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Charwell Basin Field Trip 2006 – www.paleoclimate.org.nz
1
2006 AUSTRALASIAN-INTIMATE Meeting
Charwell Basin Field Trip
Geomorphic Responses to Climate Change in the Charwell Basin
1 December 2006
Peter Almond, Soil and Physical Sciences Group, Agriculture and Life Sciences Division, Lincoln University
Matthew Hughes, Soil and Physical Sciences Group, Agriculture and Life Sciences Division, Lincoln University
Philip Tonkin Geological Sciences Department, University of Canterbury
Right and left branches of the Charwell River just downstream of the Hope Fault. Extensive
surfaces are the Stone Jug terrace.
Australasian Quaternary Association Inc.
AQUA
Charwell Basin Field Trip 2006 – www.paleoclimate.org.nz
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Itinerary
1. Lynton Downs – Loess sheet L1 and associated soil (2548890E, 5869710N).
2. Correlatives of the Quail Downs gravels above Green Burn Stream. View the trace of
the Hope Fault (2542320E, 5867500N).
3. Kawakawa Tephra site and view of Stone Jug aggradation gravels and the strath
beneath the gravels (2540010E, 5866280N).
4. Fill-cut and strath terraces beneath the Stone Jug fill terrace, Charwell River,
(2540230E, 5862180N).
5. “Stein Creek Gully” study site on Dillondale gravels – loess stratigraphy, slope
processes, paleoenvironmental reconstruction (2539120E, 5862000N).
6. Landsurface morphology on the Quail Downs gravels (2536510E, 5863400N).
Bibliographic reference: Almond, P.C., Hughes, M., Tonkin, P.J. 2006: Geomorphic responses to climate change in
the Charwell basin. Field trip guide for Australasian-INTIMATE meeting, Kaikoura, New Zealand. 29 November - 1
December 2006. www.paleoclimate.org.nz.
Material from this field trip guide may not be used in any publication or presentation
without prior approval of the authors.
Charwell Basin Field Trip 2006 – www.paleoclimate.org.nz
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Geomorphic Responses to Climate Change in the Charwell Basin
Introduction
The field trip and its title owe much to the seminal work Geomorphic Response to Climate
Change by Bill Bull (1991), which features a chapter on the Charwell basin. The trip
examines the geomorphic responses of the piedmont reach of the Charwell River and its
associated terraces and hillslopes to climatic and tectonic forcing, and considers some new
paleoenvironmental data extracted from loess deposits.
The Study Area
The piedmont reach of the Charwell River occupies a structural basin bounded to the
northwest by the steep range front of the Seaward Kaikoura Range, which here rises to around
1700 m ASL (Fig. 1). The Seaward Kaikoura Range is underlain by faulted and folded,
massive to medium bedded greywacke of the Pahau Terrane. In the basin, relief is in the order
of 200 m between the channel floor of the Charwell River and the upper slopes of the fans
adjacent to the Hope Fault. The range-bounding Hope Fault moves in a right lateral sense at a
rate of about ca 33 mm/year, which, importantly, has had the effect of preserving older valley
fills and ancestral channels of the Charwell River by dislocating them in a southeast direction
from the drainage basin and active river channel. From a dated fluvial strath, Bull (1991)
estimated the uplift rate in the basin to be 1.3 ± 0.1 m/kyr; the mountains were estimated to be
uplifting 2-3 times faster. The present precipitation in the watershed ranges between 1200 and
2000 mm with mean monthly temperatures between 1°C and 14°C. In the basin comparable
climatic characteristics are 1000 to 1400 mm and 3.5-15.5 °C.
