EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 17,39-51(1992)
THE INFLUENCE OF ROCK MASS STRENGTH ON GLACIAL
VALLEY CROSS-PROFILE MORPHOMETRY: A CASE STUDY
FROM THE SOUTHERN ALPS, NEW ZEALAND
PAUL C. AUGUSTINUS
Department of Earth Sciences, University of Waikato, Hamilton.?
Received 29 August 1990
Revised 10 March 1991
ABSTRACT
The erosional morphology in the vicinity of the Main Divide of the Southern Alps, and Fiordland, New Zealand, appears
to be a product of the interaction between Alpine Fault-inducedtectonic processes, rock mass strength of the uplifted and
eroded bedrock, and the processes acting to denude the developing mountain landscape. The magnitude of the effects of
glacial erosion on the landscape is directly controlled by the size and physical properties of the glaciers, whilst the form of
the trough is a direct consequence of the rock mass strength (RMS)properties of the slope rock. Realistic models of
development of the cross-profile shape of glacial valleys must take into consideration the R M S properties of the eroded
substrate.
KEY WORDS
Glacial valley Cross-profile morphology Rock mass strength Morphological development
INTRODUCTION
Modelling the development of glacial troughs usually involves examining only the physical properties of the
glacier (Boulton, 1974; Hallet, 1979, 1981; Roberts and Rood, 1984), largely ignoring the influence of the
properties of the eroded bedrock on the erosion processes and resultant valley form. Although it has long
been observed that troughs are deep and narrow where bedrock is resistant, and wider and shallower where it
is less resistant (Matthes, 1930), the contribution of rock resistance to the glacial erosion process has been
assessed only qualitatively (e.g. Sugden, 1974, 1978; Addison, 1981, Harbor et al., 1988). It remains a
limitation to the realistic assessment of the results of glacial erosion and the development of the glacial trough
form.
Hirano and Aniya (1988, 1989, 1990) used cross-profile data from alpine and continental type glacial
troughs to argue that the varying morphologies of glacial valleys can be attributed to the nature of the
glaciers that produced these valleys. They argued that the cross-profile of the valley eventually assumes a
shape in which glacial erosion operates most efficiently. Hirano and Aniya (1989) raised further questions
concerning the relative influence of bedrock lithology and the nature of the glacier (polar or temperate) on the
final valley shape.
Harbor (1990) questioned some of the assumptions in the work of Hirano and Aniya (1988), and argued
that to better understand the evolution of glacial valley cross-profiles requires focus on the following main
areas: (1) flow patterns through glacier cross-sections; (2) glacial erosion processes; (3) patterns of bedrock
t
Present address: University of Tasmania, P.O. Box 1214, Launceston, Tasmania 7250.
0197-9337/92/0 10039-1 3$06.50
0 1992 by John Wiley & Sons, Ltd.
40
P. C. AUGUSTINUS
resistance to erosion; (4) the evolution of valley slopes above glaciers; and temporal variations in ice
occupation. It is the purpose of this paper to further examine the model developed by Hirano and Aniya
(1988, 1989, 1990) for the development of the glacial valley cross-profileyand to offer an alternative model
using rock mass strength (RMS) and morphometric data acquired from glacierized troughs in the Southern
Alps, New Zealand.
THE STUDY AREA
To enable the study of the influence of lithological and geotechnical properties of the basement rocks on
glacial valley form in the Southern Alps, New Zealand, it was necessary to examine deglacierized troughs
developed in a variety of rock types. Troughs, which are either unmodified or have been only slightly
modified by subaerial processes since retreat of the glaciers, were studied so that rock conditions were as
close as possible to those that existed during glaciation.
Broad variations in valley morphology are apparent across the Southern Alps (Figure 1) and can be
separated into five morphological terrains on the basis of regional variations in glacial valley cross-profile
shape and the lithologies on which they are developed (Figure 2): (1) wide, flat floored valleys developed
within the greywackes of the Mt Cook region (170"15'E-43"35'S), and (2)similarly wide, flat floored, debrisfilled troughs developed in chlorite grades I1 and I11 schists of the Mt Aspiring region (168"40E-44"35'S).
