have stripped weathered rock in some places and not in others.
All survey heights were used to generate the trend surface for BD (figure 5.21) and for the hand-contoured map (figure 5.20). As explained earlier, the transects were laid out radiating from the
summit, and on the residual itself they follow the direction of maximum slope. The extension of these lines in the direction onto the subaerial pediment surface generally follows the direction of maximum slope closely
on transects 1, 3, 5, 6, and 8; however, on transects 4, 7, 9, and 10 they run obliquely to the maximum slope. The greatest elevation on
the footslope is at the nick on the western side of the residual: contours are concentric on this point on both the trend surface and on the hand- contoured map.
5.2.2 TRANSPORTED REGOLITH
5.2.2.1 Backing hillslope
The regolith on backing hillslopes varies considerably from place to place in response to lithological differences. In Spring Valley
(BB and BC) remnants of a plateau surface are preserved on agglomerate
and chert. On both sides of the valley a discontinuous free face is
developed in this agglomerate and the subjacent vesicular basalt
(figure 5.5). Weathering and erosion of the free face produce
large boulders, often several metres in maximum dimension, and these tumble down the steep hillslope and are either trapped on the way by
other boulders or accumulate at the nickline. Outcrops of vesicular
and lion-vesicular basalt are frequent on the backing hillslopes, and weather to yield debris ranging in a continuum from very fine sand to large boulder size. Mean particle size appears to decrease
downslope. Pebbles and cobbles commonly originated as well-rounded
amygdules. Many boulders are also well-rounded, but probably owe this
characteristic to their origin as corestones: some are clearly being
exposed at the present time and are often well-weathered. In contrast,
vesicular basalt forms angular boulders. All this material is frequently
varnished.
Finer material is also found, particularly concentrated in small anastomosing rills which are developed extensively between rock
outcrops. This material is frequently oxidised.
At both BB and BC weathered bedrock is generally found either at or within a few centimetres of the surface of the backing
hillslope. The varied outcrops and deposits produce very rugged micro
relief, and walking is hazardous.
On BD the characteristics on the steep slopes of the residual make it impossible to reach the summit from certain directions. Apart from an outcrop of vesicular basalt up to 2 m thick (figure 5.8), the
hillslope is formed of rotten
in situ,
basalt with a discontinuouscover of sand and small gravel with occasional pebbles and cobbles.
This material is commonly varnished, and particle size appears to decrease
downslope. When trodden upon these larger fragments slide over the
finer material below, which in turn slides on the rotten rock. The
surface feels very insecure and slippery. In contrast the summit
of the residual is blocky and large boulders are found. The rock at
the summit is very resistant and forms flat surfaces on neighbouring
mesas and buttes. However, at BD slope tetkeat has reached the stage
2
Some boulders originating from the resistant stratum have fallen to the nickline. None are found on the flanks of the residual: the slopes are too steep and irregularities are virtually absent to arrest the fall of a boulder.
As at BB and BC, oxidised fine material is also found, particularly in small anastomosing rills on the hillslope.
5.2.2.2 Nick
Near the nick there is a rapid decrease in the % volume of material of cobble size and coarser:
mean % of cobbles and
EB BC BD larger mean % 3 m upslope of x of cobbles and 60 60 80 larger at x 30 30 20
Large boulders are found at the niclcline, and occasionally a few metres beyond, and appear to be derived from the resistant rocks forming
the free face.
Sands and gravels are commonly deposited at the nick, often *
in the form of small fans opposite small re-entrants. However,
accumulations of detritus are rarely more than 0.1 m thick, and overlie weathered rock (5.2.2.3.5), or may be absent altogether (e.g. BB2,
BD1, and BD9).
