Soil Survey S taff , USDA ,  1 9 7 5 ) . If  amorphous 
materials are present the fl �oride ions replace hydroxyl ions from the 
colloidal surface with a resul tant increase in pH value . For the exchange 
complex to be dominated by amorphous materials ( if there is suffic ient c lay 
to have a I S  bar water content , of  previously dried soil , of �20%) , the 
pH ( in NaF) is required to be greater than 9 . 4  (Soil Taxonomy , 1 9 7 5 ) . 
* the so il is  dried overnight , at a temperature of  IOs- 1 1 00e .  
6 . 2 . 2  Resul ts and Discussion 
6 . 2 . 2 . 1  Particle Density (P . D . ) and Bulk 
Density (B . D . )  
Results  for particle density (P . D . )  and bulk density (B . D . ) are 
given in Table  7 .  Fig . 34 displays the results  for B . D .  graphically . 
The P . D . ' s  of  soils in the study area are influenced by three factors :  
amounts o f  tephric material , amounts  o f  non-tephric material and amounts  
of organic matter .  (The P . D . of  dominant soil minerals ,  such as  quart z  
- <  
and feldspar is near 2650kg . m  - ,  and the density of  organic mat ter i s  
- 3  -3 usually between 1 300kg .m  and 1 500kg . m ) .  Table  7 shows that surface 
horizons have the lowest values for P . D . ; the Takapari  peaty loam (Tp)  
1 14 .  
and topsoil of the Takapari hill soi l s  (TpH) having particularly low values 
due to their peaty nature .  
The B . D . o f  a soil gives some indicat ion o f  its  struc tural condition . 
Low B . D . ' s  usually indicate an open structure and therefore a large capacity 
for water storage . Heinonen ( 1 954 , cited in Marshall , 1 9 6 7 )  found tha t ,  
in clay soils ,  available water-holding capacity varies inversely with B . D  • .  
He a ttributed this to more open granular aggregates in soils o f  lower B . D . 
( see also Fig . 38 ) . The struc tural randomness of  amorphous c lay constituents 
in the soils helps to create  a loo sely packed soil struc ture , with resultant 
friable cons istency . 
B . D . values are particularly low in the Ha and Ah horizons of  Tp and 
TpH respectively , and are highest  in the Ruahine steepland soils (RuMS , 
RuS , RuVS ) .  TpH shows a large change in B . D .  with depth . I t  is 
possible that extremely low values for B . D . in Tp reflec t the presence of  
amorphous c lays as  well  as  a large amount of  organic matter present in the 
soil . 
The Dannevirke soils (D tax . , DH) have low B . D . ' s , which are a ttributed . 
to their large amorphous clay component . The Ah horizon within TpH has 
a slightly lower B . D .  value than the Dannevirke soils , due to a greater 
TABLE } :  BULK DENSITY AND PARTICLE DENSITY VALUES FOR SELECTED SOIL PROFILES OF 
THE STUDY AREA 
SOIL SOIL HORIZON BULK DENS ITY PARTICLE 
NAME SYHBOL SAHPLED CLODS CORES DENSITY 
Takapari km/m
3 
peaty loam Tp Ha l 1 90  190  1 7 90  
Ha2 460 480 2 240 
Takapari hill TpH Ah 460  2030 
soils Bg 760 2470  
Cw 1 1 1 0 2540 
mod . s teep RuMS Ah 1 1 80 2 560 
I - steep Bwl 1 2 70  2650 0-QJ phase  Bw2 1420 2650 QJ 
t\ {/) steep RuS Ah 1 1 70  2620  ,..., °M QJ 0 phase Bw 1 1 90 2650 c '" -M .t:: '"d very steep RuVS Ah 7 50 7 50 2430 C1l c ::l C1l phase Bw 830  830  2590 0:: ,..., 
Dannevirke sil t Ah 500 4 7 0  22 1 0  
loam taxadj unc t D AB 580 540 2290 
Bw 730  630 2630 
C 1 1 60  2650 
Dannevirke DH Ah 7 1 0  700 2350  
hill soils Bw 820 760 2 650 
FIGURE 34 : A HISTOGRAM TO SHOW THE BULK DENSITY VALUES FOR EACH SOIL CLASS 
-
M 
E 
� 
I-
I-
I- r--
1Q(X1-
>­I--
If) Z UJ o 
I- upper limit for Andepl� 
:lC 
....I ::> 
In 
5c<J 
I-
I-
I-
I-
l-
r--
....-
SOIL HORIZONHal Ha2 
SOIL SYMBOL Tp 
r--
r--
Ah B9 Cw 
TpH 
-
r--
-
r--
Ah Bwl Bw2 
R u MS 
-
,....-
Ah Bw 
R u S  
r--
r--
Ah Bw 
R uVS 
,..-
r--
r--
r--
Ah AB Bw C 
o tax. 
.; 
..-
,..-
Ah Bw 
DH 
-
-
0 
0 
. 
0 
0 
0 
. 
1000 
soo 
o 
1 1 5 .  
1 1 6 .  
organic matter content . The lower horizons within TpH have markedly 
increased B . D . values due to lower organic matter contents . Bg is still  
probably dominated by amorphous clay components having a bulk density of  
3 3 less than 850kg/m ; however , the underlying C horizon has a B . D .  of  1 1 1 0kg/m . 
The Ruahine steepland soils have higher B . D . values . Due to their 
occurrence on unstable sites . accumulation of low B . D . materials e . g .  loess 
(with tephric components , wea thering to amorphous clays) or organic mat ter , 
is minimal , and these soil s are dominated by the accumulation of  greywacke 
materials in a weakly struc tured soil .  
The RuVS soil selected has a relatively well  developed profile ,  with 
a thin Bw horizon . I t  is considered that  it formed in a relatively stable 
pocket ,  on very s teep terrain , surrounded by less well  developed soil s .  
Its  B . D .  values indicate that  some tephric loess may have accumulated at  
this point , and this is conf irmed by  sand mineralogical evidence .  
6 . 2 . 2 . 2  Total Porosity and Macroporosity 
Total porosity and macroporosity data for the selected soil samples 
is shown in Fig . 3 5 .  This ind icates that the range o f  values for many 
samples was considerable (between ± 0 . 0 1 5  and + 0 . 3  for total porosity 
data , and ±0 . 1  to ± 0 . 8  for macroporosity data ) . Thus ,  within any soil 
horizon total porosities (and macroporosities to a greater extent)  will  vary 
considerably over short distances . However , a certain number of  broad trends 
in the results  are evident , which are discussed below. Also , results  
are  given for  a Bw horizon , in  which nat ive earthworm activity had produced 
* 
krotovinas ,  approximately 1 5mrn in diameter . 
To tal porosity values show a general decrease with depth in most  soil 
profiles . S ince organic matter tends to improve soil structure ,  and hence 
increase  total porosity,  as mineral  grains are less tightly packed , this 
* Krotovina : an inf illed tunnel in a soil horizon generally filled 
with material  from another horizon . 
w � :J -.J o > 
-.J 
(5 
1Il � 0'4� 
z o � 
<;-J � 
0: u.. 
0·2 � 
r-t-
FIGURE 3 5 :  TOTAL POROSITY AND MACROPOROSITY OF 
SELECTED SOIL SAMPLES . 
--
r::J TOTAL POROSI TY  Q MACROPOROSI TY  
I range of va l ues 
1 1 7 .  
· 
· 
r+� . 
· 
I--' VI � I __ r- V V I-- V � .1.  VVv � � 
SO I L  HORIZON Hal Ha2 
OL.���L-���-J�� __ �--�����--�� __ ����V���� __ "��V __ ��... � 
Ah Bwl Bw2 Ah Bw Ah Bw Bw· Ah  AB Bw Ah Bw Ah B9 Cw 
TpH SOI L  SYM BOL  Tp RuMS  RuS RuVS 0 tax. DH  
*RuVS Bw horizon with krotovinas .  
Total porosity and macroporosity ratings o f  
New Zealand Soil Bureau , 1 968 . 
trend may be  related to a decrease  in organic matter content with depth . 
Macroporosity values show this trend to a lesser extent , in the majority 
1 1 8 .  
o f  prof iles , suggesting that macropores are not a s  simply related to organic 
matter values . I t  is probable that macroporosity is also affec ted by  
the presence of  roots , stones , earthworm channel s and possibly inorganic 
amorphous materials .  Macroporosity is seen to remain relatively constant 
with depth in the Dannevirke taxadj unc t  soil , probably due to the exchange 
complex being dominated by amorphous material . 
The presence o f  Ah horizon material , as  a krotovina , in the 
selected Bw horizon sample ,  gave a "high" porosity value (using the rating 
system of N . Z . Soil Bureau , 1 968) . I t  also gave a particularly large 
value for macroporosity at  22 . 6% .  I t  is highly probably that  preferential 
movement of  air  and water would occur in kro tovinas .  
o f  common occurrence in the study area . 
Such krotovinas were 
The Takapari hill soil (TpH) profile is of  particular interest . The 
percentage o f  macropores decreases in the Bg ( or Br)  horizon , and then 
increases in the Cw 0r Bw) horizon . The value for the Bg horizon is infact 
the lowest value for macroporosity in all  soils tested . This suggests  that 
drainage in the Bg horizon is poor , and that  the Cw hor izon is relat ively 
�el1 drained . This is substantiated by the soil morppology which shows 
considerable  gleying in the Bg horizon , occurring above a freely-drained 
lower horizon . 
RuMS and RuS , two soil phases of the Ruahine s teepland soil s , have 
the lowest  values of total porosity ,  having "low-medium" ratings . 
However , they have "high" values for macroporosity . This suggests  that 
such soils will drain relatively rapidly and have a relatively low storage 
capacity .  This effec t  was also noticed by McDonald  ( 1 9 6 1 )  when discussing 
some physical properties of New Zealand soils from greywacke parent  material . 
The soil profile from the third phase ,  RuVS , has a larger total porosity,  
1 1 9 .  
suggesting a greater content o f  organic matter and improved s truc tural 
development , compared with RuS and RuMS soil s .  This  evidence further 
substantiates the suggestion that the atypical nature of  the selected 
RuVS soil is due to its formation in a relatively s table pocket . 
6 . 2 . 2 . 3  Saturated Hydraulic Conductivity (Ksat )  
The hydraulic conductivity va lues for  three soil profiles are  shown 
in Table 8 and Fig . 36 . Ksat values for the Takapari hill soils (TpH) 
are of particular interes t in the present study ,  s ince they occur in a zone 
susceptible to soil creep and mass movement . The Dannevirke taxadjund soil 
profile  (D tax) was sampl ed for comparison . The D tax soils occur on 
ridge-sites at  lower elevations in the catchment , and are well-drained . 
The TpH soils occur at higher eleva tions on ridge-sites and convex creep 
slopes and are poorly to very poorly dra ined . 
Ruahine steepland soils were no t tested . I t  was cons idered that  
sampling of  these stony to  extremely stony soils  on steep to  very s teep 
terrain was impracticable for the purposes o f  the present study . Their 
morphology indicates that  they are rapidly draining , as does the macroporosity 
data . 
Ksa t values for the surface horizons of  TpH and D tax soils ( sampled 
at  a depth of 0-7 5mm) were very rapid in the majority of  cores tested , and 
3 4 - 1  of  the order o f  1 0  - 1 0  mm. day This data gives an indication of  the 
infiltration rate into the surface soil . The Ksat values for the sur face 
horizons far exceed the 24-hour one-year event rainstorm intensity of 
- 1  60mm. day , calculated for the West Tamaki River catchment , (Martin ,  pers . comm) . 
Thu s ,  it  is probable  that  the infiltration capac ity of  the surface soil 
would  seldom be a limiting factor to water intake , and the l ikelihood o f  
surface runoff  is small .  
In comparison with the D tax soil ( see Fig .36 ) ,  the TpH soils show 
a marked decrease  in Ksat in the Bg (or Br)  horizon compared with the upper 
- 1  hor izons , some o f  the cores having extremely low values ( O . lmm. day ) .  
1 2 0 .  
TABLE 8 :  SJ'.TURATED HYDRAULIC CONDUCTIVITY DATA FOR SELECTED SOILS 
SOIL DRAINAGE HORIZON Ksat (mm day-I) . PERMEABILITY SOIL 
NAME SYMBOL CHARACTERISTICS SAMPLED RANGE OF AVERAGE RATING 
Takapari  
hill soil  
Takapari  
hill soil 
Dannevirke 
taxadj unct 
TpH 
TpH 
D tax 
poorly Ah 
drained Bg 
very poorly Ha 
drained Br 
freely Ah 
drained 
AB 
Bw 
VALUES 
7,1 39-92,534 39,654 
10- 7 38 286 
1 70- 1 1,669 3,1 9 1  
0 . 1- 408 1 02 
9,67 7-26,4 70  1 5,356 
1 0,368-18,85 1  q67 1  
1,1 52-7,4 1 6  4,594 
FIGURE 36 : K sat VALUES FOR THREE SELECTED SOIL PROFILES . 
> 10 
r-
> 
r-
� 10 o 
z 
o 
u 
.. 
4 .  
3�  
u 10 
2 
� 
-I 
::;) 
<! 
� 10 > 
:::I: 
� If) 
1 
� 
A h  8 9  
TpH 
( poor ly 
drai ned ) 
Ha Br  
TpH 
(very poorly 
dra t  ned 
A h  A S  Bw 
o Ta x. 
( f reely 
d r a l r- .::- d )  
(Smith & Brown-
ing, 1 946)  
v.  rapid 
slow-rapid 
mod .-v .  rapid 
extrmly slow-
moderate  
v .  rapid 
v .  rapid 
rapid-v r�pid 
· 
-
· 
.. 
· 
S O I L H OR I Z O N  
S O I L S Y M B O L  
1 2 1 :  
This suggests that water movement through this B horizon i s  considerably 
impeded and a perched water-table may develop at its upper boundary . 
Thus , water infiltrating rapidly through the surface horizon will be  
impeded to a large extent on  encountering the  underlying Bg (or Br)  
hor izon .  This , together wi th the steepness of  slope ,  suggests that 
interflow is l ikely to occur within the TpH soils and may be  an important 
soil process governing the likelihood of s lope failure in these soil s .  
The very poorly drained TpH so il has lower Ksat values than the poorly 
drained TpH soil . The Br horizon of  the former soil has extremely slow to 
moderate  permeability,  compared with slow to rapid permeability of the 
Bg horizon of the poorly drained soil (using the ra ting system of Smith 
and Browning , 1 94 6 ) . Thus , Ksat appears to be one fac tor expla ining the 
sl ightly different morphologies of  these two soils . 
Within several of  the subsurface cores of the D tax soil s ,  krotovinas 
were present . In all cases these gave the highest Ksat values measured 
in the survey indicating that water movement through these porous infilled 
tunnel s  imparts more freely draining charac teristics to the soil . The 
Ksat  values for the AB and Bw horizons o f  the D tax soil are considerably 
larger than for the B horizons of the TpH profiles . It  is concluded that 
the different  soil morphologies of  the D tax and TpH soils is a reflection 
of dif ferent  drainage characteristics between the soils ,  expressed by the 
Ksat values measured . 
6 . 2 . 2 . 4  Soil Water Retention and Available 
Water-holding Capacity (A . W . C . ) 
Water retention values for selected soils are given in Table  9 ,  and 
illustrated in Fig . 3 7 . The A .W . C .  has been calculated as  t he difference 
between the volumetric water content , a t  a matric potential  o f  -49mb (�  
Field Capacity , F . C . ) and -15  bars ( � Permanent Wilt ing Point , P .W . P . ) .  
