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4.2.1.1 Backing hillslope

Profiles of the backing hillslope are shown in AppendixH, and the slope data for each transect are given in figure 4.10: all correlation coefficients are significant at 0.001 p.

At Springvale Pediment (GA) all transects show irregularities in detail corresponding to large slabs or boulders. The slope of the steepest segment varies from 9.1* to 27.5*. The mean value for all transects is 19.3*.

On the residual GB there are fewer irregularities in detail. The range of values for the steepest segment (10.0* to 22.1’) does not indicate a boulder-controlled slope of constant angle from place to place. The mean value for all transects is 16.2*.

Nearby on GC the hillslopes are different. On transects 3, 5, 6, 7, and 10 the steepest segment is separated from the footslope by another unit: on transects 3, 5, 7, and 10 this unit is a convex element, and on transect 6 it is a segment at a lower angle. The mean value of the steepest segment is 23.1*.

At Pompey’s Pillar (GD) large slabs of bedrock crop out immediately above the nick albng transects 1, 3, 4, and 5 to form a slope segment between the steepest segment and the footslope. The mean value of the steepest segment is 24.4*.

The low mean value of the steepest segment for the residual GB is highly significantly different from GC (at 0.005 p) and GD

(at 0.001 p), but not from GA (only at 0.20 p ) . The high mean value at GD is significantly different from GA (at 0.05 p) but not from GC (only

at 0.20 p ) . Clearly, the slope of the steepest segment varies both at one location and between locations.

4.2.1.2 Nick

The nick is angular on every transect surveyed: the angularity ratio is given in figure 4.11. The mean value at GA is 1:8, at GB is

1:15, at GC is 1:13, and at GD is 1:15. Coefficients of variation are high, except at GC, and the only highly significant difference between means is for GA and GC (at 0.01 p ) .

The nickline depressions at both GC and GD have not changed the basic angularity of the nick - indeed, it has been emphasized by the meeting of the negative slope of the footslope (looking towards the hillslope) and the positive slope of the hillslope.

The development of a nickline depression is favoured by the presence of saprolite at the niclcline. However, saprolite is found in profiles on most parts of the footslope, and the formation of a depression at the nickline in the East Kimberleys appears to

require that at least one of four situations obtains. First, a nickline depression may form by the lateral diversion of a stream flowing off the backing hillslope. Second, it may form where there is a pronounced longitudinal slope near the nickline. Third, a nickline depression may form where the nickline corresponds to a major joint. None of the cases

studied in detail showed this, but such situations were found in the area west of Bow River Station. Fourth, a nickline depression forms where a stream has been guided by a combination of structure and

topography in nearby areas as it has been extending itself headwards. At GA a small stream crossing the nickline near

8x

is diverted laterally along the nicklinq, by a small tor, but peters out after

about 10 m .

The nickline depression at GD has not effected a general reversal of the transverse slope, except on transect 9. This nickline depression appears to have formed by the lateral diversion of a small stream flowing off the backing hillslope near

lOx

which was probably favoured by the pronounced slope in a longitudinal direction near the nickline between transects 10 to 6 inclusive (see figure 4.21).

The main channel does not follow the nickline closely, but numerous small tributaries erode headwards to that point. The stream is diverted

north-westward across the footslope by a quartz reef near transect 5 (figure 4.46).

To the NE of Alice Downs No. 2 Pediment (GC) a small tributary to the Little Panton River meanders between isolated residuals and tors and is incised a few metres into the weathered rock surfaces between

them, and it appears almost by chance that the headwaters of this tributary reach the nickline at GC. The stream is actively eroding headwards

into saprolite as shown by erosion between painted nails inserted into the ground in 1971 which were examined again in 1972 (figure 4.43).

In detail the stream follows a tortuous path within the nickline depression (figure 4.44) determined partly by the outcrop of dikes, and in places

the main channel is separated from the nick by a few metres. The

channel has cut up to 2 m below a reconstructed former subaerial surface

(figure 4.45). The lowering of the former surface is indicated by

numerous gibbers on plinths, and by the emergence of corestones,

especially in the stream channel. The detailed relief of this nickline

depression is characterised by rounded interfluves (figure 4.43) and interlocking spurs, and in this respect is similar to GD.

4.2.1.3 Subaerial Pediment Surface downslope of

x

Transverse profiles are shown in Appendix 11, and data for

the mean slope between sampling points J and

x

are given in figure

4.12. All correlation coefficients are significant at 0.001 p. The mean value at GC is distorted by the negative slope (looking

towards the hillslope) in the upper parts of transects 6 to 10 inclusive, associated with the nickline depression. At GD, transects 6 to 10

inclusive also cross a nickline depression, but a negative slope has

not developed, except along transect 9. The mean value for GD is

significantly higher than GA and GB at 0.10 p, and GC at 0.001 p.

GC has a significantly lowe£ mean than GA at 0.01 p and GB at only 0.20 p. There is no significant correlation between the mean transverse slope and slone leneth except on GC, where there is a strong inverse

relationship.