Fluvial Response to Climate and Tectonic Forcing
Five valley fills are recognised in the Charwell basin. They are, from young to old, the Stone
Jug, Flax Hills 1, Flax Hills 2, Dillondale, and Quail Downs gravels (Fig. 2). They form fill
(aggradation) terraces that are progressively higher, more dissected and more modified by
slope processes with increasing age. The bedrock straths beneath the fill terraces, representing
the valley floors prior to aggradation, form a staircase reflecting the long term incision of the
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Charwell River in response to basin uplift (Fig. 3). The timings of the phases of aggradation
were estimated by the tectonic displacement of valley fill packages, assuming a constant slip
rate of 33 mm/yr on the Hope Fault (Bull 1991). Concentrations of glass shards identified as
Kawakawa Tephra (26,500 cal yr B.P.)within silty lenses in the lower third of the Stone Jug
aggradation deposits provided an additional constraint. The chronology of valley filling was
used as evidence for climate control on river behaviour, and the causes couched in terms of
changes in the ratio of stream power to resisting power and exceedence of the Critical
Threshold of Power. The ages of the straths were estimated from radiocarbon dates from
organics above the straths under the constraints established by the inferred intervals of
aggradation. These estimates were then refined by correlation to high sea level stands
identified by Chappell and Shackleton (1986). Bull (1991) assumed the climatic amelioration
responsible for eustatic sea level rise also affected the balance of stream and resisting powers
and thereby stream behaviour, rather than some causative base level effect.
River downcutting with the formation of flights of fill-cut terraces and strath terraces occurred
during periods of excess stream power as the Charwell River attempted to attain base level of
erosion. The fill-cut terraces represent temporary still stands or even short-lived aggradation
resulting from complex feedbacks among different reaches of the river (complex response)
during downcutting into the valley fill. Strath terraces form as the river accommodates
episodically the bedrock uplift accumulated during and after aggradational phases. The
variations in tempo of downcutting are exemplified by the fill-cut and strath terraces formed
after 14 ka when the Charwell incised into the Stone Jug fill (Fig. 4).
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0 100 200 km
Charwell Catchment
Charwell River
QuaternarySurfaces Faults
0 2 4 km
Hope Fault
N
(a) (b)
(c)
Figure 1 (a) Location of the Charwell area, South Island. (b) Shaded digital relief model of
Charwell basin and catchment. Late Quaternary surfaces and faults are shown. (c) Orthophoto
of Charwell basin and catchment shown in 3D perspective looking northwest. Three times
vertical exaggeration.
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Figure 2
Late Quaternary terrace surfaces in Charwell basin with loess cover indicated,
underlain by a shaded digital relief model. After Bull (1991) and Tonkin and
Almond (1998).
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Figure 3. Conceptual framework for climatically controlled aggradation and degradation of
Charwell River
Figure 4. Downcutting curve of the Charwell River from 16 k cal yr B.P. to
present.
Bull (1991) argues for different climatically influenced hillslope and stream channel
processes to account for the Flax Hills aggradation, the Stone Jug aggradation and the Post
Stone Jug degradation (Table 1). These responses were inferred from paleoclimate
reconstructions of McGlone (1998/ and written comm.) for the LGM to present period, and
from pollen analysis by D. Mildenhall on samples taken from sediments above the Flax Hills
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strath. Bull assumed a treeline lowering, from the present 1200 m ASL, of 400-600 m during
the Flax Hills 2 aggradation and 800 m during the Stone Jug aggradation.
Table 1 Hillslope and channel responses during the last two major aggradation phases and the
most recent degradation phase of the Charwell River (from Bull 1991).
Geomorphic
Response
Time
interval (k
cal yr)
Climatic
variables
Hillslope processes in
drainage basin
Piedmont reach
channel
characteristics
Flax Hills 2
aggradation
38-31 Cool but
fairly wet
Frozen substrata, periglacial
procceses, gelifluction,
landslides, debris flows,
very large sediment yields
under lowered treeline
High stream power
but exceeded by
resisting power of
very large sediment
supply
Stone Jug
aggradation
29-16 Cold, dry Frozen substratum and
periglacial proceses, but less
ppt available for sediment
mobilisation, tectonically
driven landsliding
Reduced stream
power and large
sediment supply
cause threshold of
critical power to be
exceeded
Stone Jug
degradation
14-0 Warm,
increasing
rainfall to
mid
Holocene
Reduction of snow pack,
increase veg density on
slopes, rise in treeline,
increased discharge,
Reduced sediment
supply, increased
stream power
particularly mid
Holocene – rapid
downcutting
Loess Stratigraphy and Chronology
The terraces in the Charwell basin are mantled with increasing thicknesses of loess with
increasingly complex stratigraphy from the Stone Jug to the Dillondale terrace (Fig. 5). The
Dillondale terrace has been significantly modified from its original constructional form by
dissection and drainage network elaboration. Addition of loess to this evolving landform has
resulted in redistribution of primary loess by soil transport processes. The thickness and
stratigraphy of the loess mantle on flat interfluves and broad plateaux, however, are thought to
represent a complete record. The surface on the Quail Downs valley fill retains none of its
original constructional form, having evolved a ridge and valley morphology, and loess is
absent.