TABLE 9 :  SOIL WATER RETENTION VALUES AND AVAILABLE-WATER CAPACITY (A.W . C . )  OF SELECTED SOIL SAMPLES 
SOIL 
NAME 
Takapari 
peaty  loam 
Takapari 
hill soils 
SOIL 
SYMBOL 
Tp 
TpH 
"0 C (1j 
ad . s teep- RuMS 
steep 
..-i 0-
aJ 
hase 
VOLUMETRIC WATER CONTENT (%  soil volume) 
HORIZON SATURATION 
(Omb) 
( to tal  
porosity) 
Ha l 8 9  
Ha2 80  
Ah (Ha) 7 6  
Bg  (Br)  6 7  
Cw (Bw) 5 6  
Ah 
Bw1 
Bw2 
5 1  
54 
4 5  
a t  
FIELD CAPACITY 
( -4 9mb) 
(macro­
porosity) 
7 1  
7 6  
68 
58 
4 5  
3 9  
2 7  
20 
WILTING POINT 
( - 1 5  bars)  
28 
35 
34 
42 
38 
24 
1 9  
1 4  
A.w . e .  
(FIELD CAPACITY -
WILTING POINT) 
(% soil volume) 
4 3  
4 1  
34 
1 6  
7 
1 5  
8 
6 
TOPSOIL RATINGS 
FOR A . \.J . C .  
( McDonald , 1 96 1 )  
high 
high 
medium 
aJ 00r------------------------------------------------------------------------------------------------------------.IJ ..-i  00 '8 steep 
� Cll phase 
"M ..c: (1j 
a 
I 
very steep 
phase 
Dannevirke 
sil t loam 
taxadjunct 
Dannevirke 
hill soils 
RuS 
RuVS 
D 
DH 
Ah 
Bw 
Ah 
Bw 
Ah 
AB 
Bw 
C 
Ah 
Bw 
6 1  
4 4  
6 9  
6 7  
7 7  
7 3  
7 2  
58  
7 1  
6 9  
3 2  
28  
63  
54 
68 
63 
6 1  
5 1  
60 
59 
1 7  
1 7  
1 7  
2 5  
3 5  
3 9  
3 2  
3 3  
3 5  
1 5  
1 1  
46  
29  
3 3  
22  
19  
27  
24 
medium 
high 
high 
high 
...... 
N 
N 
. 
1 2 3 .  
FIGURE 3 7 : WATER RETENTION CHARACTERISTICS 
OF SELECTED SOIL PROFILES . 
-
E 
UJ E 
-' -
1L. 0 0: Q. 
-
E 
E 
-
( C )( b )(a) 
( _ d - )  
::J: � Q. 
UJ 0 
-
E 
E 
-
2 
SOIL VOLUME ( .,. ) 
20 40 60 SO 100 0 20 40 60 80 100 o 
1R.tl 
r I 
SOI L  VOLUME ( .,. ) o 20 I[) 60 80 t)() 0 20  I[) 60 1l'l 100 
R u M S  
o �' l 
M 
SOI L VOLUME ( .,.) 
60 80 t)() 40 60 80 100 
KEY 
a = freely draining porosity ( 0  
b A .W . C .  (-50mb to - 1 5bars) 
c = water unavailable to plants 
d total porosity 
Water Content at : 
W Wil ting Point ( - 1 5bars)  
F Field Capacity (-50mb ) 
S Saturation (Omb) 
to-50mb ) 
( > - 1 5bars)  
1 24 .  
Table  9 shows that all  topsoils ,  except those o f  RuMS and RuS , have a 
high A .W . e . , being greater than 22%  (using the ra ting system o f  McDonald ,  
1 96 1 ;  N . Z . Soil Bureau , 1 968 ) . Values f?r the organic soil , Tp , are 
particularly high due to a very large organic matter content  and low bulk 
density .  The Ruahine steepland soil , moderately steep phase ,  RuMS , has 
the lowest value for A .w . e .  This soil also has lowest  values for organic 
matter and highest values for bulk density.  There is an obvious relation-
ship between A .W . e . , bulk density and organic mat ter .  
between these parameters are shown in Fig . 38 and 3 9 . 
Regression l ines 
They show a negat ive 
correlation between A .W . e .  and bulk dens ity (Fig.  38 ) , and a positive 
correlation between A .w . e .  and organic ma tter (Fig . 39 ) , significant at the 
1 %  l evel . 
The A. W . e .  is the maximum amount of  water that is available to plants .  
In  mos t  cases , the soil i s  below F . e . , and less  water will  be  available . 
Thus ,  it  appears that RuS and RuMS ( see Fig . 3 7 )  are the soils most l ikely 
to induce drought iness during a dry spel l ,  their ability to store water 
being considerably smaller than the o ther se il s .  This will in turn adversely 
affec t  plant growth, and thus decrease  the stability contributed by  the root 
system to the solum .  
Fig . 3 7  illustrates the amount of water that  i s  unavailable to plants  
held a t  matric po tentials  greater than - 1 5  bar s .  I t  constitutes more  than 
twe nty percent  of the total soil volume in all hor izons of the TpH and 
DH soil s .  The same holds for the D tax soil except for the Bw horizon . 
I t  seems l ikely that  this water is largely associated with the organic mat ter 
and amorphous �lay frac t ion . 
A .W . e . , together with Ksat rates , indicate  the rate with which a soil 
will approach saturation , a so il proper ty previously discussed as  an important 
fac tor in soil erosion s tudies . Thu s ,  a soil with a smal l  A .w . e .  and low 
Ksat is slowly drained and mos t  l ikely to  become sa turated . The RuMS 
and RuS soils  have the smallest  values for A .W . e .  However , macroporosity 
FIGURE 38 : THE RELATIONSHIP BETWEEN AVAILABLE WATER-HOLDING CAPACITY 
AND BULK DENS ITY IN SOIL SAMPLES FROH CAR PARK CREEK . 
o 
r = - Q · 8 6 
o 
u 1 0  � o 
1 2 5 .  
� O�--------------------------�--------�---------
o 5 1 0  
B U L K 0 E N  5 1  T Y ( k g I m3 ) x 1 02 
* significant at  1 %  
FIGURE 39 : THE RELATIONSHIP BETIVEEN AVAILABLE WATER--HOLDING CAPACITY 
AND ORGANIC t-1ATTER IN SOIL SA1-1PLES FROM CAR PARK CREEK . 
.......... 
(lJ � 50 
o 
� 40 
o o 
OR GA N IC MAT T E R  (010 of t o t a l  s o i l  w e i g h t ) 
* significant at 1 %  
15  
1 2 6 .  
values indicate  that they are rapidly drained . s o  that saturation o f  these 
soils may not occur . However , their small storage capacity suggests  that 
they will wet up to f ield capacity relatively quickly in the autumn months . 
In the case of  TpH , the surface horizon has a large A .W . e .  with a 
markedly decreased value in the Bg horizon . This  information together with 
macroporosity and Ksat values subs tantiate the argument that  1 )  drainage 
is impeded in the B horizon , relative to the A horizon , and 2)  a perched 
water table may develop a t  the j unc tion of  these two horizons , and in 
suf f iciently wet conditions at  the base of the B horizon (where wa ter 
movement is impeded ) at  the iron pan . This perched water table  will  
develop upwards during a wet period , thus  creating a zone o f  saturation 
within the soil , which in turn may result in greater susceptibil ity to soil 
creep or  mass movement on a slope .  
6 . 2 . 2 . 5  1 5  bar Soil Water Retention, and 
the Effec t  of Drying 
o 
The eff ec t  of air drying (40 e ,  for 24  hours )  as a pretreatment on 
the ability of  soils to retain water a t  a matric po tential of  -1 5 bars , 
1 .  e .  the "drying effec t " ,  was investigated . A provisional suggestion 
has been made by G .  Smith  ( 1 9 78 )  that  the Percentage decrease in 1 5  bar 
water retention between undried and dried so il samples is a crude estimate  
of  the amount o f  amorphous clay present . 
Resul t s  obtained are summarised in Table  1 0 .  They indicate a 
relatively large "drying effec t" in Tp , TpH and D tax soil s ,  with the smallest 
eff ec t  noted in the RuS soil . These  values for percentage decrease o f  
undried 1 5  bar water retention , ranging between 1 3% and 6 5% ,  may be  compared 
with values obtained for some yellow-brown loam soils from Taranaki , which 
gave values between 66% and 8 3% ,  (G.  Smith,  1 9 78 ) . 
The rela tionship of  soil wat er retention values , and thus  A .W . e . , to 
organic mat ter has been previously shown (Fig . 3 9 ) . The relat ively 
TABLE 1 0 : THE EFFECT OF DRYING ON 1 5  BAR WATER RETENTION VALUES OF SELECTED SOIL SAMPLES 
SOIL 
NAME 
Takapari  
peaty loam 
Takapari 
hill  soils  
'"d mod .  steep-s:: ell s teep M 
0. phase  Q) 
Q) Ul 
+.1 M  
VI .� s teep 0 
Q) II) phase  s:: '.-4 ..c: ell very steep � phase  
Dannevirke 
taxadj unct 
soil 
Dannevirke 
hill soil 
SOIL 
SYMBOL 
Tp 
TpH 
RuMS 
RuS 
RuVS 
D tax 
DH 
HORIZON 
Ha l 
Ha2 
Ah 
Bg 
Cw 
Ah 
Bwl 
Bw2 
Ah 
Bw 
Ah 
Bw 
Ah 
Bw 
C 
Ah 
Bw 
1 5  BAR WATER RETENTION "DRYING 
(gravimetric water content , minus stones) EFFECT" 
undried dried ( see  p . 7 )  
1 48 . 58 7 0 . 48 78 . 1  52  
7 5 . 52  39 . 7 1 35 . 8 1 4 7  
7 2 . 7 2  48 . 1 0 24 . 62 34 
55 . 7 7 34 . 39 2 1 . 38 38 
7 3 . 9 6 28 . 98 44 . 98 6 1  
3 5 . 1 5 28 . 04 7 . 1 1  20 
25 . 89  1 8 . 37  7 . 52 2 9  
22 . 33 1 3 . 74 8 . 59 38 
2 8 . 1 6 22 . 7 1  5 . 45 1 9  
1 6 . 4 2  1 4 . 3 3  2 . 09 1 3  
44 . 0 1 28 . 00 1 6 . 60 3 7  
3 9 . 34 2 5 . 59  1 3 . 90 35  
7 2 . 34 39 . 76 32 . 58 4 5  
58 . 28 20 . 1 0 38 . 18 65  
2 7 . 84 1 5 . 96 1 1 . 88 4 3  
55 . 08 34 . 8 5 20 . 23 3 7  
4 7 . 98 24 . 25 2 3 . 7 3 49  
I-' 
N 
-...J 
. 
high "drying effec t"  values for Tp (Table  1 0 )  indicate  that : 
1 )  organic matter may have some part to play together with amorphous 
c lays in the "drying effect" , or 
2 )  these soil s contain the greatest amounts of  inorganfc amorphous 
constituents . 
Fig . 40 shows the rela t ionship between the "drying effec t "  and 
organic mat ter va lues for all the sel ec ted soil sample s .  Correlat ion 
between these two factors ,  for Ah and Ha horizons , is 0 . 84 ,  which is 
1 2 8 .  
signif icant a t  the 2%  level . The regression line for this set of data is 
shown in Fig . 40 . The correla tion between the "drying effec t"  and organic 
matter in the B and C horizon s is no t s ignifican t .  The data therefore 
indicate  that organic matter has a significant  influence on the "drying 
effec t" in Ah and Ha horizons . However , in the underlying B and C horizons , 
where organic mat ter levels are lower , the amorphous c lays seem to have 
an overriding influence on the "drying effect" , suggesting that this 
measure for amorphous ma teria l is best suited to subsurface horizons which 
have relatively low organic matter contents .  I t  i s  also possible tha t  these 
two fac tors are interrela ted , so tha t the "drying effec t "  of the amorphous 
clays is influenced by the amoun t of organic matter in the soil . 
The particularly large ef fec t of  drying in the Cw horizon of TpH, 
and the Bw horizon of D tax ind icate tha t these subsurface horizons contain 
a signif icant amount of  amorphous clays . The "drying effec t"  values for 
Ruahine steepland so ils are seen to be minor in the RuS soil , and greatest 
(on average) in the RuVS soil . In the RuMS soil , the values are seen to 
increase with depth , which may be partially explained by a dif ference in the 
initial hydration status of the c lays . Thus , it is possible that the degree  
o f  hydration of  the soil c lays is compara tively low in  the surface horizon , 
due to air-drying effects after a long dry summer , together with a possible 
desiccating effect of  winds .  The "drying effec t"  i s  expected to be less  
for soil  clays with an  initial low hydration status , compared with those  
,..." 
OJ III 
0 70 <1J 
� 
U 
<1J 60 ""0 
0 -0 
....... r:J) 
1: t-
U 
w 40 u... 
lL.. 
W 
<.9 30 
z 
:> 
a: 
0 .. 
10 
0 
O 
0 
0 
FIGURE 40 :  THE RELATIONSHIP OF ORGANIC MATTER 
TO THE EFFECT OF DRYING ON 1 5bar WATER RETENTION 
VALUES FOR SELECTED SOIL SAMPLES . 
0 
0 
0 
• r = 0 · S 4 
o r = 0 ' 4 4  
• 
1 0  20 30 40 50 
O R G A N I C  M A T T E R  ( %  at t ot a l  s o i l  we ight ) 
o B and C h o r i zons .. r = O '  4 4 , no t s ign if i cant . 
• A and  H hor izons,  r = 0 · 84, sign if i can.t at 2% • 
"DRYING EFFECT" - for definition see 6 . 1 .  
1 2 9 .  
1 30 .  
having a higher hydration status . 
6 . 2 . 2 . 6  Loss of  Weight on Ignition 
Organic ma tter level s are closely r elated to physical charac terist ic s  
o f  the selec ted so il samples , and are approximated by loss o f  ignition data , 
shown in Table 1 1 .  The Ruahine steepland soils , which are  charac teristically 
weakly structured , are seen to have the lowest loss on ignition values .  
Soils on stable sites  contain the greatest  amounts o f  organic matter , 
as accumulation is greatest there . Thus the Tp soil o f  the summit plateau 
has developed into a peat  under adverse conditions for rapid decomposition . 
Accumulation of up to approximately 50% organic matter has occurred in the 
upperpart  of the profile . TpH and DH con tain less organic matter than their 
counterparts  Tp and D tax (which occur on more stable s ites)  as reflec ted 
in the loss on ignit ion values . 
6 . 2 . 2 . 7  �Val�es in ( 1 )  Wa ter and ( 2 )  Sodium Fl uoride 
( 1 )  The pH ( in water) values indicate that  relat ively acid ic condit ions 
occur in soils , a t  higher e levations ( i . e .  Tp , TpH ) , in which the pH is 
seen to vary between 4 . 1  and 4 . 9 , with a slight increase  with depth . These 
low values are adverse to ac tive microbial activity,  and together with 
relatively low mean annual soil temperatures « SoC) , help to explain slow 
decomposition rates in these soils . The D tax and DH soils have slightly 
higher pH ' s ,  between 5 . 2  and 5 . 7 .  In these soils there is less accumulation 
of o rganic  materials  on the surface ,  and it seems probable  tha t  they maintain 
an ac tive microbial populat ion , aiding the breakdown and admixing o f  organic 
materials .  The Ruahine steepland soils show a comparat ively wide range 
of pH values .  A general trend exists  for the pH o f  all  the soils in the 
s tudy area to decrease  with increasing age and weathering t (Table 1 2 ) . 
( 2 )  the pH ( in NaF) of soils has been used as  a measure of  amorphous material 
content (Soil  Survey Staf f ,  USDA ,  1 975 ) , with the requirement that the water 
retention of  the previously dried soil is  greater than twenty percent a t  
TABLE 1 1 :  
SOIL 
NAME 
Takapari 
peaty loam 
Takapari 
hill soils 
Ruahine 
steepland 
soils 
Dannevirke 
taxadj unct 
Dannevirke 
hill soils  
1 3 1 . 