These contrast markedly with the relatively deep and narrow troughs cut into (3) the plutonic basement
comprising the Darran Mountains region of northern Fiordland (168" 05'-44"40S), and (4) the deep, narrow
troughs eroded into chlorite grades 111, IV and garnet-oligoclase schists of the Westland region close to the
Main Divide (170" 10E-43'27'S). Within Fiordland, a marked disparity is apparent between the overdeepened troughs of the Darran Mountains, and the relatively overwidened forms of (5) Merrie Range area of
south-central Fiordland (167"05'E-45"35'S).The general geology of the southern part of the Southern Alps
and Fiordland is shown in Figure 3.
Cooks (1983) measured a range of strength parameters for cores sampled from five major rock types in
South Africa and the U.S.A., and correlated these with mean drainage density, slope angle and hyposometric
integral of the fluvially eroded morphologic terrains controlled by each rock type. The results indicate that, in
general, the valley-side slope angles of the drainage basins can be correlated directly with the strength of the
bedrock underlying them.
The dominant controls on the development of glacial erosional landforms are the factors controlling the
processes of plucking and abrasion at the ice-rock interface (Embleton and King, 1975). The processes of
plucking and abrasion involve: (1) the detachment of fragments from the rock surface; (2) comminution into
progressively smaller fragments; and (3) the abrasion of the bedrock surface as the glacier sole slides over the
bed, and is armoured with rock fragments of varying degrees of hardness. Boulton (1974) showed that there
was little difficulty in providing forces for rock failure to occur subglacially,arguing that the plucking process
may act directly on joint bounded blocks, but that on unjointed bedrock hummocks, some rock failure
mechanism must be available to fracture the bedrock.
Rock intact strength measures used in the present study were: the point load strength index and Schmidt
hammer rebound; as well as the rock index properties: rock unit weight and Los Angeles abrasion resistance.
These parameters were considered to be the most appropriate (and most easily determined) intact rock
measures of resistance to subglacial erosion (shear failure and crushing, as well as abrasion). The variation in
rock intact strength for each lithology (Table I) is controlled by the geometry of structural weaknesses, such
as the foliation in the South Westland schist. As a consequence, incorporation of point load strength index
values in the analysis is complicated by the marked strength anisotropy displayed by the schist (Table I).
Only those measures considered to be minimally influenced by rock mass anisotropy were used in the
correlation analyses.
Hence, the Los Angeles abrasion index and rock unit weight are the most applicable measures as they are
the only indices of rock strength not strongly influenced by anisotropy. Schmidt hammer rebound values are
less influenced by rock anisotropy than point load strength, and display a strong correlation with the rock
intact strength measures (Deere and Miller, 1966; Augustinus, 1988, in preparation). Hence, the Schmidt
GLACIAL VALLEY MORPHOMETRY
41
A
B
Figure 1. (a) Photo showing typical glacially overdeepened valleys in northern Fiordland; (b) photo showing relatively overwidened
Hooker Valley in Mt Cook region
42
P.C.AUGUSTINUS
I
I
168"E
1;oo
172>
1j4"
Figure 2. Location of mqrpholo&al terrains examined in this study
0Granite, gneiss
zone
v
Figure 3. Generalized map of the geology and main tectonic elements in the southern half of the South Island, New Zealand. After
Suggate (1982)
43
GLACIAL VALLEY MORPHOMETRY
Table I. Mean intact strength parameters for each morphological terrain
~~~
Study area
Rock type
South Westland Mt Cook
Greywacke/
Biotite schist
argillite
Mt Aspiring
Grey/Green
schist
North Fiordland South Fiordland
Diorite/gabbro Granite/
metamorphics
Unit weight
(kgNm-')
27.00 j, 041
27.52 f 0.61
28.89 f 1.56
Porosity (YO)
Point load
strength (MPa)
Los Angeles
Abrasion (%)
Schmidt hammer
rebound
Schmidt rebound x
unit weight
0.94 f 029
8.19 f 1-24+
2.86 f 0*2911
27.3 f 3.3
0.16 f 0.04
11.21 f 0-15
13.2 f 2.1
0 9 4 f 0.29
5-85 f 2.55 +
1.81 f 0.90''
38.1 f 6.4
29.2 f 5.9
25.4 f 3.1
51.5 f 3.1
64.5 j, 4.4
49.5 j, 6.9
58.5
61.1
+
1434 f 49
1774
25
1495 f 11.4
28.75 f 0.37
28.30 f 0 7 0
076 f 0.25
5.50 f 1-31
0 7 f 031
5.32 f 1.44
2.8
1630 & 75
4.7
1729 j, 59
refers to tests conducted perpendicular to foliation, 1) refers to tests parallel to foliation.
hammer could be used as a valid field measure of the compressive strength and hardness of the rocks studied,
especially when the Schmidt hammer rebound value is multiplied by the appropriate rock unit weight (Deere
and Miller, 1966; Augustinus, in preparation) (Table I).