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Loess sheets show systematic spatial relationships with aggradation terraces and they are
presumed to have genetic relationships to valley fills. Loess is produced in aggradational
phases when rivers are not constrained to narrow, incised valleys. The broad surfaces of the
alluvial fans forming during aggradation provide a large loess source area, and climatically
mediated erosion processes in the hillslopes such as frost shattering promote formation of silt-
sized material suitable for wind transport. Loess produced during a given aggradation event is
found on all higher terraces so long as erosion on those surfaces is minimal (Fig. 5).
Boundaries between loess sheets are marked by strongly expressed buried soils presumed to
have formed during phases of river downcutting when loess supply was mimimal. Soil
modification of the loess can be divided into an upbuilding phase with relatively weak soil
development when soil and loess accumulation occur together, and a topdown phase when
loess accumulation is minimal and strong soil modification takes place (Almond and Tonkin,
1999). Buried soils tend to have similar morphologies to surface soils except for the absence
of organic rich A horizons, i.e. a mottled, clay rich horizon with Mn-Fe nodules (Btgc) over a
dense subsurface horizon with or without significant accumulation of clay (Bxg or Btg) (Fig.
6). This morphology corresponds to Perch-gley Pallic soils of the NZ Soil Classification
(Hewitt, 1998) and Epiaqualfs of the USDA Soil Taxonomy system (United States. Natural
Resources Conservation Service., 1999). The degree of clay accumulation particularly is
presumed to be indicative of the length of time of soil formation.
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Figure 5. Loess thickness and stratigraphy for terraces of the Charwell River.
Accepting the above conceptual framework a robust chronology of loess accumulation would
be a test of the (climatically controlled) aggradation/degradation chronology of the Charwell
River.
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Figure 6. Loess stratigraphy across Charwell River terraces (Milne et al., 1995)
Morphological Evolution of the Charwell Terraces
The morphological evolution of the Charwell terraces is driven by long term base level fall as
the Charwell River responds to tectonic uplift of the basin. The base level fall signal
propagates to the rest of the landscape through tributary streams. The changes in drainage
networks, loess stratigraphy, and topography across the fill terraces suggest the following
evolutionary sequence.
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• Immediately after abandonment of a valley fill by river incision, steeply sloping and
steep-sided gullies form at terrace edges by spring sapping initiated at the gravel/strath
interface, and fluvial erosion (Fig. 7)
• A positive feedback results as an extending gully intercepts a larger area of the
groundwater and its drainage area increases. (Stone Jug)
• Loess accumulates episodically on terrace tread remnants producing a loess-mantled
terrace. (Flax Hills 1 and 2)
• Eventually drainage density is sufficient that competition between neighbouring gullies
for groundwater, throughflow and overland flow is severe enough that some become
inactive.
• Diffusion-like processes transport loess into gullies and round out gully walls to form
convex hillslopes. Some gullies infill.
• Ongoing extension and elaboration of the drainage network increases the area of
hillslopes and decreases the area of terrace tread. A loess-mantled downland results.
(Dillondale)
• Erosion rate increases as the proportion and convexity of hillslopes (driven by ongoing
incision) increase.
• Eventually hillslopes are ubiquitous and the erosion rate exceeds long term average
loess accumulation rate. Erosional ridge and valley terrain results. (Quail Downs)
• A (flux) steady state landscape results when erosion rate equals base level lowering rate
(ca 1.3 mm/yr). (Quail Downs??)
Figure 7. Evolving gullies in the Stone Jug terrace (Photograph P. Almond).