LOSS OF WEIGHT ON IGNITION DATA FOR SELECTED SOIL SAMPLES 
mod .  steep 
- steep 
phase 
very 
steep 
phase 
SOIL 
SYMBOL 
Tp 
TpH 
RuMS 
RuVS 
D tax 
DH 
HORIZON 
Ha l 
Ha2 
Ah (Ha) 
Bg (Br)  
Cw 
Ah 
Bw1 
Bw2 
Ah 
Bw 
Ah 
Bw 
C 
Ah 
Bw 
LOSS OF WEIGHT ON 
IGNITION ( %  of  dry soil ) 
49%  
32%  
2 0% ( 5 1 % )  
1 5% ( 1 5%, 
1 7 %  
15%  
8% 
5% 
18% 
1 3% 
3 1 %  
1 2 %  
5% 
1 7 %  
12%  
1 32 .  
TABLE 1 2  pH VALUES IN  ( 1 )  WATER , and ( 2 )  SODIUM FLUORIDE 
SOIL 
NANE 
SOIL 
SYMBOL 
HORIZON pH in water 
( 1 : 2 . 5 ) 
pH in Sodium 
Fl uoride (NaF) 
( 1 :  50)  
Takapari  pea ty 
loam 
Takapari hill 
soil 
Tp 
TpH 
Ruahine 
mod . steep - RuMS 
steep 
steepland 
soils  
Dannevirke 
taxadj unc t 
phase 
steep 
phase 
very steep 
phase 
Dannevirke hill 
so il 
RuS 
RuVS 
D tax 
DH 
Ha l 
Ha2 
Ah 
Bg 
Cw 
Ah 
Bw1 
Bw2 
Ah 
Bw 
Ah 
Bw 
Ah 
Bw 
C 
Ah 
Bw 
4 . 5  
4 . 6  
4 . 1  
4 . 6  
4 . 9  
6 . 2  
5 . 7 
6 . 0  
5 . 5  
5 . 7  
5 . 4  
4 . 8  
5 . 2 
5 . 5  
5 . 4  
5 . 7  
5 . 7 
7 . 4 
8 . 3  
7 . 4  
7 . 7  
9 . 3  
7 . 5  * 
7 . 5  * 
7 . 5  * 
7 . 5  * 
7 . 5 * 
7 . 5  * 
8 . 3  
7 . 5  * 
1 0 . 7 ** 
1 0 . 4  ** 
7 . 7  
1 0 . 4  
* water retent ion a t  - 1 5  bars , for previously dried soil i s  < 20% , 
thus precluding these result s from use for Soil Taxonomy 
classification uses . 
** pH in NaF is > 9 . 4 ,  and therefore amorphous material dominates the 
exchange complex ( Soil Survey Staff , USDA , 1 9 7 5 ) . 
- 1 5  bars tension . Thus it  exc ludes certain horizons o f  the Ruahine 
steepland soils . The results show that the D tax and DH soils have B and 
C ,  and B horizons respec tively in which the exchange complex is largely 
composed of amorphous materials . The presence of  amorphous materials  in 
the Bw horizon of  RuVS , which is suggested by pH ( in NaF) values , is  
supported by "drying effec t" and bulk density da ta . Relatively high pH 
1 3 3 .  
( in NaF) values a r e  also noted in subsurface horizons of  Tp and TpH . The 
high organic matter contents  of the surface horizons may be obscuring the 
presence of  amorphous materials to this test , an observation also made by 
G. Smith  ( 1 978 )  in West Indian and South American soils ,  (Table 1 2 ) . 
SECTION THREE : SOIL MINERALOGY 
6 . 3 . 1 Sand Mineralogy 
6 . 3 . 1 . 1 Materials  and Methods 
A small portion of the sand fraction ( 63-1 2 5�m) was extracted f rom 
selec ted soil samples (which had been previously air-dried , and passed 
through a 2mm sieve) in the fol lowing manner . Approximately 20g o f  this 
<2mm soil frac tion was placed in a beaker . to which hydrogen per oxide 
A more concentrated solution of hydrogen peroxide 
( '" 30% ) was required for soils with a high organic content .  The beaker 
was slowly heated on a water bath , with fur ther addit ions of H
2
02 , until  
quiescence .  The reaction was taken to completion on the water bath a t  1 000C 
for a few hours ,  and lef t overnight to cool slowly . The remaining 
< 2mm mineral frac t ions were placed in centrifuge tubes , with distilled water . 
A few drops of 1 : 1  NH4 solution was added to disperse the clays , and these 
fractions were ultra-sonically vibrated for three  to four minutes to improve 
disper s ion . 
The < 1 . 0�m clay frac t ion was then removed , by decantation o f  a suspension 
obtained by centrifuging the soil suspension a t  1 000rpm for 8 . S  minutes .  
The decantates were kept for  clay mineralogical studies . 
1 34 . 
The remaining coarse clay , silt  and sand was dried in an evaporating 
dish, at 1 0S
o
C .  The 63-1 2 S�m sand fraction was obta ined by dry sieving . 
A subsamp1e of this fraction was mounted on a microscope slide in 
Lakeside Cement , for investigation , using a petrological microscope .  
Point count analysis of  each slide  was used as  a method to  estimate per­
centages of minerals present ; with a minimum of  300 grains being counted 
on each slide .  
6 . 3 . 1 . 2  Results and Discussion 
Results  of  point count analyses are indicated in Table 1 3 .  They 
show that  quar t z ,  greywacke rock fragments and rhyolitic glass are the 
most frequently occurring grains .  
The presence o f  cummingtonite was also noted in the An horizon of the 
Dannevirke taxadj unc t (D tax) soil , which indicates one source of the tephric 
components to be  the Haraharo Rhyolite Complex in the Okataina Volcanic 
Centre (Ewart ,  1 9 68 ) . The presence of  andesitic a sh in the form of  micro1ites 
of f eldspars and maf ic s  held together in a glassy meso stasis was carefully 
examined . A few grains were seen to have this appearance in the Takapari 
peaty loam (Tp )  and D tax soils , and it seems likely that some andesitic 
ash is present in these so ils . If  greater amounts of  andesitic ash were 
originally present in the soil , they have been largely converted to o ther 
minerals ,  such as a110phane,  or possibly weathered to a smaller size fraction . 
Contamination of  these soils by andesitic tephras is a lso suggested by the 
presence of hornblende (which is common in intermediate igneous rocks , such 
as andesite ,  but rare in greywacke and rhyolite) , and also hypersthene 
(common in andesitic rocks ) . 
The composition of  plagioc lase f e ldspars was investigated , where 
possible ,  using the Michel-Levy Method ( see e . g .  Kerr , 1 959 ) . Oligoclase 
(An 2S%)  and albite (An 1 0%)  were identified in the Takapari  hill soils 
TABLE 1 3 :  SAND MINERALOGY OF SELECTED SAND FRACTIONS OF SAMPLES FROM THE STUDY AREA 
GREYWACKE 
SOIL HORIZON QUARTZ FELDSPAR MICA ROCK FRAG- RHYOLITIC HYPERS- HORN- MAC-N-
MENTS GLASS THENE AUGITE BLENDE ETITE 
% o f  sand fractiou 
Dannevirke Ah C c - a a S S R R 
taxad junct Bw a c - C C c S R S 
C a c - a c S R R R 
(Dtax) 
Dannevirke Ah C C R a C S S S R 
hill soil Bw R C S S R S a c a 
(DH)  
Takapari Ha l C C R S a c c S S 
peaty Ha2 C C S S a c c S S loam 
(Tp )  
Takapari Ah a C R c a S S S R 
hill Bg C C S R S a c - c soil 
( TpH) 
Cw a C - C C S R - S 
Ruahine Ah a C - C C S S R -
steepland Bw a C R C C S S R S soil 
(RuVS )  
Ruahine Ah C C S a C S R - R 
steep land Bw a c S a S S R - -soil 
(RuMS) 
colluvial 
pocket  C S C S A S S R - S 
beneath  (Tp) 
-
ZIRCON 
R 
-
-
R 
-
-
R 
-
-
- -
-
-
-
-
-
-.  -----
FREQUENCY LEVELS 
OF MINERALS IN 
SAND FRACTIONS 
very 
A = > 50% abundan t 
a = 30-50% abunda n t  
C = 
c = 
S = 
1 0-29%  very common 
5-9% common 
1 -4%  scarce 
R = < 1 % rare 
(after : Fieldes & 
Weatherhead ( 1 9 6 6  
..... 
w 
V1 . 
) 
1 3 6 .  
(TpH) and Dannevirke hill seils (DH) soils , respectively . These 
plagioc lase feld spars , r ich in potassium , are considered to be  abundant 
in acid-intermediate rocks such as greywacke and rhyolitic ash,  (Fieldes 
and Weatherhead ,  1 966) . Sanadine , an alkali feldspar considered to be 
charac teristic of volcanic rocks such as  rhyolites (Kerr , 1959 )  was noted 
in a Ruahine steepland soil sample by its  characteristic low relief and 
birefringence .  
Rhyolitic glass  was observed in all samples investigated . I t  was 
abundant ( 30%-50% of sand fraction ) in the Tp soil , in which hypersthene 
and augite were also common ( 5%-9% ) . This soil has the largest proportion 
of tephric components , probab ly because there is relatively l ittl e addition 
of greywacke-derived loess and colluvium . At lower elevations , on relatively 
gentle slopes , greywacke-derived loess is  the main constituent with a smaller 
tephra component in the sand frac t ion . On less stable sites , where soil 
creep and mass  movement occur , greywacke col luvium is  also added to the soi l ,  
"diluting" both the greywacke-derived loess and tephra components . I t  
may b e  that  the tephra component at  a l l  eleva tions is  of  s imilar volcanic origin , 
but whereas at high alt itudes little "dilution" has occurred due to little  
greywacke-derived loess  deposition ; a t  lower altitudes , the tephra 
component has been inundated by greywacke loess and colluvium . 
The Ruahine steepland so il sampl es conta ined 70% quartz  and feldspar (derived 
predominantly from the underlying greywacke bedrock) and greywacke rock 
f ragments . These soils have the smallest proport ions of tephra mineral s .  
The D tax soils contain common to abundant amounts  of  rhyolitic glass ,  
together with between 4%  and 1 1 % mafic grains (augite , hypersthene and horn­
blende) . Fieldes and Weatherhea d ( 1 966)  described the sand mineralogy o f  
a Dannevirke s il t  loam , from Eketahuna , which contains more quartz and less 
glass .  I t  seems l ikely that during Post-glacial t imes (Aranuian S tage) 
less greywacke loess has been deposited in the mountainland , compared with 
the terraced floodplains of the rivers , travers ing the lowlands ,  which are 
the major  source o f  Pos t-glac ial loess . 
A small pocket o f  colluvial material  exposed in a recent cut ting 
on the summit plateau was al so investigated for princ iple  sand mineral 
fractions . The colluvial pocket occurs between the greywscke bedrock 
1 3 7 . 
and the base of  Tp . I t  was found to contain predominantly greywacke rock 
fragments .  However 2% to 3% o f  the sand fraction i s  hypersthene and 
augite suggesting that andesitic ash may once have been present , but has 
subsequently been weathered away . The presence o f  2%  magnetite ,  which is  
very resistant  to  weathering and remains in  the soil when other components  
have been physically and chemically altered also suggests  the former presence 
of a greater tephra component . Rhyclitic glass is scarce in this depo sit , 
in contrast to the overlying soil , where it is abundant .  Thus , this 
colluvial deposit which predates the overlying soil (about 4 600 years old)  
and contains  markedly less rhyolitic tephra ,  suggests  that : (a )  there was 
less  rhyolitic volcanic ac tivity when this colluvial mater ial was deposited ,  
o r  (b)  i t  was depo sited , and buried by an overlying depo sit , before i t  could 
receive a significant quantity of  rhyolitic tephra grains , or  (c) the 
original tephras have been largely weathered to amorphous materials .  
6 . 3 . 2  Clay Mineralogy 
6 . 3 . 2 . 1  Introduction 
The clay mineralogy of soil s ,  from selected sites within Car Park Creek 
subcatchment was investigated using 4 techniques : 
X-ray diffrac t ion (XRD) 
Differential Thermal Analysis (DTA) 
Infrared Spectroscopy ( IRS ) 
and Transmission Electron Microscopy (TEM) 
XRD techniques are used to identify crystalline phyllosilicate minerals 
present  in soils , as  well  as other crystalline mineral s  such as quart z  and 
feldspar .  Amorphous materials , undetected by  XRD, may be investigated using 
DTA and IRS . These techniques combine to give a detailed p ic ture of the 
1 3 8 .  
constituents of  the soil clay frac tion , for  comparison of  hor izons within 
a soil profile ,  and between different soil profiles . 
TEM provides additional informat ion about minor const ituents , which 
may be present in amounts  too small to be detec ted positively by the o ther 
techniques . I t  also provides positive evidence for the presence of  
amorphous materials ; and allows a visual assessment of  the proportions 
of easily identified constituents of the clay frac t ions . 
6 . 3 . 2 . 2  Materials and Methods 
The soil c lay frac tion ( � l �m) was separated from the selec ted soil 
samples , a s  described in 4 . 2 . 1 .  Pretreatment of  the clays was kept to 
a minimum to ensure minimum alteration of  the inorganic components .  
(a)  X-ray Diffrac tion , (XRD) 
+ 
X-ray dif frac t ion pat terns were obtained for NH4 saturated clay 
samples by drying a small al iquot of  the clay suspension on a glass  slide,  
and rotating between 40 and 400 of  29 in a Philips X-ray diffraction 
apparatus . 
Th 1 d f h d . f · 11 · M 2+ e norma proce ure or t e etec t l0n 0 montmor l onlte : g 
saturation of  the c lay ,  and introduction o f  a drop of  5% glycerol in ethanol 
which expands the basal spacing of the c lay to about 1 7 . 7R, was followed . 
To investigate  the proportions and struc ture o f  vermiculite  and 
+ 
pedogenic chlorite ,  the c lays were then K satura ted , and sequentially 
heated through 3000C , 4500C to 5500C ,  ob taining an XRD pattern for each 
sample , at each stage . This  procedure also enabled investigation for the 
presence of kaolinite ,  which is characterised by a basal reflec t ion a t  
7 . 1 8R . With K
+ 
satura t ion , the vermiculite  structure collapses to a mica-
l ike struc ture ;  with a basal reflection at about l oR. An aluminous 
vermiculite ,  however , will not collapse completely with K+ saturation;  but 
o does collapse when heated to 550 C ,  for 3 0 minutes . The amount o f  collapse 
of  structure from l 4g to l oR at 3000 C and 4500C indicates the amount o f  
substitution o f  the exchangeable  cations between lat tice layers by  the 
1 39 .  
hycroxy aluminous groups . I f  interlayering of the vermiculite  by hydroxy 
aluminium polymers  is complete ,  the mineral is pedogenic chlorite .  This 
does not collapse when heated to 5500C ,  and infac t the 14R peak is  normally 
intensified . 
Investigation of  the 14g and 7R peaks a fter the three stages of  seq­
uential heating also showed the relat ive contribution , if any , of  kaol inite 
to the 7R peak. 
(b)  Differential Thermal Analysis , (DTA) 
The D . T . A .  method depends on the detec tion of  heat given out or 
absorbed by a substance due to thermal changes such as dehydration and 
dehydroxyla t ion , as its  tempera ture is raised uniformly .  Pretreatment 
of the so il samples with hydrogen peroxide is  not necessary for D . T . A .  
a s  oxidat ion reac tions of the organic mat ter are suppressed by using a nitrogen 
atmosphere .  A clay sample , in fine powder form .is  placed i n  a specimen 
holder , in contact with a set of  thermocouples . An inert substance is 
placed in an adj acent speciman holder , also with a set of  thermocouples . 
Whilst  no reac tions occur within the unknown clay sample as  the temperature 
is ra ised , the t empera ture of both the clay sample and inert s�bstance will 
be the same . When a reac t ion involving a heat change occurs , the t emp-
erature of the clay sample will deviate from that  of the inert substance .  