The relative importance of subglacial abrasion resistance to glacial erosion is not known (Drewry, 1986).
However, geotechnical methods of abrasion resistance estimation can be used as a surrogate of abrasion
resistance. Los Angeles (LA) abrasion resistance is a standard measure of rock abrasion resistance (Brown,
1981). Besides the laboratory measurement of the intact strength of samples collected in the field, the in situ
rock mass strength was evaluated for each terrain where appropriate.
Hence, to test for possible relationships between the strength of the eroded bedrock and glacial erosional
landform development, a range of rock strength properties was measured from each terrain for comparison
with various measures of glacial valley morphometry. The morphological variables quantified were: valley
cross-profile form ratio (FR); shape factor (j);
glaciated valley density; and the coefficients a, b, and c from the
quadratic equation that best fits the valley cross-profile shape.
INTACT ROCK STRENGTH
Large oriented rock samples were collected in the field, and intact strength was measured in the laboratory.
Unfortunately, frequent difficulty of access and removal of rock material allowed only limited amounts of
sample to be retrieved, so that only a limited geotechnical testing programme was attempted. Standard
I.S.R.M. methods were followed as per Brown (1981), and detailed in Augustinus (1988), and will not be
described here. The mean value of the intact strength parameters for each morphological terrain are given in
Table I.
ROCK MASS STRENGTH ASSESSMENT
Cooks (1983) study of rock strength control on landscape development ignored the influence of bedrock
jointing on rock strength. It has long been recognized in engineering geology studies that the jointing of a
rock mass is the major control on rock slope development and hence on erosion (Hoek, 1983). Such factors as
the state of weathering,joint width, continuity and spacing were found to be critical in controlling in situ rock
mass strength, and hence slope stability, erosion resistance, and geomorphic development (Selby, 1982,1987;
Augustinus and Selby, 1990). The RMS method has not previously been applied to alpine-type glacial valley
slopes. Thus, a rock mass strength assessment of rock slopes in recently deglaciated troughs was undertaken
for each morphological terrain.
44
P.C. AUGUSTINUS
Table 11. Summary of rock mass strength (RMS) and valley shape data
Study Area
Rock Type
South Westland Mt Cook
Biotite schist
Greywackej
argillite
Mt Aspiring
Grey/green
schist
North Fiordland South Fiordland
Dioritelgabbro Granite/
metamorphics
Form ratio (FR)
cov
Shape Factor (f)
cov
Drainage
density (km- ’)
cov
Coefficient ‘a’
cov
Coefficient ‘b’
cov
Coefficient ‘c’
1.56 f 0.21
13.4
0.48 f 0.04
8.0
0.56 f 0.07
1.77 f 0.19
11.0
052 k 0-02
4.4
0.40f 0.06
1.79 0.25
14.2
0.48 f 0.04
7.4
0.36 f 0.07
1.08 & 0.13
12.2
0.43 f 0.04
8.9
0.71 f 0.13
12.8
6231 f 634
10.5
1.37 f 0.26
19-9
1.89x 10-4
f 0.71 x 10-4
38.9
83.1 k 7.4
90
14.2
7015 k 574
18.8
6267 f 873
14-5
1.23 k 0.27
22.6
1.11 x 10-4
f 0 3 1 x 10-- 4
29.2
78.7 f 8-5
10.8
17.9
5592 f 1140
19.3
2.14 f 0.44
21.3
302 10-4
f 0.78 x 10-4
26.5
86.2 If: 5.1
6.0
cov
RMS
cov
8.5
1.16 f 0.17
15.5
1.15x 10-4
0.36 x 10-4
32.1
79.1 f 5-3
6.7
+
1.39 0.22
15.4
046 f 0.04
8-6
4372 f 1126
26.6
1.45 f 033
23.1
1.80 10-4
f 0-47 x 10-4
26.3
85.6 f 3.1
3.6
CoV = coefficient of variation.
This study uses Selby’s (1980) classification, with further breakdown of the measured parameters using the
modified RMS chart of Moon (1984). Eight parameters are incorporated in the classification system. These
are: intact rock strength, joint width, joint spacing, continuity, joint orientation, state of rock weathering,
joint roughness, and groundwater outflow. To each of these is allotted a weighting dependent on the relative
influence of that parameter according to the modified scale of Moon (1984). For the graphical representation
of the system, the total RMS rating is plotted against slope inclination at each site. Superimposed on the plot
is the RMS envelope as refined by Abrahams and Parsons (1987).
For each morphological terrain, the mean rock mass strength (RMS) values were calculated for
comparison with the mean of the morphometric indices for that terrain. The mean RMS data for each
morphological terrain are listed in Table 11. Individual RMS values were not plotted against individual site
shape indices due to: (1) considerable variations in strength properties and consequent lack of representativeness of samples taken from individual sites; (2) variations in form between and within adjacent valleys
apparently unrelated to RMS; and (3) the small-scale maps (1 :63 360) from which the morphometric data
were derived are of dubious accuracy. The mean value from each region was considered to reduce the
compound error and allow comparison between the study regions.
The existence of a genetic relationship between rock strength and shape of the appropriate erosional
landform must be demonstrated for the rock intact and mass strength data to be applied to the examination
of the erosional landforms. To this end, a variety of morphometric properties of glacial valleys were
investigated.
GLACIAL VALLEY MORPHOMETRY
Valley-side slope inclination was of limited utility in the description of the glacially moulded rock slopes due
to the pervasiveness of structural controls on slope form. An attempt was made to fit parabolic curves to the
slopes to enable their numerical description following Svensson (1959), Graf (1971), Doornkamp and King
(1971), and Aniya and Welch (1981). However, structural controls and difficulty in matching the valley
profiles reduced the utility of this method. Another problem was the nature of the topographic maps
available for the analyses. The small scale and frequent inaccuracy of the NZMS Series 1, 1:63 360
45
GLACIAL VALLEY MORPHOMETRY
topographic maps reduced their suitability for detailed morphometric analyses. Insufficient time and
resources were available to allow detailed field surveying of the slopes.
However, the small scale of the maps was sufficient to allow the generation of quadratic curves to fit the
complete cross-profile,without the need to treat the opposing valley sides separately following the method of
Wheeler (1984). The curve:
Y = a + bX c X z
(1)
+
can extend below the zero datum of the coordinate system used and its turning point can be both
altitudinally and laterally shifted relative to the mid-point of the valley through the general character and
asymmetry of the cross-section (Wheeler, 1984). Better fit to the valley cross-profiles can be obtained by
fitting third or fourth-order polynomials, but the problems in interpreting the derived equations outweigh
their usefulness. Good quality aerial photographs were used to identify the surface materials and to
supplement the maps with limited ground control. Within each study region, the relatively uniform structure
and lithology holds the effects of geology constant. At least 15 curves were fitted to representative troughs
from each region and the mean value of the coefficients a, b, and c are given in Table 11.
The coefficient c is sensitive to the slope of the parabola, so that if the parabola is a good fit to the glacial
valley cross-section, then c is a good indicator of the variation in slope inclination between the terrains. An
increase in c reflects a steepening and narrowing of the curve, whilst a and b control the location of the curve
turning-point and are interrelated. Consequently, the exponent c appears to be the most appropriate
exponent for application as a valley shape index. This is supported by the relationship between form ratio,
shape factor, and the coefficient c (Table 111).
The form ratio ( F R ) and shape factor (f)were also measured for the same profile for which the quadratic
equation best fitting the profile was derived. F R is a useful measure of trough form, but it is not of itself an
adequate shape descriptor as ‘ U and ‘V-shaped troughs can have the same F R (Graf, 1971; Andrews, 1972).