Data for the slope between sampling points

g

and

x

are

given m figure 4.13. All correlation coefficients are significant

at 0.001 p. The negative slope on profiles crossing the nickline

depression at GC is reflected in the low mean value, and to a lesser

extent this may also be important at G D . The low mean value at GC

is significantly different from GA at 0.001 p, GB at 0.005 p, and GD

at 0.05 p. The low mean value at GD is significantly different from

GA at 0.005 p. The value for the residual GB is only significantly

lower than GA at 0.20 p. Finally it should be noted that (a) the mean

slope between

g

and

x

is greater than the mean slope between

j

and

x

at GA (significant at 0.001 p) and GB (significant at 0.10 p), and

(b). the mean slope between

g

and

x

is less than the mean slope between

Data for the slope between sampling points

j

and

g

are

given in figure 4.14, and all correlation coefficients are significant

at 0.001 p. The highest mean value is at Pompey’s Pillar Pediment

(GD) and is highly significantly different (at 0.001 p) from other cases: there are no other significant differences between the other

three'cases. The mean slope between j and

g

is less than the mean slope

between j and

x

at GA (significant at 0.01 p), Gß (significant at only

0.20 p ) , and GC (0.50 p); at GD the values are the same. The mean

slope between J and

g

is less than the mean slope between

g

and x

at GA (significant at 0.001 p), and GB (significant at 0.005 p ) .

The mean slope between j and

g

is greater than the mean slope between

g

and

x

at GC (significant at 0.10 p), and at GD (0.40 p ) .

Values of the coefficient

b

for best-fit regression to

bx

the exponential regression y = ae and the correlation coefficient

r for the goodness of fit are also given in figures 4.12, 4.13, and

4.14. Values of

b

are low indicating that the exponential increase in

slope near the nick is very weak or may be absent altogether. However,

if the mean correlation coefficient for y = mx+c and the mean correlation bx

coefficient for y = ae are compared for each case, it is found that

(a) for the slope between j and

x

the exponential

equation gives a significantly higher correlation on GA at 0.01 p, on GB at 0.001 p, on GC at 0.20 p, and on GD at 0.10 p.

(b) for the slope between

g

and

x

the exponential

equation gives a significantly higher correlation coefficient on GA at 0.001 p, on GB at 0.001 p, on GC at 0.20 p, and on GD at 0.70 p.

(c) for the slope between

j

and

g

the exponential equation

gives an identical mean correlation coefficient on GB,

GC, and G D . On GA the mean correlation coefficient

for the exponential equation is higher, but is

only significant at 0.80 p. Clearly, a linear

relationship describes the profile of the lower three-quarters of the footslope just as well as an exponential relationship.

The footslope must also be considered in longitudinal profile. With the exception of the channel from the nickline depression at GD which makes an abrupt turn across the footslope (figure 4.21), linear erosion is confined to the nickline and the end of the footslope.

Longitudinal profiles are remarkably rectilinear over great distances

(as shown in Appendix TjJ) . The slope of longitudinal profiles is

influenced by the regional slope of the land, usually in the direction

of flow of the footstream. Occasional irregularities may be formed

by isolated piles of boulders (e.g. on GB ) .

Figures 4.15, 4.17, 4.19, and 4.21 are hand-contoured maps of GA, GB, GC, and GD respectively, and figures 4.16, 4.18, 4.20,

and 4.22 are the corresponding third-degree trend surfaces. For cases

GC and GD the detailed level data of the nickline depression are not used in the trend surface analysis for two reasons: first, the nickline depressions are local features and are not part of the trend we are seeking to identify; second, a cluster of closely-spaced data points (relative to the remainder of the data points) exerts too strong an influence on the

generation of a best-fit surface. However, the data for the nickline

depressions are used for the hand-contoured maps.

On GA the direction of maximum slope on the subaerial pediment surface is not collinear with the direction of maximum slope on the backing

hillslope. Near the nickline the transverse slope is steeper than

the longitudinal slope. However, as distance from the nickline

increases, the maximum slope becomes predominantly longitudinal in direction.

*

On GB (figure 4.17 and 4.18) the slope of the subaerial

pediment surface is more complex. The presence of the other isolated

residual to the south of GB (shown in figure 4.7) is implicit in the

hand-contoured map (figure 4.17). (Although this is also implied on

the trend surface map (figure 4.18) it is wrong to use it to infer the

form and position of an earlier surface since "any least-squares regression

surface must3 by definition_, slice through the known data points"

(Tarrant 1970). The reasoning which follows is therefore based on the

hand-contoured map).

The trend of the contours suggests that the two isolated

residuals once formed a single residual. At the present time certain

transects radiating from the summit of GB follow the direction of maximum slope closely (e.g. transects 3, 4, and 5), whereas others

do not (e.g. transects 8, 9, and 10). However, if we imagine a

network of transects radiating from an assumed centre of a former single residual (e.g. XA and XZ on figure 4.17), the direction of

most transects would approximate to the direction of maximum slope. On GC (figures A.19 and 4.20) the maximum slope of the subaerial pediment surface is collinear with the direction of maximum slope on the backing hillslope on transects 1, 2, and 3 only. The lack of collinearity on other transects was not detected in the field.

On GD (figures 4.21 and 4.22) the trend of the subaerial pediment surface is particularly striking. The third-degree surface

(CD=0.976) is only very slightly different from the linear surface (CD=0.965). The contours do not radiate from the hill as might be expected, but form an almost planar surface of remarkable regularity. Again, this was not evident in the field, partly on account of

restricted visibility due to trees, and partly because slopes are gentle.

4.2.2 TRANSPORTED REGOLITH

4.2.2.1 Backing Hillslope

The regolith on the backing hillslope comprises boulders, gibbers, and grus. The boulders originated as corestones and are generally varnished and rounded, although split corestones with unvarnished faces are also found (figure 4.23, and Whitaker 1974a). There is considerable variation in boulder size from less than a metre

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