The t emperature difference between the two sets o f  thermocouples is recorded 
automatically , as  a series of peaks on graph paper . The direc tion of  the 
peaks indicates whether a heat change is exothermic or endothermic , while  
the approximate  temperature at  which a heat change commences , reaches a 
maximum , and declines can be read off  the graph , and the magnitude o f  the 
reaction noted . The pat terns ob tained can be interpreted to iden tify the 
presence of  cer tain clay minerals  or amorphous constituents . However , 
Mackenzie ( 1 9 7 2 )  s tresses tha t  D . T . A .  on its  own solves few problems in 
Soil Sc ienc e ,  and only by integrat ing the results  obtained with those  
from other investigational methods can detailed information be  obtained . 
+ 
The clay sample for analysis was in an air-dried , crushed , NH4 
1 40 .  
saturated form ,  of  which 1 5mg was required . To ensure comparable hygro-
scopic moisture contents at  analysis , the samples were equilibrated for a t  
least 3 days at  5 6%  relative humidity ,  using a saturated solution of  
o A heating rate of  1 0  C per minute and a nitrogen a tmosphere 
were used for all  samples . 1 0mg of AlZ
03 were used as  the standard 
inert substance .  
( c )  Infra-Red Spectroscopy, (IRS ) 
Clay mineralogical studies using IRS depend on IR absorption , the 
wavelength of which is equal to the wavelengths of the bonds between or 
within molecules . Absorption typically occurs in the 4000-400cm- 1  
wavenumber r egion , (wavelength Z . 5-Z5�m) . Thus bonding between and within 
molecules , and between ions of crystals ,  can be characterised . In this 
study the sample for investigation was prepared by weighing accurately Zmg 
+ of dried , c rushed , NH4 saturated clay , which was then mixed with 1 70mg 
of  po tassium bromide (K Br)  and ground to a homogeneous mass  in a ball  mill .  
The powder was then inserted into a sample holder , evacuated , and pressed 
into a nearly transparent 1 3mm disc for analysis in the IR beam . The 
o prepared disc was left  in an oven overnight , a t  50 C .  A double-beam 
spectrophotometer was used for analysis , one beam passing through the 
prepared KBr disc , while the reference beam passes through air .  This  
makes possible differential analysis , thus eliminating errors due to 
variation in radiation source and to absorption by atmospheric water vapour 
and carbon dioxide . 
In some cases it  was necessary to dilute the s arrple in the KBr disc 
sixfold , to resolve peaks at  lower wavenumbers . In this case , the disc 
was removed from its  holder and crushed . Approximately one-sixth ( 3Omg) was 
mixed with a further 140mg of  KBr , and a second disc prepared . 
1 4 1 .  
(d )  Transmission Elec tron Microscopy (T . E . M . ) 
T . E .M .  employs a beam of  electrons . focused magnetically onto the 
specimen . The elec trons are scattered by solid obj ec t s ,  such as clay 
particles , casting a "shadow" on to a screen or photographic plate .  
The el&tron micrographs produced reveal the approximate size  and shape 
of the clay particles . They can also be  used for positive identification 
of certain substances such as amorphous materia l s .  However ,  unlike the 
3 previous techniques no struc tural detail is detec table , unless elec tron 
dif frac tion procedures are used . 
The sample  reqt:.ired for invest igation needs to be in a dry s tate ,  when 
introduced into the transmiss ion electron microscope ,  as  it is viewed in 
an evacuated lens column and specimen chamber . I t  must also be no greater 
than O . 2�m thick ,  this being the l imit of  penetration of  the electron beam . 
+ 
In this investi3ation a very dilute suspension of  the NH4 saturated clay 
fraction was prepared , which looked slightly opaque when held to the light . 
One drop of  this suspension , af ter being fully dispersed both chemically 
and ul trasonically was taken , using a clean pipette ,  and mounted on a carbon 
film supported by a 3mm copper grid . The specimen was allowed to dry in 
an oven at  500C prior to investigation with the elec tron microscope .  
6 . 3 . 2 . 3  Resul ts and Discussion 
(a )  X-ray Diffraction (XRD) 
+ Initial diffrac tion patterns obtained for the NH4 saturated clays , 
indicated that the crystalline components were in all  cases dominated by 
14g vermiculite and/or pedogenic chlorite ,  with varying degrees of inter-
stratification , ( see Fig . 4 1 ) . Fig.  4 1  indicates that the 1 4g peak is 
particularly well defined in the samples of  TpH (Cw horizon) and DH (Bw 
horizon) . I t  is also clearly evident in the samples of  the RuMS and 
RuVS soils (Bw horizons ) ;  which also have well defined 1 09 peaks produced 
by mica . The absence of  clearly defined l oR peaks in the o ther patterns 
in Fig 4 1  is noted . This suggests that  mica if present  originally (which 
FIGURE 4 1 : X-�\Y DIFFRACTION PATTERNS OF NH+ SATURATED 
CLAY SA11PLES FF.0M SELECTED SOIL PROFILES , SH��nNG THE 
PREDOl lINANCE OF 1 4 R VERMICULITE AND 1 2R INTERSTRATIFIED 
MATERIALS . 
SOI L 
NAME 
TAKAPAR I 
z o N -.J ­_ 0:: 0 0  If) :t:  
PEATY  - Hal LOAM 
TAKAPARI 
H I LL  SOIL 
8g 
o 
3 ·32A o 
7A 
J� 
1 4 2 . 
Cw 1J�fW\»f1\. / 1\ I, . �� I :' � IV ., ; I I I I \ A .1 \  I 
RUAHINE
.
/\;J \ . ..wi"'" '. .r! � STEEPLAND 8 r""'" �� � 
SOI L  - w 
(moderately 
steep phose) 
RUAHINE 
STEEPLAND 
SOI L  -Bw 
(very steep 
phase ) 
DANNEVIRKE 
HILL  SOIL -Bw I. o 0 0 
7A leA l2A lL.A 
1 4 3 .  
is  l ikely due t o  the nature of  parent  materials ) has been al tered t o  some 
extent , so that a clearly defined 109 is not ob tained . The shifting of the 
14g peak toward 1 2g, in the case of Tp (Ha l )  and TpH (Bg) , in Fig . 4 1 ,  
indicates a considerable  amount of  interstratification o f  the vermiculi tic 
material . 
Chlorite  charac teris tically gives a small 14g peak and large 7g 
+ peak in XRD pa tterns of  NH4 sa turated clays . Figs . 4 1  and 42  show the 
1 4g peak to be larger , indicating that it  is  due to vermiculite  rather than 
chlorite . 
The more weakly weathered Ruahine s teepland soils also show quartz  
and feldspar to  be minor constituents in all  samples inves tiga ted ( see 
Figs . 4 1 ,  4 3 )  al though Tp (Ha l )  gives a particularly large quartz  peak at  
3 . 32g (Fig . 4 1 ) . This latter peak may be due , in par t ,  to an aeolian 
contribution ,  as well as a contribution of  c lay-sized quartz  weathering 
from coarser frac tions , in this wet ,  acidic environment . 
To characterise material contributing to the l4g peak , XRD patterns 
+ of K saturated clays were obtained , sequentially heated through 3 stages 
additional to the no heat treatment . Examples of patterns thus obtained 
are shown in Figs . 42 and 4 3 .  In no sample did the l4g peak collapse to 
109 on K+ saturation alone , without heat treatment ,  which would be  the case 
if vermicul ite were presen t .  Thus , an intergrading aluminous vermiculite ,  
or  pedogenic chlorite ,  was considered to be  present  in the clay samples . 
o Pedogenic chlorite retains its  structure even when heated to 550 C 
and then usually gives an enhanced f irst  order spacing , at  about l4R . 
The clay frac tion of the Bw horizon o f  the D tax soil  gave a pa ttern indicating 
a small  amount of pedogenic chlorite ,  ( see Fig . 4 2 ,  arrowed ) .  Clay frac tions 
from the Ah horizon of D tax and from the Bw horizon of RuVS , when 
sequentially heated to 550
0
C ,  produced patterns in which the l 4R peak 
collapsed to about l oR ,  indicating the presence of  aluminous vermiculite , 
( examples are shown in Fig . 4 2 ,  43) . 
In the case of  the more weakly weathered Ruahine s teepland soil s ,  the 
FIGURE 4 2 :  X-RAY DIFFRACTION PATTERNS OF A 
DANNEVIRKE TAXADJUNCT SOIL PROFILE INDICATING 
THE PRESENCE OF A SMALL AMOUNT OF PEDOGENIC 
CHLORITE , (arrowed) . 
CD N H Z  satu ra ted c l ay  
@ K + 
Q) .. 
0 "  
® .. 
II 7A 
. .  . heated to 300 °C .  
.. 450 °C . 
o 14A 
® 
CD 
, ° .. 550 C .  
o 7A o 14A 
144 . 
FIGURE 4 3 :  X-RAY DIFFRACTION PATTERNS OF NH: AND K+ 
SATURATED CLAY SAMPLES FROM A Bw HORIZON OF A 
RUAHINE STEEPLAND SOIL (RuVS) FOR THE INVESTIGATION 
OF VERMICULITE COMPOSITION AND THE PRESENCE OF 
KAOLINITE . 
0 0 0 
7 A lOA l4 A 
G) NH,4 saturated c lay 
Q) K + 
0) . .  
@ 
® 
. .  
" 
. .  
I 0 heated to 300 C . 
o 
450 C .  
I I  o 
550 C . 
@ , I 
o 
7A 
o o 
lOA l4A 
1 4 5 .  
14R peak had largely collapsed a t  3000C ,  indicating less replacement 
of  the interlayer cations in the vermiculite by hydroxy alumina polymers , 
than in the o ther soil s .  One example  is shown in Fig . 4 3 .  
1 4 6 .  
I n  two Ruahine steepland soil profiles studied the 7 R. peak appears to 
retain much of its intensity at 3000C (arrowed in Fig . 4 3) , al though the 
1 4R peak has begun to collapse .  From this evidence ,  it  was considered 
that a small  amount of kaolinite may have been contributing to the 7 R  
peak . 
The possible occurrence o f  al lophane being concentra ted on slippage 
planes at the soil-rock interface was investigated at the head of Cats  Paw 
Scar , a rocksl ide  in Car Park Creek . Workers in North Westland , New Zealand 
(O ' Loughlin and Pearc e ,  1 9 76 )  have reported one of the principle causes 
of  instability on slopes to be an allophane-rich organic clay layer at a 
regolith-sandstone interface .  Thus , a c lay sample  was taken f rom the soil-
rock interface , at  the head of  the rockslide ,  and its XRD patterns were 
compared with those of c lay samples taken from the Ha and Br  horizons of the 
same soil , a TpH . The c lay sample  was found to contain a significant amount 
of crystalline material , identified as aluminous  vermiculite .  The 
XRD patterns indicated that its clay mineralogy was s imilar to the o ther 
2 samples studied , i . e . the Br and Ha horizons o f  the same soil . 
In summary, the XRD patterns show that the crystalline fraction of the 
soil clays in the s tudy area is dominated by aluminous vermiculite,  with 
small  amounts  of  pedogenic chlorite ,  mica , quart z ,  feldspar ,  and possibly 
kaolinite in some of  the horizons . The relative proportions of these 
constituents , and the composition of the vermiculites charact erise the 
extent of weathering within these soils . 
(b)  Dif ferential Thermal Analysis , (D . T . A . ) 
Further characterisation of  the soil clays was carried out by D . T . A. 
This method of  investigation was used on c lays suspec ted of containing a 
significant inorganic amorphous component . Fig . 44  shows some of  the 
D . T . A . curves obtained . Curves produced by TpH samples . from the Bg and 
1 
2 
3 
4 
5 
X 
FIGURE 44 : D . T . A .  CURVES OF SELECTED SOIL CLAY 
SAMPLES FROM CAR PARK CREEK . 
® 
40 100 200 300 400 500 600 700 100 Il1O 
TEMPERAT�E ( 'C )  
Takapari peaty loam ,  Tp , Ha2 horizon 
Dannevirke taxadj unct .  D tax . , Ah horizon 
" " " C horizon 
Takapari hill soil , TpH , Bg horizon 
" " " " Cw horizon 
200 C ,  Y = 450 C .  
1 4 7 .  
• 
148 . 
Cw horizons , show a doublet on the initial low temperature endo therm, 
indicating the presence of  two phases , whilst the Ah and C horizons o f  the 
D tax soil , and the Ha2 horizon of Tp , show this doublet  to a . lesser 
extent . This endotherm is due to dehydration of  non-crystalline materials 
with some contribution from dehydration of  the vermiculite group . By 
cross  reference with the other techniques it is l ikely that the low 
temperature component of  the doublet ( 70-960C )  is due to loss of  water , 
weakly bonded to vermiculite , whereas that at  1 18- 1 280C is due to loss o f  
water from allophane .  The comparatively large low temperature endotherm 
o 
at  1 2 8  C ,  produced by c lay in the Cw horizon of  TpH suggests a greater 
amount of amorphous material in this horizon than the overlying Bg  hor izon . 
o Interstratified phyllosilicates may give an endotherm a t  600 C and 
a sigmoid peak system between 8000C and 9000C (Mackenzie,  1 9 7 2 ) , but there 
is no evidence o f  such curves on the thermal patterns presented here . 
Indeed , the l ack of  clearly defined large peaks in all  the curves 
indicates that crystalline minerals do not dominate the clay f ractions . 
D . T . A .  is not suited to identification of  the chlorite group , as  chlorite 
gives various thermal responses depending on its structure . 
The small  endotherms , between 2000C and 4S00C (between X and Y in 
Fig . 44 )  may indicate  dehydroxylation o f  aluminous polymers , in the inter-
layer space of aluminous vermiculite , although a possible contribution by 
iron oxides cannot be ignored . Non-crystalline and crystalline materials 
also contribute to an exotherm , between 93S
o
C and 9 790C ,  in all  5 curves 
in Fig . 44 . Thi s exotherm is attributed to the formation o f  mullite ,  a 
spinel phase , from amorphous materials ,  which are transformed at  lower 
temperatures , possibly from phyllosilicate clays . 
The D . T . A .  curves give only a l imited amount of  information about 
the so il clay frac tion , which must  be used in conj unction with information 
obtained from the other investigational procedures .  However ,  i t  c learly 
1 4 9 .  
indicates that  al though there is  a well defined crystalline fraction . 
amorphous materials are also present in appreciabl e amounts  and are mod-
erately hydrated .  
( c )  Infra-Red Spectroscopy ( IRS ) 
Fig . 45  indicates the results  obtained from IRS studie s .  Diagnostic 
vibration patterns of  layer silicate minerals .  due to cons tituent units  
include :  
constituent unit  
hydroxyl group 
silicate anion 
oc tahedral cations 
vibration waveno (em-I ) 
3400 - 3 7 50 
700 1 200 
150  - 600 (coupled with Si-O 
bending vibrations ) 
(Farmer . 1 9 7 4 )  
The patterns obtained from the selected soil samples . contain these 
characteristic peaks . thus indicating the presence of layer silicates . 
Two broad absorption bands at  3400 and 3200 cm- 1 ( indicated by dashed 
lines in Fig . 4 5 ) , due to OH stretching vibrations , are most  clearly 
defined in the curves produced by Tp (Ha2 ) , D tax (Ah) and TpH (Cw) . That 
at  3400cm- 1  is due to allophane , whereas that  at  3200 cm
- 1 is probably due 
to interlayer wa ter in the vermiculite  component  of  the clay.  The sharp 
-1 -1 bands a t  3 700cm and 3625cm of  Tp (Ha2 ) . are due to bonded OH in the 
oc tahedral planes of  the c lay crystall ine components .  
- 1  - 1  The strong absorption bands in the 3400cm to 3200cm region . together 
- 1  -1  with s trong absorption in the 1 700cm to 1 600cm region due to OH- bending 
vibrations , indicates a dominance of highly hydrated materials .  such as  
allophane ,  together with contributions from the int erlayer water of  verm-
iculitic materials ,  in Tp (Ha2 )  TpH (Cw) and D tax (Ah) soil s .  A shoulder 
at  1 1 00cm-1  indicates a considerable s iliceous component in the allophane .  
and is present in all  selected samples , except the Bw horizon of  the Ruahine 
steepland soil , being most  pronounced in the Tp and D tax soil sample s .  
Absorption in the 800cm
- 1 region i s  indicative o f  the presence o f ' s il ica . 
1 
2 
3 
4 
5 
6 
FIGURE 4 5 : INFRARED SPECTRA OF SELECTED SOIL 
CLAY SAMPLES FROM CAR PARK CREEK . 
® 
® 
40 31 32 28 24' 20 
Takapari peaty loam, 
,. 14 10 8 
WAVENUMBER ( e m  - I )  
Tp , Ha2 horizon , 
6 
Danneyirke taxadjunct ,  D tax . •  Ah horizon 
" " " C horizon 
Takapari hill soil , TpH , Bg horizon 
" " " " Cw horizon 
Ruahine s teepland soil , very steep phase , 
RuVS , Bw horizon : 
4 x100 
1 50 .  
1 5 1 .  
- 1  A peak at  9 1 Scm ( "A" in Fig . 45 )  due t o  AI-OR vibrations within the 
oc tahedral part of the crystalline structure is not strongly in evidence 
in any of the patterns . This indicates Al-OR bonds in crystall ine mineral s  
are not a maj or constituent o f  the sample , and AI-OR i s  probably in the form 
of  amorphous gels or AI-OR polymers .  
Peaks occurring a t  1400cm-1 are due to the vibrations o f  cat ions which 
occupy exchange positions . 
+ 
These cations include NH4 introduced by pre-
treatment , together with K
+ 
and A1
3+ and possibly small amounts of  other 
cat ions which are thought to occupy interlayer positions of the vermiculitic 
materials .  The broadness o f  the peak is indicative o f  the variety of  
cations which are  present ; and its  strength is indicative of  the cation-
exchange capacities (C . E . C . ) of the clays . Clay from the Ra2 horizon 
of the Tp soil produces a comparatively large peak, indicat ing that the C . E .C . 
of  its  c lays is  quite apprec iable .  The Ah horizon of the D tax soil also 
has a large pEak a t  1 400 cm-1  which is larger than that  for the C horizon , 
and it therefore has a larger C . E . C .  In the TpR soil , the clay o f  the Cw 
horizon appears to have a larger C . E . C .  than that of  the Bg horizon , which 
may be  due to  a larger amount of  amorphous material in the Cw horizon . 
-1 - 1  -The peaks at  2850cm and 2900cm indicate  OR bands , contributed by 
humus , with a possible contribution also from allophane-organic complexes .  
These are particularly noticeable in Tp , TpR (Bg) and D tax CAh) . 
The spectra indicate  a significant amount o f  hydrated amorphous material  
in  the  soil s ,  especially in Tp , TpR (Cw) and D tax (Cw) , together with a 
layer silicate component . The presence of  a highly s iliceous gel is 
indicated particularly in Tp , TpR and D t ax .  RuVS contains some amorphous 
constituents ,  being less highly hydrated than in the o ther soil s .  
(d)  Transmission Electron Microscopy (TEM) 
Table  14 summarises the findings o f  TEM studies of the clays .  These 
s tudies enabl edidentif ication of  some phyllosilicat es and amorphous materials , 
and a visual assessment of  their relative proportions in the samples . 
SOIL 
NAME 
Takapari 
peaty loam 
Takapari 
hill soils 
mod . steep 
"0 -steep I=! phase cd ...-i 0.. 
OJ steep OJ 
.jJ en phase "' ...-i 
.� OJ 0 � \I) 
.� ..c: very steeJ cd ;:l phase � 
Dannevirke 
taxadjunct 
Dannevirke 
hill soils 
SOIL SOIL PRESENCE OF AMORPHOUS UNIDENTIFIED PHYLLO-
SYMBOL HORIZON MATERIAL HALLOYSITE KAOLINITE IMOGOLITE LATHS S ILICATES 
PRESENT ABUNDANT 
Tp Ha l + + + + + 
Ha2 + + + + +-
TpH Ah + + + + 
Bg + + 
Cw + + + + 
RuMS Ah + + 
Bw + + 
RuS Ah +- -+ + + + 
Bw + + + + 
RuVS Ah + + + + + 
Bw + + + 
Dtax Ah + + + + 
Bw + + + 
C + + + 
DH Ah + + 
Bw + + + + 
TABLE 14 : RESULTS OF TRANSMISSION ELECTRON MICROSCOPY : VISUAL IDENTIFICATION OF MINERAL AND 
AMORPHOUS MATERIALS . 
OTHER MINOR 
CONSTITUENTS 
volcanic glass  
diatom� 
volcanic glass 
diatoms 
volcanic 
glass 
volcanic 
glass 
..... 
VI 
N 
I I 
O·5�m 
Fig . 46 : Kaolinite and tube-like 
halloysite (Tp , Ha l ) . 
1 5 3  
.. , 
.. 
Fig . 4 7 : Chunky cylindrical 
(c) and tube-like ( t )  
halloysite  forms , (Tp , Ha l ) .  
Fig . 48 : Weathering Volcanic 
Glass (Tp , Ha l ) . 
Fig . 49 : Amorphous 
gel (Tp , Ha2 ) .  
, 
• 
" . .. . , 
i . 
. . , . 
Fig . 51 : Representative 
micrograph of a RuS soil 
(Ah) showing crystalline 
f lakes , with minor amounts 
of amorphous material . 
1 54 
Fig . 50 : Amorphous 
(arrowed)  and crystalline 
material , and unidentified 
laths , (RuVS , Ah) . 
Fig . 52 : Imogolite , 
and unidentified laths 
(RuVS , Ah) . 
1 55 
Fig .  53 : Representative micrograph 
of Dtax (Ah)  showing finely 
comminuted amorphous-dominated clay 
mat erials . 
Fig . 55 : Aokautere Ash 
showing amorphous materials  
(arrowed) in  a fibrous gel 
ground mass (G) . 
Fig . 54 : Representative 
micrograph of Dtax (C)  
showing coarser amorphous­
dominated clay material s . 
Fig . 56 : Greywacke sample 
showing moire pattern (M) . 
1 56 . 
Volcanic glass ,  halloysite , kaolinite , imogolite  and d iatoms were recognised 
in several of  the grids as l isted in Table 14 . 
are presented , to illustrate  selec ted examples . 
S everal electron micrographs 
Inorganic amorphous materials are abundant in all  horizons of the 
D tax , DH , Tp and TpH soils , commonly occurring as  a filmy gel . This  
f ilmy gel form was particularly abundant in Tp (Fig .  4 9 ) . Amorphous material 
was also observed in RuVS (arrowed , Fig . 50) together with the o ther Ruahine 
steepland soil s .  Crystal l ine minerals  predominate  in all  the Ruahine 
steepland soil s ,  as might be expected ( see e . g .  Fig . 5 1 ) . 
Kaolinite (Fig . 4 6 ) , halloysite (Figs 4 6 ,  4 7 )  and imogolite  (Fig . 52) , 
which were not revealed by the other instrumental techniques ,  were noted in 
several soil samples , part icularly in Tp indicating that they are present 
in minor amounts . Halloysite occurred in the well-documented tube-like 
form (Bates et al . 1 950) , and also in the chunky cylindrical form (Kirkman , 
1 97 7b ) , as  shown in Fig . 4 7 .  Kaol inite was recognised a s  c learly defined 
hexagonal crystals (Bates , 1 9 59 ) , and imogolite  as smooth,  interweaving threads ,  
(Gard , 1 97 1 ;  Parfitt  and McHardy, 1 9 74 ) . Weathering volcanic glass was 
noted in a number of clay samples , an example being shown in Fig . 4 8 .  
The presence of  platy micaceous and feldspathic mat erial in the Ruahine 
steepland soils , which contained large amounts of crystalline flakes , seems 
probable ,  al though they could not be positively identified .  
pa tterns indicate that these minerals  are present . 
However , X . R . D .  
Long lath-like crystals ,  up to 5-6�m i n  length,  occurred in the majority 
of  soil clay samples , generally in small  amounts . In two samples  o f  the Ah 
horizons of  Ruahine steepland soils they were observed in appreciable amounts , 
(Fig . 50 , 5 1 ,  52 ) . These lath-like crystals are a t  present  unidentified 
but exhibit  a similar appearance to the zeolite  mordenite ,  which occurs 
sporadically in rhyolitic materials (Gard , 1 9 7 1 ) . 
they are a type o f  zeolite . 
I t  seems probabl� that 
Electron micrographs , shown in Figs . 53 and 54 indicate the relat ive 
1 57 .  
abundance  o f  c rystalline and amorphous materials ,  within the Ah and C 
horizons of  the D tax soil . The Ah horizon is shown to contain more� 
f inely comminuted material , presumably due to greater physical , chemical 
and biological  breakdown of soil particles in the upper horizons . 
These  micrographs may be compared with those shown in Figs . 55 and 5 6 ,  which 
illustrate the form of the clay frac t ion of  a sample of  Aokautere Ash and 
�reywacke , respectively . The c lay frac t ion of  the Aokautere Ash, i s  
seen to  be largely amorphous with a grain-like appearance (arrowed in Fig . 55) , 
together with a grot�nd mass  which is largely a f ibrous gel ( labelled G) . 
Fig .  56  is an elec tron micrograph o f  a greywacke pebble ,  ground to clay 
size .  I t  is  seen to be largely crystalline , with some o f  the flakes 
* 
exhibiting a moire pa t tern ( labelled M) . 
T . E . M . studies were thus particularly useful for a visual assessment 
of the rela tive proportions of constituents of the c lay-sized frac tion . 
It  was also possible to positively identify kaol inite ,  halloysite ( tube-
l ike and cylindrical forms ) , imogolite  and wea thering volcanic glass . 
6 . 4  CONCLUSIONS 
Soil-water charac teristic s ,  and the mineralogy of  a number of  selec ted 
soil samples has been investigated . The resul ts  obtained enable a detailed 
comparison o f  the properties and genesis of  the soils mapped , together with 
an understanding of the processes occurring within these soil s ,  such as 
soil water movement ,  soil water retention and the extent  of  weathering . 
In the case of  the Ruahine steepland soils , three profi les  were 
investigated . The RuMS and RuS samples were taken f rom representative 
profiles of a moderately steep-steep and steep phase , respec t ively ; whil s t  
RuVs samples were taken from a profile  with better horizon development than 
the mod al prof ile o f  the very steep phase .  
* A moire pat tern is  tha t  produced on an elec tron micrograph due to 
overlapping of platy crystals ,  producing sets of parallel f ringes (Garct , 
1 9 7 1 )  
1 58 .  
Data obtained for the RuMS and RuS soils showed them t o  be  characteris-
tically different from the other soil mapping units .  The results  show that 
they have the largest bulk densities , smallest to tal porosities , and largest 
macroporosities ( together with the samples of the Takapari . peaty loam ,  
Tp) . They are also characterised by very small A .W � C �  ' s  and the least  
accumulat ion o f  organic matter . These soil s ,  therefore , are freely 
draining with a low water storage capacity ;  and during a heavy rainstorm ,  
water will move rapidly through the solum .  
Cohesion in soils is a bonding together o f  particles due t o  attractive 
forces ;  mainly contributed by c lays , organic matter and the surface tension 
eff ect  o f  air-water interfaces . S ince the Ruahine steepland soils have 
low clay contents (being weakly weathered) ,  low o rganic matter contents and 
weak s tructures , they will tend to become cohesionless on drying , which in 
turn will decrease their shear strength . Due to their low A .W . C .  ' s  and 
rapid drainage , the soil water status of  these soils will change relatively 
rapidly compared with the other soils s tudied in the survey . Thus , 
a t  the beginning of  a wet period , for example  during the autumn months , these 
so ils with low cohesion wil l  become wet rapidly ( i . e .  a t ta in a low soil 
water matric po tential ) ,  and at  this stage may be  particularly susceptible 
to erosion . At the end of  a wet period ,  they will drain rapidly to attain 
8. relatively high soi l water matric potential . This effect will be  enhanced 
by a l ack of organic matter to retain moisture at  the surface and in the 
solum so that these soils , in particular , are suscept ible to droughtiness 
which in turn l imits plant growth.  
Mineralogical studies of  RuMS and RuS , and the "drying effect" indicate  
that they contain the smallest  amount of amorphous clays with respect to the 
other soils in the subcatchment . Sand mineralogy studies indicate  1-3% 
mafics (hornblende ,  hypersthene and augite)  and 3-13%  rhyolit ic glass , a 
smaller percent than in the o ther soils . The presence of  mica , feldspar and 
quartz  shown by X-ray diffraction s tudies , together with aluminous vermiculites  
with only a moderate amount of interstratification , indicate that  the 
soils are weakly weathered . 
The properties of the RuVS soil differ markedly from those of the 
RuMS and RuS soils , in ' a  consistent fashion . In this soil , a marked 
1 5 9 .  
decrease in bulk density is  no ted together with an increase  in the following : 
total porosity,  A .W . C . , the "drying effect" , pH ( in NaF) of  Bw horizon , organic 
matter levels and proportions of  mafics  ( 3-4%)  and rhyolitic glass ( 1 8-24%) . 
Transmission electron microscopy studies of  the clay f raction showed the 
presence of  some imogolite , halloys ite and amorphous material , as  well as 
aluminous vermiculites and mica .  This data suggests  that this soil has 
developed on a more stable site than the RuMS and RuS soils ; so that there has 
been suf f icient t ime for greater amounts  of tephric loess and organic matter 
to accumulate and for a Bw horizon to develop . Thus on the steep to very 
steep slopes of  the subcatchment where many erosion scars occ�r , there are 
also sites which have remained stable for some considerable  period of  t ime . 
Because the soil-water characteristics and mineralogy o f  the soil s in the 
Ruahine s teepland soil mapping unit  are rather varied , it  seems probable 
that their susceptibility to erosion also varies considerab ly between sites . 
In comparison with the Ruahine s teepland soils , the Dannevirke taxadjunct 
(D tax) and related Dannevirke hill soils (DH )  display features indicative 
of a large amorphous clay component . The D tax soils have ( 1 )  pH ( in NaF) 
values > 9 . 4  in all subsoils analysed , and ( 2 )  a large "drying effec t" . 
The DH soils show these ef fects  to a l esser extent . Both soils have large 
A. W . C .  ' s o  The saturated hydraulic conductivity values for the D tax soil 
indicates that infil trat ion rates are seldom , if ever , l imiting so that  
the soil is  freely draining wi th little or no surface runoff . 
The sand mineralogy of  the Dannevirke soils indicate 4 - 1 1 %  mafics  and 5-36% 
rhyolitic glass , together with �uart z ,  feldspar , greywacke f ragments  and 
other minor constituents . Thus , the parent material may be assumed to be  
derived from a mixed source provenance of  greywacke material and aerial  
v olcaniclastic ej ecta from the rhyolitic and andesitic vol canoes of  Central 
1 60 .  
North Island . The sand mineralogy , the "drying effec t"  values , bulk 
densities and pH ( in NaF) values for respec tive horizons all sugges t  that 
the proportion of  tephra components  and thus amorphous c lays in the Bw 
horizon is greater than in the C horizon of  the D tax soil . 
The Dannevirke soils are more strongly weathered than the Ruahine 
steepland soil s .  X-ray diffraction pa t terns indicate  that the crystalline 
clay frac t ion of both soil s are dominated by interstratified vermiculite ,  
however , those o f  the Dannevirke soils are  more  highly aluminous . D tax 
soils contain a small amount of pedogenic chlorite .  Visual assessment o f  
the elec tron micrographs indicates that  amorphous material contributes to  
at l east 50%  of  the c lay frac t ions in  the Dannevirke soils .  Mica and 
feldspar have been largely destroyed by weathering in these soil s .  Thus , 
due to large water s torage capacities , considerable amounts of amorphous 
clays and organic matter ,  the Dannevirke soils  show properties tha t  render 
them less susceptibl e to erosion than the Ruahine steepland soils .  This 
is expressed in their prof ile  development where the freely draining , friable ,  
wel l  developed profiles have a low bulk density and a high porosity.  
The Takapari peaty loam (Tp) and Takapari hill soils (TpH) are 
distinguished by their pea ty nature ,  t"ogether with an amorphous clay content  
largely in the form of  highly s il iceous gel (as  shown by infra-red spectro-
scopy) . The peaty topsoil of  TpH , and the Ha horizons of  Tp have very low 
bulk densi ties , and large A . W . C .' s meaning that the soil  is  able to store 
large amounts of  water . The porosity and macroporosity values of  Tp are 
large , and the "drying effect"  on i ts water retention properties is also 
large (probably a combined effect of  amorphous clays and organic mat ter) . 
Tp has the greatest  proportion of  mafic and rhyolitic  sand grains o f  all  the 
soils examined in the subcatchrnent , const ituting up to 60% of the total sand 
frac t ion . Quartz  and feldspar are also common . Thus the parent  materials  
for  this so il include , in order of  abundance :  peat , t ephric loess , rhyoli tic 
1 6 1 .  
tephras  and weathering greywacke . 
The position of  Tp on the summit plateau o f  the Ruahine Range abrogates 
the effects  of erosion processes , such as  so il creep and mass movement ,  which 
dominate soil formation on the steeper slopes to the east . 
The TpH soil occurs on slopes , just  below the s·urnrnit plateau . The 
topsoil (Ah , Ahg or Ha horizon) has low bulk densities and relatively large 
porosity and A .W . C .  values . In the Bg  horizon a marked increase in bulk 
density and decrease in porosity and A .W . C .  occurs . This  horizon has the 
smallest macroporosity values of all  samples studied . Saturated hydraulic  
conduc tivity stud ies show that infiltration into the surface horizon is 
-1 typically rapid , and of the order of several thousand mm. day • However , 
a very marked decrease in rate of  vertical water movement is noted in the 
underlying Bg horizon . I t  seems that  interflow is likely to occur in this 
s ituation . The contrast ing properties of  the Bg  horizon compared with the 
Ah horizon also suggest that as 'water movement through the Bg horizon is 
impeded it will accumulate at the j unc tion of these horizons , so that with 
time in  a suffic iently wet environment , a perched water table  will  develop . 
This  saturated zone ,  with its  subsequent decreased shear s trength,  will  
act as  a plane of  weakness in  the  soil  and i s  l ikely to become the shear plane 
for mass  movement to occur . 
Below the Bg (or  Br) horizon is a thin placic horizon , overlying a Cw 
horizon. The Cw horizon shows a marked increase in ( i) rnacroporosity , 
( ii)  the "drying effect"  and ( iii) pH ( in NaF) values , compared with the Bg 
horizon . This  information together with c lay mineralogy studies ,  indicates 
that the Cw horizon is  more  freely draining and contains a larger 
proportion of amorphous materials .  Thus , the investiga t ions show that the 
gleyed or completely reduced B horizon of the TpH soils has different soil-
water characteristics  to ( 1 )  the overlying Ah or Ha horizons and to ( 2 )  
the underlying Cw horizon . Thus , each horizon has d iagnostic soil-water 
1 62 .  
movement and soil-water retention properties , producing a soil with a 
high suscept ibility to eros ion processes . 
A secondary observation in this study was the large nat ive ear thworm 
ac t ivity in most soil profiles (with the excep t ion of  Tp) indicated by the 
presence of large burrows , infilled with material often from another 
horizon . The investigations indicate that these burrows aid good drainage 
and aeration of  the soil profiles , as  well as assisting the breakdown and 
incorporation of organic matter within these soils , improving s truc tual 
stability.  
In  conclusion , the extent of weathering and amount s  of  amorphous 
constituents  within the clay fractions of the so il s ,  t ends to increase  in 
the order : Ruahine steepland soil s ;  Dannevirke hill soil s ;  Dannevirke 
taxadjunc t soils and Takapari hill soils ; Takapari  pea ty loam . The 
Ruahine steepland soils are characteristically rapidly draining soils ,  with 
low water storage capaci t ies . The Dannevirke soils ,  in contrast have much  
greater water s torage capacities , but are  also free-draining , and have a 
large amorphous clay component . The Takapari hill soils are characterised 
by drainage impedance in the B horizon with soil-wa ter propert ies markedly 
different to those of  the horizons above and below it . The Takapari peaty 
loam exists  on s table  sites of  the summit plateau , where interflow does 
no t occur . Instead , in this area of  highest  amounts  o f  rainfall , waterlogging 
of the so il occurs as it  becomes saturated upwards f rom i t s  contact with 
the underlying greywacke bedrock.  This resul ts  in ( i )  low decomposition 
rates of the organic mat ter , producing a peat , and ( ii )  a moderately acid , 
wet environment for mineral weathering to occur . 
CHAPTER SEVEN 
FINAL DISCUSSION OF RESULTS AND EROSION 
PROCESSES , WITH CONCLUSIONS 
163 . 
CHAPTER SEVEN 
FINAL DISCUSSION OF RESULTS AND EROSION 
PROCESSES , WITH CONCLUSIONS 
The fluc tuation and magnitude of  eros ion rates in the Southern Ruahine 
Range through geological time , and in recent decades,  has been discussed 
in the Literature Review .  This indicates tha t :  
( 1 )  erosion rates have been highly variable since the Range began to be 
uplifted ,  and ; 
(2 ) in recent  decades , the erosion rates are increasing . 
Mosley ( 1 97 7 )  provides the following estimates of erosion rates (which he 
states are based on some very restric tive assumptions ) for the southeastern 
Ruahine Range , thus substantiating ( 2 ) above : 
(a ) 3 -1  - 1  a minimum erosion rate  o f  10m  ha yr  on  a long-term basis , 
(estimated from the volume of  rock removed during valley incision , assuming 
the Range to be 1 . 5  million years old ) . 
(b ) 3 -1  - 1  an eros ion rate of  28m ha yr between 1 94 6  and 1 974 (estimated 
by the change in area of  erosion scars during this period) . 
Introduced wildlife , a cl imatic change , earthquakes , the clearing o f  
natural deposition areas , s teepness o f  slope and the nature of  the bedrock 
have been suggested as fac tors involved in the recent increa ses in erosion .  
However , the argument a s  to whether increased erosion rates observed today 
are part  of a natural cycle ,  or whether they are accelerated by one or more  
of these factors ,  mentioned above,  is  disputed . Hence ,  the aim o f  this 
study has been to invest igate the extent and types of erosion and the related 
soil resources with a view to establishing l ikely mechanisms and reasons 
for the erosion . 
Erosion types which are related to the soil resources are considered to 
be (a ) mass movements ,  which involve soil materials together with rock 
fragments  and organic matter , and (b ) soil creep , which is generally a slower 
1 64 . 
form of  erosion . 
A detailed soil survey of Car Park Creek showed that  the distribution 
of the so ils is associated with different geomorphic surfaces,  which have 
been classified according to the nine-unit landscape model (NULM) of  
Conacher and Dalrymple ( 1 97 7 ) . I t  is  likely that  the soils of  Car Park 
Creek are representative of  soils along the eastern side o f  the Range ,  and 
that a similar pat tern of soils will exist  in other catchments ,  their 
distribution being related to the landsurface  unit  and alt itude .  
This s tudy has shown that  the mineral soil parent material s  are : 
in s itu greywacke-derived materials , with additions of  greywacke loess and 
volcanic ash . The Dannevirke and Takapari series , on more s table s ites 
than the Ruahine soi l s ,  contain the greatest amounts  of loess together with 
a large tephra component , producing a deep so il . Field observat ions and 
sand mineralogy studies of the Takapari peaty loam indicate tha t  tephra 
additions include lapilli from the Waimihia and Taupo Pumice erupt ions .  
These tephras indicate a minimum age of  soil development for the Takapari 
peaty loam of ca. 3440 years ( extrapolated to ca . 4600 years assuming a 
constant rate of  peat  accumulation in the lower part o f  the profile) . This  
suggests that  the summit plateau of  the Southern Ruahine Range was s tripped 
bare prior to this date , during an erosive period . The presence o f  a third 
tephra ,  the Aokautere Ash ( erupted ca . 20 , 000 yrs .  B . P . ) in a gravel depo sit , 
underlying the Dannevirke taxadjunc t soil , indicates that  ac tive erosion 
and deposition was occurring alongside the main West Tamaki River channel 
a t  this t ime . 
The primary mineral assemblage and extent of  weathering indicated by 
the sand and clay mineralogy studies suggest tha t  the Takapari hill soils 
are of  a comparable  age to the Dannevirke soils ,  al though the former are 
subj ect to more int ense chemical weathering . The Takapari peaty loam contains 
highly aluminous vermiculites and minor amounts  of Kaol inite and 
hal loysite ,  which have formed in the profile under very wet ,  acidic condi t �ons . 
1 6 5 .  
This indica tes tha t  this soil i s  al so subj ect  t o  relatively intense 
chemical weathering . The primary mineral assemblage o f  the Ruahine 
steepland soils indica tes tha t  they contain the least  amount s  of tephra 
components  o f  the soils studied , whilst their clay mineralogy shows tha t  
there has been insufficient t ime for all  o f  the mica and feldspar in these 
soils to have been removed or altered . This is to  be expected as  the 
Ruahine steepland soils occur on the most  uns table slopes in the subcatchment , 
and their profile  development is modified by rejuvenation processes . 
However , the occurrence of  s table pockets within this steepland soil mapping 
unit  has been noted , indicating that the erosion susceptibilities of sites 
within the mapping unit  are varied . 
Virtually all  o f  the erosion within Car Park Creek occurs in the areas 
mapped as  Takapari hill soils (TpH) and Ruahine steepland soils (RuMS , RuS ,  
RuVS ) . A number o f  soil parameters  were measured in the subcatchment to 
investigat e :  
( a )  why these soil s ,  in particular , are susceptible  t o  erosion , and 
(b)  which soil parameters  are most closely related to erosion processes . 
The Takapari  hill  soils occur in  an area in  which most  of  the deep­
seated erosion occur s .  Their soil morphology i s  a peaty top , underlain by 
a grey , gleyed Bg (or in local depressions a completely reduced Br)  horizon, 
which can be  of  thicknesses up to 0 . 5  metre . Below this a discontinuous ,  
thin iron pan occurs , underlain by a more  freely-draining horizon . This 
soil has been described in Chapters  5 and 6 ,  and in the latter the l ikel ihood 
of interflow through the permeable surface horizo� is proposed . I t  is  
also  suggested that  a perched water table may form at  the upper surface of  
the  Bg (or Br)  horizon in wet periods , thus creating a saturated zone . A 
decrease in shear strength of  this saturated zone occurs on wet t ing , due to  
loss  of many cohesive bonds ( contributed by air-water menisci)  and a 
decrease in fric tional forces between particles . Thu s ,  a zone of  weakness  
1 6 6 .  
will develop forming a probable shear plane for mass  movement result ing 
in , for example ,  debris avalanches and slides . Also , a slower , possibly 
imperceptible ,  soil creep may be  occurring not only at  this plane but also 
in the underlying struc turally weak, gleyed B horizon . Other factors which 
favour soil creep in this zone include :  (a) the slope gradient , of between 
1 20 and 30° , (b )  a high mean annual precipita tion of approxima tely 240Omm , ( c )  
a vegetational stand , unheal thy in places , and with a l imited root 
distribution , and (d )  any factor causing a disturbance of  the soil particles , 
such as  earthquakes . 
Creep , not only of  surficial  soil layers , but also o f  underlying 
colluvial and shattered bedrock materials may be also occurring . This i s  
favoured by the fac tors (a ) ( b )  and ( c )  lis ted above together with : ( e )  
the struc ture and lithology of  the greywacke bedrock , which i s  j o inted , 
faul ted and shattered , and consists of  alternating beds of  sandstone and 
argillite  which have differing resis tances to wea thering . Evidence for 
a more deep-sea ted creep movement is the presence of  large scale (approximat ely 
100 metres in length) , deep terracettes on the convex creep slope,  within 
Car Park Creek, orientated parallel to the r idge-top ( see Fig . 7) . These 
appear to be similar in nature ,  although on a smaller scale ,  to the small 
discontinuous scarps occurring in indura ted greywacke and argillite ,  in the 
Southern Alps  of  the South I sland , New Zealand , (Beck , 1 968) . The latter 
are formed by gravity adj ustment , often triggered by earthquakes , in a t errain 
oversteepened by glaciat ion . In the Southern Ruahine Range , where slopes 
are oversteepened by fault movement ,  and earthquakes are common , i t  is 
l ikely tha t  a s imilar gravity adjustment occurs ,  which would thus explain 
these deep terracette features .  
Indeed , the presence o f  terracettes within the zone in which Takapari  
hill  soils occur is an  indication of  slow valleyward movement o f  the  ent ire  
area , which is enhanced by  undercut tinga t  the  stream bed . Thu s ,  terracettes 
may form either as in (a )  or (b )  (Fig .  57) . However , i t  i s  possible tha t  
Fig . 5 7 : An Illustration of the Possible Origin of Terracet tes ,  
Observed at  the Head of  Car Park Creek . 
- - �'- ......­
ac tive 
undercutting 
(a )  terracette  formation due  to  creep of surficial  soil  materials , 
over the bedrock . 
active 
undercutting 
(b)  terracette  formation due to  deep-seated slumping within the 
bedrock . 
1 6 7 .  
1 68 .  
the terracettes o f  (b) form simul taneously with a more superficial  soil 
creep ( forming smaller terracet tes , as in (a) ) superimposed on its surfac e .  
Similar t erracette  features are al so found at  the head of  the Rokaiwhana 
S tream ,  to the south of Car Park Creek . 
The Ruahine steepland soil s  also have a high erosion susceptibil ity.  
Soil fac tors which enhance this susceptibility include weak s tructural 
development and low organic matter levels .  The RuS and RuVS phases exist  
on  transportational midslopes , with angles of  slope characteristically 
o greater than 30  . This steepness of  slope is a major  factor contributing 
to their erosion susceptibility . The RuMS phase exists  on a more  stable 
slope uni t ,  the colluvial footslope .  However , some mass  movement undoubt-
edly occurs in  this  uni t ,  which is  also affected by  f luvial erosion processes . 
I 
The alluvial toeslope , where Recent soils occur,  has similar susceptibilities 
to erosion as  the colluvial foo tslope . The soils on this surface are also 
commonly weakly structured with low organic mat ter levels ,  and occur in a 
zone susceptible to fluvial erosion processes . 
The Takapari peaty loam and Dannevirke taxadjunct soils occur on level 
to gently sloping topography where mass msvement seldom occurs . The 
Dannevirke hill soils occur on the f lanks of the interfluves , below the 
7 50 metre contour , in an area where slight erosion occurs . These latter 
soils are  l ess  well developed than their more s table counterpart s ,  the 
Dannevirke taxadjunc t soils . 
The present  study suggests that  a number o f  soil parameter s  are related 
to the erodibility o f  a soil which,  together with o ther fac tors  such as  
slope and aspec t , determine its  erosion susceptib ility . These soil 
parameters are : 
(a)  Struc tural Development - During pedogenesis , the individual soil 
particles become clus tered into aggregates by bonding together by c lay 
particles and humus , with cracks and pores developing as roots  and soil 
1 69 .  
fauna penetrate the soil . Thus, a well  structured . soi1 contains stable 
soil  aggregates and sufficient cracks and channels to provide good aeration 
and drainage.  Soil structure can be des troyed or seriously reduced by : 
( 1 )  wetting a soil when it is  dry , or 
( 2 )  compression and shearing when it  is wet .  
The stability o f  the structure depends on the amount o f  structural bonds 
within the soil , provided by the clay fraction , sesquioxide cements ,  ·.humus 
and microbial gums . 
Thus , in the case of the Ruahine steepland soils and Recent soils which 
are relatively young and weakly weathered ( in the majority of cases) , clay 
and organic matter levels are low with resulting weak s tructures . The 
Dannevirke and Takapari  soils have better developed s tructures , al though 
the hill soils associated with each of these series have s truc tures that  
are  less strongly developed . 
Destruction or reduction of  soil structure will increase  soil sus-
ceptibility to erosion on a slope .  The Ruahine steepland soil s ,  which 
have weakly developed structures , also dry out considerably during the 
summer months (having a rapid drainage rate , inferred from a large percent 
of macropores , and a small A . W . e . ) .  Thus ,  when the dry soil becomes wetted 
the soil i s  l iable to lose much of  its  structural stability . This is  
enhanced by the position of  these soils  on s teep slopes , where they are  
subj ected to large shearing forces . 
In the Dannevirke hill soils , the soil-water characteristics do not favour 
such rapid drainage of the soil , and it is l ikely that  these soils do not 
dry out to such an extent as the Ruahine steepland soils . Thu s ,  these 
weakly to moderately structured soils are less susceptible to loss of  struc­
tural stability.  The Takapari hill soil s ,  having poorly to very poorly 
drained profiles , seldom dry out , even during a long dry summer . The 
1 7 0 .  
weakly structured B horizon , which in places is completely reduced , i s  
l iable to lose i t s  struc tural stabil ity when subj ec ted t o  shearing forces . 
In contrast ,  the Takapari peaty loam, which occurs  on level to gently 
sloping surfaces , is not subj ected to these shearing forces . Likewise ,  it  
seldom dries out  to any great extent , so  tha t  fac tors within this soil 
do not favour the loss of  struc tural stability.  
(b)  Organic Mat ter - The amount of  organic matter in soil is intimately 
associated with a number o f  soil physical properties . Its  effec t on 
structural development has been mentioned in the preceding pages . I t  is 
also related to bulk density and available water-holding capacity , as 
discussed in Chapter 6 . 2 . 2 .  The presence of  an organic l i tter on the soil 
surface aids rainfall  acceptance and decreases the l ikel ihood o f  soil splash 
and surface  compac tion - fac tors of  maj or importance to the l ikelihood o f  
erosion of  the surface soil layers . In this s tudy, horizons with high 
organic mat ter content  are found to have large macroporosities . 
The Takapari hill soils , which have peaty surface horizons with large 
macroporosities , are a case in point . Satura ted hydraulic conductivity 
measurements  on the surface horizon of this soil together with macroporosity 
data suggest that infiltration and drainage are not l imited by the drainage 
characteristics  of the surface horizons . Thu s ,  the c hange of  surface 
soil losses due to  runoff ,  or the ponding of water on the surface where 
infiltration is l imiting , is small . 
The Takapari peaty loam soils also have a high organic mat ter content , 
but in t hese so ils where vertical water movement occurs and soil depths are 
generally about O . Sm ,  the soil becomes saturated upwards from its contact 
with the basement  gre�wacke , which enhances its  waterlogged nature . 
The Ruahine steepland soils , in contrast , have relatively low organic 
mat ter l evels ,  with little  or no accumulation on the surface . Macroporosity 
data indicate tha t  they are rapidly draining soil s ,  which is due to their 
1 7 1 .  
textural coarseness  and stoniness . However their relatively cohesionless 
surface horizons are l iable  to soil splash and dispersion during intense 
rainstorms , in an area where there is little or  no organic l it ter protec t ing 
the so il surface . 
The Dannevirke taxadjunc t ,  and Dannevirke hill soils  to a l esser ext ent , 
have high organic matter l evel s and an apprec iable  accumulation of  organic 
j 
litter on the surface . The large amorphous c lay component in these soils  
contributes to  their f riable consistence and low bulk density , while at  the 
same t ime facilitating the formation of amorphous clay-organic complexes . 
Thus , these soils are freely-draining ,  and their surfaces  are protected by 
organic matter against soil spla.sh and dispersion . 
( c )  Rooting  Depth - A soil  on a slope is  strengthened by  the presence of  
tree roots  and maximum strength is gained when these roots extend throughout  
the  solum, . This is  the case in all  of  the soil mapping units  which occur 
on slopes within Car Park Creek , with the exception of TpH , as  shown in 
the table below :  
TABLE 1 5  AVERAGE SOIL AND TREE ROOTING DEPTHS OF EACH SOIL 
SOIL 
Takapari hill  soil 
Dannevirke hill soil 
Ruahine 
steepland : 
soils  
MAPPING UNIT , WHICH OCCURS ON A SLOPE 
SOIL AVERAGE SOIL* AVERAGE TREE* 
SYMBOL DEPTH (cm) ROOTING DEPTH 
(cm) 
TpH 70 50 
DH 70 70 
RuMS 60  60  
RuS 45  4 5  
RuVS 35  3 5  
TREE ROOTING DEPTH 
AS PERCENT OF SOIL 
DEPTH (% )  
approx . 70 
1 00 
1 00 
1 00 
1 00 
* average soil and tree rooting depths are estimated as  an average depth 
of  the to tal numberof soil profiles described in the s tudy .  
1 72 .  
The Bg ( or Br)  horizon of TpH being waterlogged provides adverse 
conditions for roo t growth , with a marked decrease in root abundance and 
therefore a marked decrease in shear s trength in this horizon, compared with 
the overlying hor izon . 
creep within this soil . 
This may subsequently increase the likelihood of soil 
(d) Presence of  an Impermeable  Horizon - In  the Takapari hill soil s ,  a t  the 
base of the Bg (or Br) horizon , a thin iron pan has developed , below which 
no roots  penetrate . This pan increases in thickness and cementation with 
an increase  in al titude and a resul tant increase  in mean annual rainfall .  
Thus ,  iron pan development is more pronounced on the convex creep s lope at  
the top of the catchment than on the s loping interfluves which extend to 
lower elevations . As well as impeding root penetration , the iron pan will  
also impede vertical water movement ,  thus enhancing the  waterlogged nature 
of  the slowly permeable Bg (or Br)  horizon . 
Where the bedrock occurs close to  the so il surface ,  which i s  the case 
in a number of  Ruahine steepland soils and Takapari peaty loam so il s ,  it 
will offer resistance to roo t penetration and water movement .  Thus ,  on 
a level surface ,  for example in the case of the Takapari  peaty loam soils , 
a perched water-table may form above the bedrock and develop upwards . Where 
the surface of  the bedrock is  sloping and c lose to the surface ,  for example  
in certain soils  of  the RuS and RuVS soil phases of the Ruahine steepland 
soil s ,  water may move along this plane and have a lubricating eff ec t  at  
this poin t ,  i . e .  where the base of the soil i s  in contact with the underlying 
bedrock . 
(e )  Soil  Water Permeability - The ease with which water pas ses through the 
soil has been measured direc tly as  saturated hydraulic conductivity (ksat )  
and indirec tly as  macropososity .  A soil becomes saturated and loses shear 
s tr ength ( increasing its  l iability to movement downslope) 
1 7 3 .  
if suff ic ient water drains through i t  a t  a slower ra te than i t  i s  being 
received . I t  is observed that Ksat will  decrease with t ime during a 
rainstorm , due to fac tors such as clogging o f  pores by the inwashing of  
finer particles . Also , the ra te of  movement of  water through the soil is 
l imited by the horizon of  slowest Ksat values , so that even if the surface 
horizon is freely draining the presence of  a slowly draining subsoil may 
result  in waterlogging , and possible saturation . 
Ksat and macroporosity measurements indicate that the Ruahine steepland 
and Dannevirke soils are freely draining , so that saturation of the soil 
profile is unlikely . The extremely stony Re cent soil s ,  which frequently 
occur on several metres of  gravels will  thus also be freely draining . 
However ,  in the case of  the TpH soils , a rapidly draining topsoil is  
underl ain by a B horizon having a marked decrease in  permeability.  This 
is indicated by Ksat measurements and substantiated by macroporosity data . 
Thus , as  a considerable amount of prec ipitation falls  at  this al titude 
(approximately 2800mm annually) , the Bg (or Br)  horizon , which has vertical 
Ksat values ranging between 0 . 1 and 400mm per day, will l imit soil drainage . 
This ,  together with the presence of  a thin iron pan makes the B horizon o f  
TpH particularly susceptible t o  saturation , and may lead t o  the formation 
of a perched water table at its  upper boundary , thus increasing the erosion 
susceptib ility of this soil . 
In the present study ,  these 5 soil parameters are o f  particular 
relevance in the assessment of  soil erodibility. The erosion susceptibility 
of a soil on a slope depends not only on its  inherent erodibility ( governed 
by its internal properties ) but also on external fac tors such as  slope and 
aspect .  The likelihood o f  the shear stress o f  a soil overcoming its  shear 
s trength increases as the angle of slope increases . 
by the equation : 
This is  illustrated 
T sin 0;; cos 0;; 
where T tangential stress ( in Pa) 
a: slope angle 
-3 Yd = dry unit weight of  soil (kN . m  ) 
z = soil depth . 
1 74 .  
Thus , a comparative assessment of  eros ion susceptibilities may be 
made by comparing the five soil parameters and angle of  slope for each soil 
class . These are summarised in Table 1 6 .  The notation "E" in the bot tom 
right-hand corner of a box indicates that this factor is  l ikely to be  a 
major cause of  decreasing a soil ' s  stability on a slope , due to the reasons 
previously discussed . The summation of  "E" fac tors is used as an index for 
comparison of soil erosion susceptibilities , ( see Table 16 ) . The data 
suggests that the erosion susceptibilities of  the soils in the s tudy area , 
decreases in the order : 
TpH > RuS and RuVS > RuMS , DR and R > Dtax and Tp . 
Indirect factors , which may modify this order ,  based on a comparison o f  
soil and s lope parameters include : 
(a)  weaknesses in the bedrock, which may result in deep-seated mass  
movements , 
(b)  fluvial erosion processes , affecting in particular RuMS and R ,  
(c )  c limatic fac tors - Erosion likel ihood is increased in the upper 
reaches of  the catchments where the mean annual precipitation is greater than 
at lower altitudes , (mean annual rainfall /altitude gradient is approximately 
1 5 1mm/ 10Om , Martin , 1 978 ) , and where increased rainfall intensities are 
expected , although little reliable data is available to substantiate this . 
Thus , the Southern Ruahine Range is an area inherently susceptible to 
very high natural erosion rates ,  due to : 
TABLE 1 6  FACTORS AFFECTING THE EROSION SUSCEPTIBILITY OF THE SOILS IN CAR PARK CREEK SUBCATCHMENT 
Takapari  Takapari  Dannevirke Dannevirke 
PARAMETER peaty loam hill soils taxadjunct hill soils  
(Tp) (TpH) (D tax) (DH) 
STRUCTURAL DEVELOPMENT weak - weak -
(of  total soil)  moderate  moderate  moderate moderate 
E 
ORGANIC MATTER LEVEL * 
(of  surface horizon) v .  high v .  high high medium 
ROOTING DEPTH 
( %  o f  soil depth) - appro x 70  - 100  
E 
PRESENCE OF 
IMPERMEABLE HORIZON - + - -
(+/ ) -
E 
SLOPE < mod mod steep - <mod mod steep -
steep steep steep steep 
E 
RATINGS FOR MACRO- medium -
PERMEABILITY POROSITY" high low medium medium 
OF LIMITING **� 
HORIZON K sat - slow rapid- -
VALUES E v rapid 
SOIL EROSION 
SUSCEPTIBILITY INDEX 
( summation o f  E ' s ) 0 5 0 2 
* using t he rating system of  Metson ( 1 956) 
** " " "  " " New Zealand Soil Bureau ( 1 968)  
*** " " "  " "  Smith & Browning ( 1 946 )  
Ruahine s teep1and soils 
RuMS RuS RuVS 
weak -
moderate  weak weak 
E E E E 
low - low -
medium medium medium 
E E 
100 100 1 00 
- + + 
E E 
mod steep steep v . s teep 
steep 
E E E E 
high high medium 
- - -
2 4 4 
Recent 
soils 
(R) 
weak 
E 
low -
medium 
E 
-
-
<mod 
steep 
high 
-
2 
( 1 )  geological factors ,  such as the type and shattered nature of the 
lithology, together with active faulting and uplifting of  the Range , 
( 2 )  steepness o f  topography, largely controlled by ( 1 ) , and enhancing 
mass movement due to gravity adjustment . 
1 7 6 .  
( 3 )  climat ic stresses , such a s  intense rainstorms and winds ,  which affec t 
in particular the south-eastern part o f  the Ruahine Range . This is illust­
rated by : (i) the distribution of soils . The Takapari hill soi1sextend down 
to the 750m contour on the east side of the Range , and to the 1070m contour 
on the west side , ( Rijkse , 1 9 7 7 ) . This suggests that adverse conditions 
producing a gleyed soil , together with slow organic matter decomposition 
rates are more prevalent at lower altitudes on the eas t  flank of the Southern 
Ruahine Range , compared with the west flank , (Rijkse,  1 9 7 7 ) . 
( ii )  certain vegetation zones , in the Southern Ruahine Range which 
show a definite drop in their upper limit , in comparison to vegetation further 
north in the Range ,  ( see Fig . 9) .  
( 4 )  certain soil parameters , which render the Takapari  hill soils and 
Ruahine steep1and soils particularly susceptible to erosion . 
Thus within any catchment of  the Southern Ruahine Range very high 
natural erosion rates are to be expected . However , the present  study of  
the West  Tamaki River catchment has indicated that within a single catchment 
there are areas of contrasting erosion potential . Fig . 58 il lustrates the 
extent and distribution of  erosion surfaces within this catchment , and 
delineates areas which are considered to have high , medium and low risk 
of  erosion , based on the evidence presented in this study . 
1 7 7  • 
The soil mapping units  are delineated in Fig.  58 by extrapolation o f  
information obtained from ( 1 )  the detailed soil survey o f  Car Park Creek 
and ( 2 )  observations and prof ile descriptions from the main channel of  the 
West Tamaki River . I t  is considered that soil mapping units  are associated 
with particular geomorphic surfaces and altitudinal zones within the 
Southern Ruahine Range . Thus ,  with the aid of  a 1 : 5840 scale aerial 
mosaic of the West  Tamaki River catchment area , soil mapping units have 
been extrapolated throughout the catchment .  Thus , it  may be  seen that 
areas of  equal erosion risk correspond to soil mapping units on a basis of 
their erosion susceptibility indices , wi th the following modifications :  
( 1 )  The "high risk" area includes the Takapari hill soils (erosion 
susceptibility index 5) only where they occur on the convex creep slopes just  
below the  summit plateau . The lower extent o f  these soils is excluded because :  
( a )  the peaty nature of  the A horizon , the thickness and gleyed nature 
of the B horizon ,  and the cementation of the iron pan are all poorly 
expressed , thus decreasing adverse soil-water propertes ;  (b )  the sloping 
interfluve areas , where these soil s occur at lowest elevat ions are considered 
to be relatively s table landsurface units compared with the convex creep 
slopes and s teep valley-sides . 
( 2 )  The "high risk" area includes the RuMS soil phase (erosion susceptibility 
index 2 )  o f  the Ruahine steepland soil s ,  and the Recent soils (R) ( eros ion 
susceptibility index 2) as well as the RuS and RuVS soil phases (erosion 
susceptibilities 4 )  because :  
(a )  RuMS and R are particularly liable to fluvial erosion processes , 
such as bank erosion , and (b)  RuMS and R receive erosion product s  from the two 
phases , RuS and RuVS , which occur above them ,  o ften producing buried 
horizons . 
Thus the "high risk" area , delineated in Fig . 58 contains the Ruahine 
-o 
'- .  '- ...... ...... 
' '''"--. ' - , -
' -
. 
- ' - . ........ . ...... ......... 
........ ....... 
NTI AL MAP . EROSION POTE 
HMENT. 
F IGURE 58 . 
MAKI  R IVER CATC E WEST TA OF TH 
SCALE 
"1'00 1000 
metres 
" 
" . ..... . ..... . 
........ . ........ . ' . ....... . ....... . 
• 
Gr id  North 
KEY 
Eros ion Risk 
high 
soil erosion 
medium 4 SUSC epti bility 
low index 
/ soil boundary DH  soil symbol 
� channel bed .. eroded surface 
'.-...0 catchment boundary 
'. ..... 