Form ratio was defined as the width of the trough at the trim-line, divided by twice the valley depth below the
trim-line. The term ‘trim-line’ refers here to the break in slope between a broad upper valley form and the
lower glacial trough. At least 15 FRs were measured for each region and their mean values calculated. The
Table 111. Correlation matrix for rock field and laboratory strength versus valley shape measures
FR
-
f
Density
a
b
C
RMS
ShxUwt
LA
Schmidt
~
FR
f
Density
a
b
C
RMS
Sh x Uwt
LA
Schmidt
1.o
0.89
p < 0.05
- 0.97
p<01
0.62
ns
- 0.95
p < 0.05
-097
p < 0.01
-0.93
p < 0.05
0.12
ns
-011
ns
- 0.21
ns
1.o
- 0.81
ns
070
ns
- 0.88
p = 0.05
-086
p < 0.07
-082
p < 0-1
0.17
ns
-055
ns
0.17
ns
1.o
- 078
ns
092
p < 0.1
099
p <002
099
p < 0.005
0.08
ns
006
ns
0.10
ns
1.0
- 042
1.o
ns
- 043
ns
- 078
ns
0.19
ns
- 029
ns
- 0.10
ns
097
p < 001
078
ns
0.02
ns
022
ns
0.1 1
ns
1.o
0.86
p < 0.06
0.02
ns
0.12
ns
0.09
ns
1.o
0.1
ns
007
ns
0.20
ns
1.o
- 071
ns
0.96
p<o.o1
1.0
- 0.84
p<01
1.0
~~
ns = not significant at 90 per cent level, p = level of significance.FR = form ratio,f= shape factor, density = glacial drainage density,
a = exponent a, b = exponent b, c = exponent c, RMS = rock mass strength, LA = Los Angeles abrasion resistance, Schmidt
= Schmidt
hammer rebound, Sh x Uwt
=
Schmidt hammer rebound x unit weight.
46
P. C. AUGUSTINUS
height of the trim-line was estimated in each region from the upper level of glacial planation. The results are
given in Table 11.
The shape factor (f)is defined as: the cross-sectional area divided by the height of the trimline above the
thalweg multiplied by the trough perimeter (Nye, 1965; Matthews, 1967).The mean value for each region was
calculated from at least 15 readings from each morphological terrain, and the results given in Table 11.
Glacial drainage density (density of glacially moulded troughs) was assessed for at least 14 troughs from
each region (Table 11). Glacierized troughs were identified on the basis of examination of aerial photographs
and topographic maps with minimal field checking. The glacial drainage density for Fiordland represents the
mean value for both northern and southern Fiordland. Drainage densities were not calculated separately for
the regions due to the observation that the drainage densities were not significantly different statistically,and
to the problem of the small number of drainage basins available from each area.
RESULTS
The mean intact rock strength and index values were plotted against the appropriate morphological
variables (Table 111). No clear trends are developed, with considerable scatter being the rule. For the point
load strength index, any relationship between the morphometric variables is complicated by the effect of rock
anisotropy on the results, and consequent difficulty in interpreting the data. Rock unit weight, Schmidt
hammer rebound, and Schmidt hammer rebound weighted by the appropriate unit weight, were correlated
with the morphometric measures and the relationships were found to be non-significant (Table 111).
The lack of a significant relationship between the strength and morphometric variables suggests that the
variations in intact strength properties of the host bedrock have minimal impact on the mode of erosional
development and enlargement of the glacial troughs. The scatter of the results also reflects the importance of
rock mass anisotropy on the results of the strength tests, a factor that was neglected in the study of Cooks
(1983).
A trend towards increasing drainage density with increasing RMS is identifiable (Figure 4a, Table 111).
This is an apparently anomalous result if comparisons with fluvial terrains are valid, for drainage density
would be expected to be highest in the weakest rocks (Cooks, 1983).The relationship between the a, b, and c
coefficientsand the appropriate RMS value is given in Table 111. Correlations significant at the 90 per cent or
higher level are only developed for the relationship between the coefficient c and the mass strength of the
bedrock from each morphological terrain (Figure 4b). The strong positive correlation between c and RMS
reflects a narrowing and steepening of the trough with increasing RMS. Further, the strong correlation of c
with glacial drainage density (Table 111) indicates a narrowing and steepening of the valleys with increase in
the glacial drainage density. This reflects the obvious auto-correlation between the two factors and RMS
control on the development of the landscape.
Similarly, a strong negative correlation was obtained for the relationship between the shape factor (f)and
RMS significant at 90 per cent level ( I = - 0.82, Table 111, Figure 4c), improving to r = - 0.93 when FR
rather thanfwas used in the analysis (Table 111, Figure 4d). These trends confirm the trend to deeper,
narrower troughs with increasing RMS.
The relationship between the valley shape measures and rock mass strength confirms the presence of a
strong causal relationship between the two sets of factors. However, it must be realized that these are only
general trends with marked departures from the mean regression lines being apparent. Further, the standard
error of the data about the mean regression line is large for the RMS of the foliated schist and greywacke
(Table 111). This is interpreted as a reflection of the influence of foliation on the strength of the rock.
DISCUSSION
The trend confirms that in weaker, highly jointed rock masses, such as the Aspiring schist and Mt. Cook
greywacke, broader, flatter glacial troughs develop. The RMS properties of the slope rock control the
amount and rate of slope retreat. Preferential widening of the troughs and consequent slope destabilization
occurs except where structural controls are paramount. Conversely, for the northern Fiordland plutonic
47
GLACIAL VALLEY MORPHOMETRY
0.54I
(C)
*y=-3.17+0.05~
1
y= 1.109-0.008x
0
h
0.6
0.5
0.4
LOP/
0.42
7
2.0,
3 . 2 ~ 1 0- ~
c
50
5
-
y=-o.001+ 1
R=0.8!3
I
0
8
I
2 8 4
RMS
I
8
6
8
(d)
8
i
y=7.97-o.o8x
1.8 >OO
1.4 1.2 -
1.6
:o 1.6x1O4'
O
0
/
,oo
o.8x104
8
0
.e1O*X
.- 2.4X1o4
I
8
'
'
'
' ' '
'
RMS
'
-
I
I
7
8
8
0
8
1
I
2 8 4
RMS
8
6
8
8
Figure 4. Relationships between rock mass strength (RMS) and various measures of glacial trough rnorphornetry: (a) relationship
between glacial drainage density and RMS, (b), (c), and (d) relationships between RMS and c coefficient, shape factor (A, and Form
Ratio (FR) respectively
rocks, the high erosion resistance, plus high level of stability with respect to subaerial processes, has given rise
to deep, narrow troughs where deepening has apparently exceeded widening. The high RMS of the slope rock
in northern Fiordland allows the troughs to maintain their deep, narrow form. The morphological variation
displayed in southern Fiordland is considered to be a consequence of the contrast in erosional response to
differing near-surface stress-fields (Augustinus, in preparation).
No clear relationship between rock intact strength and valley morphology was evident, indicating that
intact strength properties of the host bedrock alone are insufficient to explain the mode of erosional
development of the troughs. This is contrary to the experience of Cooks (1983) who argued that the intact
strength properties of the rocks had a strong control on the morphometric properties of the fluvial drainage
basins he studied. However, Cooks (1983) ignored the input of rock mass anisotropy and jointing to erosion
and landform development, whereas in the present study, strong correlation has been demonstrated between
the field rock mass strength and shape of the glacial troughs. Thus, studies of rock control on landform
development should examine the RMS properties of the bedrock, as intact rock strength properties alone are
probably inadequate for the realistic in situ rock strength characterization for the purposes of evaluating rock
resistance to erosional processes.
The relationship of glacial drainage density to RMS (Figure 4a) can be explained with reference to the ages
and stabilities of the slopes in each morphological region. The relatively high glacierized drainage density
obtained for the Fiordland region is probably a consequence of the high RMS of the host plutonic and high
grade metamorphic rocks allowing only slow landscape modification following repetitive glaciation/deglaciation cycles. Consequently, the present glacierized drainage of Fiordland is a palimpest of the equilibrium
glacial and pre-glacial fluvial drainage systems, with trough deepening being the primary morphological change. Conversely, in the relatively low RMS rocks of the Mts Cook and Aspiring regions,
erosion and denudation rates are significantly higher (Adams, 1980; Whitehouse, 1987), allowing divide
48
P.C. AUGUSTINUS
reduction, drainage capture, and the development of large outlet glacial troughs along lines of structural
weakness (Augustinus, in preparation). This has resulted in the progressive reduction of the glacial drainage
density.
It is possible that if the glaciers have simply exploited a pre-existing valley system, in which the troughs are
deep and narrow, then the final valley shapes may simply be reflecting this earlier influence rather than
having their shape controlled by their rock mass strength. Consequently, glaciers flowing in a deep narrow
valley would produce a deep, narrow valley after deglaciation. Certainly, the pre-existing valleys are a factor
in constraining the flow of the glaciers, and hence are an influence on the final trough cross-profile form,
although the rapidity of uplift and erosion of the central Southern Alps (Whitehouse, 1987, 1988) indicates
that the troughs slopes and interfluves are young, especially in Westland (Basher et al., 1988), and are
undergoing continual modification balanced by rapid tectonic uplift. Hence, the valley slopes will be
undergoing continual rapid subaerial modification controlled by the mass strength of the slope rock, so that
the low RMS terrains of the central Southern Alps are able to maintain broad, open troughs with relatively
low angle slopes irrespective of any pre-existing fluvial valley form.
In fact, the zone of highest uplift and erosion on the western side of the central Southern Alps displays
higher glacierized drainage density than the greywacke terrain on the eastern side of the Main Divide (Figure
4a and Table 11). This pattern is largely a reflection of rapid uplift, with associated rapid fluvial and glacial
erosion preferentially exploiting zones of structural weakness on the western side of the Southern Alps
(Figures 2 and 3). Consequently, the Westland valleys adjacent to the zone of greatest uplift and erosion are
muchyounger than those developed in the Mt Cook region on the eastern side of the Alpine Fault where the
uplift rates are of an order of magnitude lower (Whitehouse, 1987, 1988; Augustinus, 1988).
IMPLICATIONS FOR THE HIRANO AND ANIYA (1988) MODEL
Hirano and Aniya (1988) developed the ‘b-FR diagram, plotting the b exponent from the power equation
Y = axbagainst the appropriate form ratio (FR)defined as channel depth divided by twice the width. Using
data from the Canadian Cordillera and from Graf (1971) they found a clear linear trend, with the b value
becoming larger with increasing FRs. One type of development, represented from the alpine data, is from a
shallow, wide ‘V’ shaped trough to a deep ‘U’ shaped trough. For this model to be tested in the glacierized
troughs in the New Zealand Southern Alps it must be assumed that the valleys being examined are of the
same age and developed under the same glacial regime. Morphological data obtained from deglaciated
valleys in Southern Alps were examined to test the ‘b-FR diagram’.
Figure 5 indicates the relationship between b exponent and FR for the Southern Alps glacial troughs. The
FR data has been recalculated from that used in Table IT to conform to the equation employed by Hirano
and Aniya (1988). The Southern Alps data displays a similar trend to that obtained for alpine-type glacial
systems by Hirano and Aniya (1988). If we accept their argument that the ‘b-FR diagram’ gives a
developmental trend for glacial valley cross-profile shape, then the results for the New Zealand Southern
Alps suggest a trend in development from a shallow, wide ‘V’ shaped, to a deep ‘U’ shaped valley similar to
that apparent from the Canadian Rocky Mountains.
Certainly, such a trend seems to be apparent in Fiordland, where deep narrow troughs are preserved due to
the high RMS of their slope rock, but in the older, lower RMS terrains of the Mts Cook and Aspiring regions,
it appears that the trend is for valley widening with increasing age of the landscape. This latter trend is a
product of the relatively low RMS and instability of the valley walls, rapid removal of the erosion products,
as well as the contemporary tectonics and pattern of uplift (Augustinus, 1988).
Thus, whilst superficially it appears that the present study supports Hirano and Aniya’s (1988) results and
model for glacial valley development, it is argued here that a simple comparison of FR and b values is not
sufficient. This is because the age, as well as the mode and rate of rock slope development of the valleys whose
morphogenesisis being evaluated, is critical to the analysis. For example, whilst the Southern Alps data seem
to complement the results from the Rocky Mountains, due to rapid late Cenozoic uplift and erosional
response of the bedrock, the genetic model applicable to the Southern Alps is not a simple one of progressive
glacial valley development controlled by subglacial erosion working towards minimizing friction between ice
GLACIAL VALLEY MORPHOMETRY
49
Form ratio (FR)
Figure 5. The ‘&FR diagram’ calculated from the valley shape data from the New Zealand Southern Alps
and bedrock. This is because the glacial valleys developed in Southern Alps are of different ages and have
undergone contrasting uplift and erosional histories, the latter being controlled by the rock mass strength of
the eroded substrate, and the former by the differing tectonic regimes. Clearly the age of the landform is
important, as is rock type (hence RMS), so that the modification of the slopes and hence that of the valleys,
will occur more rapidly in low RMS bedrock that is being rapidly uplifted and eroded, and less so in regions
of higher RMS bedrock.
CONCLUSIONS
The rock mass strength properties of the slope rock are an important control on the cross-profile shape of
glacial valleys, with the trough slopes having forms closely influenced by the mass strength of the bedrock.
The steep rock slopes formed on plutonic rocks in Fiordland have high RMS, and remain stable, displaying
no evidence of deep-seated rock slope failures, so that modification of the rock slopes is slow. Conversely, the
lower RMS closely jointed and foliated schist and greywacke are relatively unstable and have their slopes
controlled by the orientation of the foliation, and modification of the rock slopes is rapid. This is further
supported by the low rates of talus accumulation apparent at the trough walls and floors in Fiordland
relative to the far higher rates on the central Southern Alps greywacke and schist (Whitehouse, 1987).
The relationship between intact rock strength and glacial trough form is poorly developed. In particular,
rock abrasion resistance was found to be a non-significant factor with respect to final trough shape. RMS,
particularly the joint properties of the bedrock, best explained the variations in trough form. The
greywacke/argillite in the Mt Cook region displays a high abrasion resistance, but the close joint spacing
gives it a low RMS that should a h w rapid subglacial erosion by plucking as well as greater subaerial slope
instability and potential for slope retreat.
Rock slope modification would be active during both glaciations and interglaciations, with rock slope
failure being most prominent following glacier retreat when removal of the buttressing glacier promotes
destabilization of the rock slopes due to the redistribution of internal rock stresses (Bovis, 1982).As well as
promoting slope failure and consequent modification of the slope morphology, weakening of the slope rock
would have the effect of providing zones of weakened rock that would be exploited by a readvancing glacier,
50
P. C.AUGUSTINUS
so that the mode and direction of enlargement of the glacial valley cross-profile will depend on the location of
the stress-induced rock weakening. The influence of in situ rock stresses on glacial valley cross-profile shape
will be considered in a subsequent paper (Augustinus, in preparation).
It is apparent that the RMS and age of the landscape are the predominant controls on the morphology of
the glacial valleys in the tectonically active New Zealand Southern Alps, with the glacier providing an
important mechanism allowing the removal of the weakened material. However, it appears that glacial
erosion was not the prime control on glacial valley cross-profile shape in the Southern Alps and is therefore
unlike the situation argued for alpine-type systems elsewhere by Hirano and Aniya, (1988, 1989).
Hence, whilst Hirano and Aniya (1988) showed on theoretical grounds that the erosion of the cross-section
of a glacial valley will proceed from a ‘V’ to a deep ‘U’ shape, the model did not consider the erosion
resistance of the eroding substrate and the rock mass strength of the valley walls that will clearly have an
important influence on the final glacial valley cross-profile morphology. Erosion resistance was considered in
the study of Harbor et al. (1988)in modelling the enlargement of a ‘V’ shaped channel provided with a zone of
material at the valley base twice as erodible as the surrounding rock. This input to the model allowed the
glacier to excavate a deeper trough than a model run with channel profile bedrock of uniform erosion
resistance, although it is not clear what property of the rock mass causes the material to be more erodible.
A prerequisite for the development of a realistic model of glacial trough development is the input of the
strength properties of the rock mass that actually control the glaciers’ ability to erode the bedrock substrate,
rather than some ideal index of rock erosion resistance that might have little influence on the erodibility of
the rock mass. Further, such a model must also consider the RMS properties of the slope rock, since this
parameter will control the rate and direction of glacial valley slope development and modification, and hence
on the shape of the glacial valley cross-profile. This is particularly relevant when the slopes are destabilized
after the retreat of the glacier that previously buttressed the trough wall.
ACKNOWLEDGEMENTS
This study was undertaken whilst the author was the recipient of a Commonwealth Scholarship and
Fellowship Plan Award at the Department of Earth Sciences, University of Waikato. The Chief Rangers of
Westland, Fiordland, and Mts. Cook and Aspiring National Parks are thanked for allowing me to undertake
field work in their domains, as well as for free use of park huts.
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