5.2.2.3 Footslope
5.2.2.3.1 Coarse surface lamina
A loose surface lamina of sand-size material up to 1 cm thick,
but generally less than 2 mm , is common on basalt footslopes. It
comprises fragments of rock which are usually friable and weathered. Although influenced locally by ant activity and microtopography such as tussock mounds, the distribution of this material shows a general decrease in % area covered downslope:
Sampling Points a - f 9 h i 3 BB 75 70 71 53 40 BC 80 68 62 41 37 BD 71 65 51 36 19 Mean 75 68 61 43 32
The material is poorly sorted near the nickline where it is part of a continuum of comminuted material varying from boulder to fine
earth size. Further away from the nick sorting improves, and although
additional coarse material is probably derived from the weathering of gibbers and bedrock exposed on the subaerial surface the material becomes smaller in size (data are in phi units):
Sampling Points a - f 9 h i 3 BB -0.83 -0.80 -0.46 -0.21 -0.10 BC i o Cn - 0 4 6 -0.62 -0.43 -0.21 BD -0.62 -0.34 -0.18 -0.02 0.36 Mean -0.80 -0.67 -0.42 -0.22 0.02 ________________________ i____________________________________________________________
This lamina is easily distinguished from the deposits immediately below: the latter include larger proportions of fine earth and are generally
cemented together in the dry season to form a hard surface. In many
places there is a thin lichen-covered patina to this surface and on BD cracks are found. The coarse surface lamina forms a loose deposit on this hard surface.
5.2.2.3.2 Gibbers
The gibbers found on basalt footslopes are predominantly comprised of various types of basalt. Depending on location varying proportions of chert, agglomerate, quartz,^amethyst, and amygdules
(e.g. chalcedony, rock crystal, prehnite, quartz, and agate) are also found (see figure 5.22). 'lost gibbers are weathered, and some basalt gibbers can be broken by hand.
On the east side of Spring Valley (BB) vesicular and non- vesicular basalt gibbers are found in approximately equal proportions
(38% and 35% respectively). Gibbers of agglomerate and chert forming
23% of the total sample are probably derived from the caprock on
the backing hillslope. Gibbers of quartz, rock crystal, amethyst,
chalcedony, and agate together form less than 4% of the total sample. Agate is found only along transect 7, and mainly around
On the west side of Spring Valley (BC) amygdules of agate and
quartz comprise nearly 36% of the gibber sample. Vesicular and
non-vesicular basalt comprise 52% (in approximately equal proportions), and chert and agglomerate (also apparently derived from the topmost stratum exposed on the backing hillslope) together comprise the remaining
12%.
On BD vesicular and non-vesicular basalt gibbers together comprise 99.6% of the total sample (15.5% and 84.1% respectively); prehnite and quartz form the remaining 0.4%.
Gibber density is weakly correlated with distance from
the nickline in a least squares regression. The correlation coefficients ■
are higher for an exponential regression, especially at BB:
BB BC BD v for y=mx+c *> -0.385 -0.131 -0.110 significance level (p) 0.02 0.50 0.50 bx r for y=ae -0.660 -0.312 -0.285 significance level (p) 0.001 0.05 0.10
The exponential relationship for BB explains 44% of the total variation in the data, but for BC explains only 10%, and for BD explains only 8%.
2
The third-degree trend surface of log q °f gibber density/m
for BB explains 58% of the variation in the data: mean gibber density decreases logarithmically away from the nickline (figure 5.23).
An unusual pattern is found on the fourth-degree surface
of BC (figure 5.24). At the northern end of the footslope gibber
density increases logarithmically away from the nickline. In contrast,
towards the southern end of the footslope gibber density decreases logarithmically from the nickline to the footstream.
2
The mean gibber density/m’ at BB=23.1, at BC=12.3, and at
BD=16.6.
The maximum values of mean gibber size at sampling points
with six or more gibbers at BB=1742 cc , at BC=1515 cc , and at BD=783 cc . Mean gibber size in cc is poorly correlated with distance from the nickline although the correlation coefficients are significant at BB and BC:
BB BC BD T for y=mx+c -0.416 -0.372 -0.172 significance level (p) 0.02 0 i 10 0.40 .. bx v tor y=ae -0.518 -0.399 -0.201 significance level (p) 0.02 0.05 0.40
The fourth-degree surface of l o g ^ of mean gibber size on BB shows
a general logarithmic decrease in mean gibber size away from the nickline
(figure 5.25). However, at the extreme downslope ends of transects
5, 7, and 9 especially there is a local increase in mean gibber size. This is mirrored in part on the other side of the valley (BC) where the downslope end of transect 7 also shows a local downslope increase in mean gibber size (figure 5.26).
No systematic trends in mean gibber size are shown on the trend surfaces for BD.