'. 
1 7 9 . 
steepland soils ,  Recent soils and the upper part  of  the Takapari hill soil s 
mapping unit . This area contains many of  the existing erosion surfaces . 
The Ruahine steepland soils occur on the steep valley-sides  and upper reaches 
of the subcatchments . These weakly structured . relat ively cohesionless 
soils ,  together with the Recent so ils ,  are found in a wide range o f  soil 
developmental stages with correspond ing vegetation pat terns also in various 
seral stages , both being controlled by erosion processes . 
In contrast to the Ruahine steepland soil s ,  the zone where Takapari 
hill so il s occur contains less erosion scars .  However ,  where these soils 
occur on convex creep slopes they have a high suscept ibility to erosion in 
the form o f :  (a)  mass  movement ,  initiated within the solum or underlying 
bedrock and (b)  creep , of  soil and underlying colluvial materials .  Many 
erosion scars occur at  the boundary between these two soil . mapping units,  
the Ruahine steepland soils slowly encroaching on the Takapari hill soil s 
as erosion takes plac e .  Fig . 7 shows a rockslide which occurs in Car Park 
Creek (marked D in Fig . 58) at the boundary between these two soil 
mapping units .  As well  a s  the deep terracette  features which are seen in 
this area , Fig . 7 also shows an oval-shaped area bounded by a scarp which 
is about one metre in height ,  occurring above this rockslide .  There i s  a 
high probability of  this whole area responding to gravity collapse during . 
for example , an intense rainstorm . Thus revegetation programmes which 
attempt to revegetate existing eroded areas should be concentrated at sites 
such as this one . Revegetation of  this site would subsequently increase 
the stability of  the surface of the rockslide,  decrease the shear stress o f  
the oval-shaped area above it , and thus decrease the l ikelihood of  erosion . 
I t  is important to note that  this rocksl ide may be governed by a deep­
seated slippage plane (as illustrated in Fig . 57b) which occurs .at a weak­
ness in the bedrock lithology or structure ,  and in this case any attempt 
to s tabilise the surface of  this inherently very unstable  site would need 
to reduce shear stress within areas of bedrock exposure . 
180 .  
The Takapari hill soils extend down interfluves which . bound the 
catchments of the Southern Ruahine Range ,  and in Car Park Creek extend down 
approximately to the 7 50 metre contour . Below the 750 metre contour the 
Dannevirke hill so il s ( erosion susceptibility index 2) occur with freely-
draining pro f iles .  The interfluve areas where these soils occur express 
a marked decrease in the number of existing erosion surfaces ( see Fig . 58) . 
Also , they are not susceptible to fluvial erosion processes . Thus , this 
area is cons idered to have a "medium risk" of erosion , being less susceptible 
than the convex creep slope and steep valley-sides ,  al though more sus­
ceptible than the level to gently roll ing areas of the catchments . 
The most  stable portions of the catchment are :  
( 1 )  the interfluves toward the base o f  the catchment , and their gently 
sloping toeslopes , where Dannevirke taxadjunc t soils form, and 
( 2 )  the summit plateau , of  the Southern Ruahine Range , where the Takapari 
peaty loams occur . 
Table 1 6  indicates that these soils have lowest soil erosion susc eptibility 
indices , and Fig . 58 shows that they are in an area where virtually no 
erosion surfaces occur , although the Dannevirke taxadjunct soils ,  where 
they extend to the channel wall ,  may be liable  to some f luvial erosion in 
the form of channel widening and bank undercutt ing . These areas with their 
associated soils are considered to have a "low risk" of erosion . 
In conclusion , much of the erosion which occurs in the Southern Ruahine 
Range is assoc iated with specific areas within catchments ;  the loca t ion of  
which is r elated , in part ,  to the nature of  the geomorphic surfac e (which 
is in turn related to the geology) and the characteristic s  o f  the soils on 
this surface .  The convex creep slopes of the catchments where Takapari 
hill soils occur , are inherently highly susceptible to erosion in the form 
of creep and deep-seated mass  movements . The very steep valley-sides of  
catchments  where Ruahine s teepland soils occur are  severely eroded . 
18 1 .  
Evidence suggests  that erosion occurs recurrently at  the same 
vulnerable sites ;  bo th ( 1 )  on the valley-sides and ( 2 )  at  the . heads o f  
the catchments particularly where the Ruahine steep1and soils bound the 
Takapari hill soils . Thus maximum benefi t  from future revegetation 
programmes at tempting to stabilise eroded and eroding areas will  be  gained 
to concentrating efforts specifically at these sites of maximum erosion 
susceptibility . This together with j udicious planning of  gravel s torage 
in channel beds and at valley throats appears to be the method best suited 
to protection of the productive farmland downstream . 
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182 . 
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Abbreviations used in Soil Profile ' Descriptions 
org . organic 
decomp . decomposed 
dk . dark 
I t . light 
v .  very 
wk . weak 
mod .  moderately 
dev . developed 
f .  fine 
mdm . = medium 
cse . coarse 
extmy extremely 
sl . slightly 
irreg . irregular 
gwk . greywacke 
1 9 2 .  
APPENDIX 1 :  CLASSIFICATION OF LANDSLIDES /ABBREVIATED VERSION 
TYPE OF TYPE OF MATERIAL 
�OVEMENT BEDROCK SOILS 
FALLS ROCKFALL SOILFALL 
Few Units Ro tational Planar 
Planar Rotational 
SLUMP BLOCK GLIDE BLOCK GLIDE BLOCK SLUMP 
SLIDES 
--- DEBRIS FAILURE BY 
Many Units ROCKSLIDE SLIDE LATERAL SPREADING 
ALL UNCONSOLIDATED 
ROCK 
FRAGMENTS SAND OR S ILT MIXED MOSTLY PLASTIC 
Dry Rock Fragment Sand Loess 
--
Flow Run Flow -- --
--
FLOWS 
RAPID DEBRIS SLOW 
EARTH FLOW AVALANCHE EARTH FLOW 
SAND OR SILT 
Wet FLOW DEBRIS FLOW MUDFLOW --
COMPLEX COMBINATIONS OF MATERIALS OR TYPE OF MOVEMENT 
(Varnes , 1 9 58) 
1 9 3 .  
APPENDIX II : SOIL CHRONOSEQUENCE IN THE WEST TAMAKI RIVER CATCHMENT 
( to accompany Fig . 1 6 )  
SOIL SYMBOL 
(see Fig . 1 6 )  
soil A 
( see f ig .  1 9) 
on terrace 
opposite No . 
1 Creek 
Soil B 
on terrace 
opposite 
No . 2 Creek 
SOIL DESCRIPTION LOSS ON IGNITION 
cm (depth) -% of dry so11- . 
o +2-0 organic lit ter & 
humus 
Ah 0-10 dk brown ( 1 0YR 
3 /3 ) ; v .  s tony 
sil t loam ; well 
dev . mdm nut 
struc ture ;  firm; 
many roots . 
AB I0-29 brown/dk brown 
( l  OYR 4 / 3) ; V .  
stony silt loam ; 
friable ; mod .  dev . 
mdm nut s tructurej 
common roots .  
Bw29-88 dk yellowish brown 
( l OYR4 /4 ) ; stony 
2 5  
1 8  
sil t  loam; friable 1 3  
with gritty feel ; mod .  
dev . c s e  blocky ; few 
roots .  
C greywacke gravels  
R greywacke bedrock 
o +4-0 org . litter and humus 
AhO-1 7  dk brown ( lOYR 3/3 ) ; 
s tony silt loam ; well 
dev .  mdm nut s tructure 
friable ;  many roo ts 
AB 1 7-32 brown/dk brown ( 1 0YR 
4 / 3 ) ; s tony silt l oam 
friable ;  mod dev mdm 
nut structure common 
roots 
Bw32-60 dk yellowish brown ( 10 
2 1  
1 5  
YR 4/4 ) ; stony sandy loam ;  
friable with gritty feel ; 
wk . dev .  cse .  1 1  
blocky s tructure ; 
few roots  
C 60+ greywacke gravels 
VEGETATION 
podocarp-
hardwood 
{many podo-
carps have 
been milled 
out) 
" 
ESTIMATED 
AGE 
> 7 7 0yrs 
" 
1 94 .  
APPENDIX I I  contd . 
SOIL SYHBOL SOjL DESCRIPTION 
cm (depth) 
LOSS ON IGNITION VEGETATION ESTlHATED 
Soil C 
on No . 2 
Creek fan 
Soil D 
( see figure 
2 1 )  
on 
Whiteywood 
Creek fan 
Soil E 
on No . 1 
Creek fan 
o +2-0 
AhO-1 2 
org . litter , 
dk brown ( l OYR 
2 / 2 ) ; extmy s tony 
silt loam : firm ; 
mod .  dev .  mdm 
nut struc ture ; 
many roots  
-% of dry soil  -
28 
Bw1 2-52 dk brown ( l OYR 3 /3 ) ; 
extmy stony silt  
C 52+ 
0+2-0 
AhO- 1 5  
loam;  firm ;  mod .  1 0  
dev . mdm blocky 
structure ;  few 
cse roots  
greywacke gravel s  
org . l it ter 
v dk brown ( 1 0YR 
2 / 2 ) ; extmly stony 
sil t loam ; firm : mod .  
dev . mdm nut struc ture; 
many roo ts 
20 
Bw1 5-55 dk brown ( l OYR 3/3 ) ; 
C 55+ 
extmy stony sil t loam ; 7 
firm ; wk dev .  mdm 
blocky struc ture ; 
few coarse roots 
greywacke gravels 
0+3-0 org litter 
AhO- l0  v dk greYish brown 
( 10YR 3 / 2 ) ; stony loamy 
sand : soft-hard ; 1 0  
wk dev .  f. crumb s tructure ;  
many roots 
1 (C)  1 0-4 6 
greywacke gravels 4 
Ahb46-4 7  v dk greyish brown 
( l OYR3/ 2 ) ; stony silt 
loam ;  firm with gritty 8 
feel ; mod .  dev . f-mdm 
nut structure; few dead 
roots . 
Bwb 6 7- 1 04 
" YC " n /cU ' O"ln  ( l 0Yr, .I �l ) · 
::'o t ( ny f ;. n °. \  0 , C), ; f -1', ; , 1, (  
de\ . n(lT � l oc . :�.' ·, t r  [ Co t·o .ce . :: 
AGE 
podocarp­
hardwood 
( including 
rimu , rata , 
kamahi , 
coprosmas , 
pseudopanax , 
ferns , 
lancewood) 
approx . 
7 70yrs 
podocarp­
hardwood 
(rimu ' s  
dominate ;  
with rata , 
ferns , lianes , 
pepperwood)  
" 
approx . 
98 yrs . 
podocarp­
hardwood 
(podocarps 
with buried 
bases ; many 
small shrubby 
plants ) . 
195 . 
APPENDIX I I  contd . 
SOIL SYMBOL SOIL DESCRIPTION LOSS ON IGNITION VEGETATION ESTIMATED 
r 
cm (depth) -% of dry soi1- AGE 
Soil E contd . ( 2 )  Cb 1 04+ 
Soil F 
on Hut Creek 
fan (S tan­
field Hut )  
Soil G 
on terrace 
below Hut 
Creek 
Soil H 
on terrace 
below No . 1 
Creek 
Soil J 
on Car 
Park Ck 
fan 
Soil K 
on terrace 
below Car 
Park Creek 
fan 
greywacke gravels 
0+3-0 org litter 
AhO- 14  v dk greyish 1 3  
C 14+ 
brown ( l OYR3 / 2 ) ; 
extmy stony loamy 
sand ; sof t-s1 hard ; 
wk dev . f .  crumb 
struc ture ; many 
roots . 
greywacke gravels 
0+3-0 org litter 
AhO-22 v dk brown ( 10YR2/2 )  
4 
extmy stony loamy sand ; 1 0  
loose ; wk dev f crumb 
s truc ture ; many roo ts 
C22+ fresh greywacke gravels 
0+1"'0 org lit ter 
AhO-1 2  v dk brown ( 1 0YR2 / Z )  
extmy stony loamy sand ; 
loose ;  wk dev f crumbs ; 
many roots  
C IZ+ fresh greywacke gravels 
0+3-0 org litter 
AhO- 1 2  v .  dk greyish brown 
3 
9 
3 
( 10YR3/ 2 ) ; extm1y 1 0  
C I Z+ 
0+9-0 
stony loamy sand ; sl hard; 
wk dev v . f-f crumb struc­
ture ; manyfroots  
fresh greywacke gravel s 
org l i tter 
3 
AhO- l 0  v dk grey ish . brown ( lOYR3/2 ) ; 
extmly stony loamy sand ; 4 
loose , wk dev .  f crumbs around 
roots  o therwise structureless ;  
many f roots 
C I O+ fresh greywacke gravels  3 
Red beech 
(Nothofagus 
fusca) 
approx. 
98 yrs 
mahoe dominate  
with pepper-
wood ,  grasses , 98 yrs 
1 ianes ; this-
tle ; foxgloves 
" " 
mahoedominate ,  < 98 yrs 
with lemon- > 40 yrs 
wood and various 
broadleaves 
pasture with 
some remaining 
lem�nwood , 
coprosmas , 
horopito 
" 
1 9 6 .  
APPENDIX IV NITROGEN MINERALISATION DATA FROM A LABORATORY EXPERIMENT 
FOR THE TAKAPARI PEATY LOAM, (Tp) 
DEPTH OF * 
SOIL INORGANIC-N % OF INORGANIC-N 
SAMPLE NH -N 4 
N0
2
+ N0
3
-N 
( 3  replicates) 
* 
average range average range average range 
pgN/g air-dry soil % 
0-20cm 434 422-447  9 7  92-99 3 1 -8 
20-50cm 108 1 03- 1 14 9 7  97-98 3 2-3 
(Macgregor ,  pers . comm. ) 
o Inorganic N was measured af ter incubation for 1 4  days , at 20  C 
using the method of  Keeney and Bremner ( 1967 ) . 
NOTES 
( 1 )  The above data indicate that substantial amounts o f  nitrogen are 
present in the soil , po tentially ava ilable for plant  use .  
( 2 )  In contrast t o  normal agricultural soils , a large percentage o f  the 
inorganic-N produced is in the ammonium form . 
( 3 )  Nitrificat ion of  the ammonium form to nitrite and nitrate  can be  
inhibited , which may be due at least  in  part to the following adverse 
conditions for microbial activity : 1 )  a low soil pH , being approximately 
4 . 5 , 
2 )  waterlogging , during the winter 
months , 
3 )  the presence of  certain tannins , 
tannin derivatives , phenolic acids 
and flavonoids , produced by the 
organic matter (Rice and Pancholy , 
1 974)  . 
1 9 7 .  
APPENDIX V PHOSPHATE RETENTION VALUES FOR THE DANNEVIRKE TAXADJUNCT , 
SOIL 
N�E 
Dannevirke 
hill soils 
Dannevirke 
taxadjunc t 
AND DANNEVIRKE HILL SOILS 
PHOSPHATE RETENTION VALUE 
HORIZON % 
Ah 83  
Bw 90  * 
Ah 9 7  * 
Bw 99 * 
C 7 9  
* Phosphate retention values > 90% ; a value suggested b y  Blakemore ( 1 9 7 7 )  
a s  a condition which i s  satisfied when the soils exchange complex i s  dominated 
by amorphous material . 
NOTES 
( 1 )  High phosphate retention values for the Dannevirke soils suggest that 
a large proportion of  their exchange complexes are dominated by amorphous 
material s .  
( 2 )  I t  is l ikely that these amorphous materials are  weathered from volcanic 
ash,  within the soil . 
(3 )  When the exchange complex of  the soil is dominated by amorphous materials ,  
the soil may be classified as an Andept (Soil Survey S taf f ,  USDA, 1 9 75 )  
or  Andisol (G . Smith , 1 978) . This presumably equates with the ye1low-
brown loam soil group (alvic soils)  of  the New Zealand genetic soil 
classification .