Charwell Basin Field Trip 2006 – www.paleoclimate.org.nz
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The above qualitative description can be quantified using terrain attributes derived from a
DEM. The slope angle and curvature data shown in Fig. 8, although compromised by a low
resolution DEM, show a trend toward increasing average slope angle, greater range of slope
angles, and an increasing variation in curvatures as terraces become older.
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Perc
ent F
req
uen
cy
0
1
2
3
4
5
0
5
10
15
20
25P
erc
ent
Fre
que
ncy
0
2
4
6
8
10
0
5
10
15
20
25
30
Perc
ent
Fre
que
ncy
0
2
4
6
8
10
0
5
10
15
20
25
30
Perc
en
t F
requ
en
cy
0
1
2
0
5
10
15
20
Perc
en
t F
requ
ency
0
2
4
6
8
10
0
5
10
15
20
25
0 10 20 30 40
Perc
ent F
req
uen
cy
0
1
2
-8 -6 -4 -2 0 2 4 6 80
5
10
15
20
Stone Jug
N=1436
N=15309
late Flax HillsN=3676
early Flax HillsN=871
DillondaleN=4819
Quail DownsN=5746
Slope (°) CurvatureConvex Concave
post Stone JugN=1436
post Stone Jug
Stone JugN=15309
late Flax HillsN=3676
early Flax HillsN=871
DillondaleN=4819
Quail DownsN=5746
(a) (a)
(c) (d)
(e) (f)
(g) (h)
(i) (j)
(k) (l)
Figure 8. Frequency distributions of slope (°) and curvature for the late Quaternary terrace surfaces in
Charwell basin, derived from a 25 m digital elevation model. Data describe terrain characteristics for
post Stone Jug (a, b); Stone Jug (c, d); late Flax Hills (e, f), early Flax Hills (g, h); Dillondale (i, j);
Quail Downs (k, l).value after 10 k cal yr B.P..
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Transport processes and slope evolution
A small gully (Stein Creek gully) on the Dillondale terrace has become the focus of detailed
studies of soil transport processes, slope evolution, and paleoenvironment. Roering et al
(2002; 2004) used the distribution of Kawakawa Tephra in soils along a hillslope transect
from interfluve to the floor of the gully to investigate the kinds of soil transport processes
operating, and to parameterise a soil transport model with the aim of quantifying erosion.
Interfluve sampling site
Hollow sampling site
N
Figure 9. Location of Stein Creek gully and sample sites.
At the interfluve Kawakawa Tephra occurs as dispersed glass grains showing a clear peak
concentration, assumed to be the primary emplacement horizon, at about 0.8 m depth.
Downslope, as curvature increases, the tephra peak is progressively exhumed until, at about
one third the way down the slope it becomes completely dispersed in the top 40 cm (Fig. 10).
This pattern is consistent with a slope dependent transport process (soil creep) driven by
bioturbation involving the upper 40-50 cm of soil. Roering et al (2002) concluded the
bioturbation mechanism was most likely tree-throw.
Slope dependent transport is modelled by an equation of the form:
x
zKqs
∂
∂=
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where qs (L2T
-1) is the soil flux, K (L
2T
-1) is a transport coefficient, and
x
z
∂
∂ (LL
-1) is the slope
gradient.
Roering et al. (2004) showed that the concentration and distribution of glass downslope was
best explained by a history of soil transport characterised by two different K values: a low K
value for between 26.5 k cal yr B.P. and about 10 k cal yr B.P. and a much higher K from 10
k cal yr B.P. to present. The pattern of glass distribution they took as evidence for a reduced
intensity of bioturbation and soil transport under LGM-late glacial grassland and an increased
intensity of bioturbation and soil transport after the Holocene reforestation. This finding
contrasts with the scenarios advanced by Bull (1991) for temporal variation of sediment
production in the mountainous drainage basin of the Charwell River. In this part of the
landscape grasslands during glacial climates were inferred to have increased sediment supply,
whereas recolonisation of slopes by trees in interglacials reduced it. It appears that different
parts of the landscape respond differently to climatically induced vegetation changes.
Figure 10. Slope morphology and patterns of tephra distribution in soils along a slope transect into
“Stein Creek Gully” on the Dillondale terrace.
Paleoenvironmental History of Stein Creek gully
The PhD study of Matthew Hughes has focussed on reconstructing the paleoenvironmental
history of Stein Creek gully. The study has involved examination of the soil chemistry and
biogenic silica (phytoliths and diatoms) from the loess on the interfluve and the reworked
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loess in the gully (Fig. 9). Age control, of varying veracity, has been provided by Kawakawa
Tephra, luminescence dating, and correlation to an established biostratigraphic datum.
The loess on the interfluve shows three loess sheets typical of the Dillondale terrace. The
surface soil, formed in L1 (1.8 m thick), is a Pallic soil with a well developed fragipan and
evidence of perching of water above that horizon in the form of a Bg horizon. The Kawakawa
Tephra primary depositional layer is at 70 cm. The first buried soil, formed into the top of L2,
has a similar morphology, but L2 is only 0.8 m thick. L3 is about 2 m thick and the soil
formed in it has very high clay content, many strongly cemented Mn nodules, and well
developed clay skins in the upper 1 m. This soil stands out as being more strongly developed
than the surface soil or the soil in L2 (Fig. 11).
Phytolith data, summarised in terms of shrub/tree types and grass types, show:
• Grasses dominated during accumulation of L1, until sometime after 26.5 k cal yr B.P.
when trees/shrubs returned to the landscape.
• L2 accumulated under a more shrub/tree rich flora than L1
• L3 started accumulating under a grass rich flora which was replaced by probably tall
forest. The upper part of L3 marks the transition to the more grass rich flora of L2. .
The phytolith data in combination with the soil phosphorus data, which show strong depletion
of P in L3, suggest the soil in L3 formed for a long period under a climate regime that
supported tall forest. Tentatively, we assign part of the soil development in L3 to MIS 5 and
at least the lower part of the loess sheet to MIS 6. Hence, L2 and L1 both formed in the last
glaciation (MIS 4-2).
We have no reliable age constraints on this loess sequence other than Kawakawa Tephra. An
OSL age from below the tephra replicated the age under-estimation problem luminescence
dating seems prone to in the South Island (Almond et al., 2007). Total element analyses from
glass grains extracted from deeper in the section showed no clear affiliations with older
tephras than Kawakawa, although the results are tantalising (Figs 12 and 13).
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Dep
th (
m)
0
1
2
3
4
4.6
Soil Stratigraphy& Loess Sheets
Ah
Bg
Btg(x)
bBw(g)
bB(x)(g)
b2Btg1
b2Btg2
b2Btgc1
b2Btgc2
b2Btg3
L1
L2
L3
b2Bt
600
Total Phosphorus µg g-1
0 10020 40 60 80
P Fractions % of Total
PCa PFe/Al POcc POrg
0 200 400
PT
Trees/Shrubs & Grasses (%)
Grasses
Trees & Shrubs
OSL18.1±1.3
Fig. 11. Summary diagram of interfluve soil and loess stratigraphy (left), P fractions (centre) and
phytoliths (right). Stratigraphic positions of Kawakawa Tephra (Kk) and OSL date are indicated.
Fig. 12. Plot of CaO vs. FeO contents of glass shards from the interfluve and hollow loess deposits
(open circles) and CaO vs. FeO contents of known North Island late Quaternary rhyolitic tephras.
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Fig. 13. Plot of CaO vs. FeO contents of glass shards extracted from specific depth increments in the
interfluve loess deposit, and CaO vs. FeO contents of known Kawakawa Tephra glass shards.
Depth variation of phytolith abundances from a core taken from the gully floor (hollow) show
a similar but expanded pattern to that of L1 on the interfluve. This suggests the gully fill
represents loess transported from the slopes since the beginning of accumulation of L1 to the
present day. The expansion of the record from 1.6 m depth to 7 m gives much higher
resolution by reducing the effects of mixing of phytolith zones and blurring of boundaries by
bioturbation. Age control for the section is again problematic. Kawakawa Tephra appears as a
glass peak at 2.6 m depth. An OSL age immediately above underestimates the tephra age by
as much as 13 ka and an age 2 m below under-estimates the tephra age by ca 6 ka. We
consider an OSL age of 28 ka at 5.6 m also to be an under-estimate. The OSL age of 8.75 ka
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at 1.1 m appears to be reasonable. This depth marks the transition from grass dominated flora to
a flora with an increasing tree/shrub component. Pollen analysis and radiocarbon dating from local
sites place this transition at 10,900 ± 300 cal yr B.P. (McGlone et al., 2004). This datum and that
provided by Kawakawa Tephra can be used to test the hypothesis of Roering et al. (2004) that soil
transport under grassland was slower than under Holocene forest. Rates of soil transport can be
estimated from the rate of gully infilling. Gully fill volumes between the surface and the Holocene
forest transition datum (HFTD), and between the HFTD and Kawakawa Tephra indicated filling rates
were 0.11 ± .04 mm/yr and 0.05 ± 0.03 mm/yr, respectively.
Phytolith ZoneBiogenic Silica
Grasses
Trees & Shrubs
Diatoms
AhBg
Bgc
Bwg
Bg2
Bwg2
Br
Bg3
bBwg
Soil Stratigraphy
OSL8.75±0.73 ka
OSL13.1±1.1 ka
OSL20.6±0.1 ka
OSL28.3±1.6 ka
Fig. 14. Summary diagram of hollow soil stratigraphy (left) and biogenic silica (centre). Phytolith
zones are shown on right. Stratigraphic positions of Kawakawa Tephra (Kk) and OSL dates are
indicated.
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Chronostratigraphic Summary
Interval (k cal yr) Valley fill
Bull
(1991)
Revised
MIS Loess
sheet
Interval
(k cal yr)
Loess sheet
drapes
Stone Jug 16-29 L1
Flax Hills 1 31-38 L2
Flax Hills 2 43-49 L3
Dillondale ?
Quail Downs ?
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References
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tephra isochron-based chronology for Birdlings Flat loess at Ahuriri Quarry, Banks
Peninsula, Canterbury, New Zealand. Quaternary Geochronology.
Almond, P.C. and Tonkin, P.J., 1999. Pedogenesis by upbuilding in an extreme leaching and
weathering environment, and slow loess accretion, south Westland, New Zealand.
Geoderma, 92: 1-36.
Bull, W.B., 1991. Geomorphic responses to climate change. Oxford University Press, New
York, 326 pp.
Chappell, J. and Shackleton, N.J., 1986. Oxygen isotopes and sea level. Nature, 324(6093):
137-140.
Hewitt, A.E., 1998. New Zealand soil classification. Landcare Research science series,.
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McGlone, M.S., Turney, C.S.M. and Wilmhurst, J.M., 2004. Late-glacial and Holocene
vegetation and climatic history of the Cass Basin, central South Island, New Zealand.
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Milne, J.D.G., Clayden, B., Singleton, P.L. and Wilson, A.D., 1995. Soil description
handbook. Manaaki Whenua Press, Lincoln, N.Z., 157 pp.
Roering, J.J., Almond, P., McKean, J. and Tonkin, P., 2002. Soil transport driven by
biological processes over millennial timescales. Geology, 30: 1115-1118.
Roering, J.J., Almond, P., Tonkin, P. and McKean, J., 2004. Constraining climatic controls on
hillslope dynamics using a coupled model for the transport of soil and tracers:
Application to loess-mantled hillslopes, South Island, New Zealand. Journal of
Geophysical Research, 109: F101010, doi: 10.1029/2003JF000034.
Tonkin, P.J. and Almond, P.C., 1998. Using the soil stratigraphy of loess to reconstruct
landscape histories of north-eastern and western lowlands of South Island, New
Zealand, 8th Biennial Conference of the Australian and New Zealand Geomorphology
Group, Goolwa, South Australia, pp. 55.
United States. Natural Resources Conservation Service., 1999. Soil taxonomy : a basic system
of soil classification for making and interpreting soil surveys. U.S. Dept. of
Agriculture Natural Resources Conservation Service : For sale by the Supt. of Docs.
U.S. G.P.O., Washington, DC, 869 pp.
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