*
Trend surfaces of gibber shape are not shown here. In most
cases they are not significant at 0.20 p . The few surfaces which
are significant at less than 0.10 p are correlated with the lithology
of the gibber sample rather than position on the footslope. For example,
gibber roundness appears to be correlated with the relative proportions of vesicular basalt and amygdules in the gibber sample, and these
proportions do not appear to be related to position on the footslope.
5.2.2.3.3 Surface samples
The mean particle size of the surface samples (in phi units)
decreases away from the nickline. The best-fits of mean particle size
against distance from the nickline to linear and exponential regression have the following correlation coefficients, all of which are highly significant:
BB V for y=mx+c significance level -w -p bx r for y=ae significance level +0.410 (p) 0.01 +0.523 (p) 0.001 BC BD +0.537 +0.545 0.001 0.001 +0.546 +0.538 0 . 0 0 1 0 . 0 0 1
The third-degree surface for BC shows mean particle size decreasing downslope and reaching a minimum value of 3.5 phi (very fine
sand) near 7j (figure 5.27). However, the trend has a major directional
component other than normal to the nickline: the coarsest materials (0.5 phi, i.e. coarse sand) are found near the nick on transect 3, and particle size decreases radially from here.
The third-degree surfaces for BB (CD=0.367) and BD (CD=0.354) also indicate that mean particle size decreases downslope, but directional components of the trend other than normal to the nickline are also
important.
Trend surfaces of % gravel, % sand, % fine earth, mean phi of the gravel fraction, mean phi of the sand fraction, and mean phi of the fine earth fraction show general agreement with their respective surfaces of mean particle size. However, the coefficients of determination are almost always lower for these other measures than for mean particle
%
size, suggesting that mean particle size is generally the best measure of the size of the surface material.
5.2.2.3.4 Form of the transverse profiles and isopach surfaces There is a discontinuous cover of transported regolith on
bedrock pediment surfaces, varying considerably in depth from less than a centimetre to a maximum of 3.80 m on BB6 as shown in Appendix II.
In nearly all instances the upper (i.e. subaerial) surface of the transported regolith is more regular than the surface where it meets
bedrock. On BB the transported regolith is generally less than
0.5 m thick at the nickline (except on transects 4 and 6) and often for
considerable distances downslope (e.g. transect 8). In places it is
totally absent (e.g. transect 2). The transported regolith is usually
On BC and BD the transported regolith is generally less than
0.5 m thick, *except at places on BC2 and 3D5 and 8. It is totally
absent locally (e.g. BC5, and BD2), and along transect BD7 it is nowhere thicker than 0.05 m.
Isopach surfaces of the depth of transported regolith have lower coefficients of determination than the corresponding trend surfaces of subaerial pediment height and bedrock pediment height for reasons given earlier (4.2.2.3.4).
The fourth-degree surface for BB (figure 5.28) shows an
increase in depth with distance from the nickline at only the northern end of the footslope. At BC on the other side of the valley (figure 5.29) the depth of the transported regolith is everywhere less than 0.90 m ,
and the trend shows that the depth bears no general relationship to distance from the nickline.
Depths are also shallow on BD (figure 5.30). The greatest
depths are at the southern end of the footslope.
5.2.2.3.5 Soil profiles
Soils on basalt footslopes are generally shallow and skeletal, although cracking clays with uniform texture profiles and friable red earths with gradational texture profiles are also developed. Along major drainage lines alluvial soils are characteristic.
The soils on BB are typical of basalt footslopes with colluvial
transported regoliths. The profiles at the nickline are generally
thin and skeletal. Saprolite is usually found within 0.5 m of the surface
and is generally light grey in colour (10YR 7/1) although a mottled
zone is occasionally found. The junction with the colluvial layer
is everywhere sharply defined. The colluvial material comprises
large angular and rounded rock fragments, gravels and sands, with
finer material forming about 20% of the total sample. It is dull brown
in colour (7.5YR 5/4). At 4x and
6x
there are deeper accumulations ofsuch weathered colluvial material. Thin sections indicate that eluviation
is important and some evidence of compaction is given by the orientation of individual grains.
The soils on the lower part of the footslope are of two sorts: