Minor forms developed on steep slopes - Landforms and Geology of Granite Terrains - page 190

rocks, whereas the backing ridge consists of granite. In many other instances, however, no such
lithological contrast can be detected. On the contrary, the bedrock in the floor of the depressions
is apparently of the same type as that exposed on the adjacent hills. Minor scarp-foot depressions
are undoubtedly caused by runoff scouring the inner fringe of the regolith where it laps up against
the bare rock hillslope, as, for instance, at Chilpuddie Rock, on northwestern Eyre Peninsula,
South Australia. Cotton (1942, p. 42) pointed out that rivers in some arid and semi-arid regions
“ consistently hug the mountain bases in a manner that cannot be fortuitous ” and went on to
suggest that the linear depressions are river channels. There is here, however, a possible confusion
of cause and effect. Moreover, accepting the notion of scarp retreat, Cotton (1942) argued that the
mountain fronts and river channel depressions must recede together. This hypothesis is difficult to
sustain, for very few of the depressions observed in Australia and North Africa are occupied by
streams. In those that are, it may be a case of the drainage channels running in the depressions
rather than excavating them. Furthermore, some of the depressions stand at the base of residuals
too small to generate streams, and others occur where no streams debouch from the uplands.
The most plausible explanation for scarp-foot depressions is that runoff from the hills saturates
the piedmont zone, which is in consequence, intensely weathered. There is ample evidence of such
chemical attack in a wide variety of lithological settings. Some products of weathering are evacu-
ated in solution. Some fines may by flushed out, or deflated by the wind. There may be a volume
decrease, compaction and settling, and surface subsidence. The water table is lower ( Fig. 8.6).
Additionally, and as doubtless happens in some places, once formed, the depressions may have
been deepened by intermittent or episodic surface streams. Whatever agent is responsible, however,
the scarp-foot zone is lowered and a topographic depression aligned along the hill base is formed.
The comparatively great breadth of some depressions relative to the associated residual hill, like
those featured from central Australia ( Fig. 8.16b), suggests that the process of scarp-foot rotting
and lowering must be of long duration.
Some regard the forms as having developed in the humid tropics. They believe that where these
scarp-foot depressions are found in arid lands they are inherited from former humid climatic
phases, but this suggestion is sustained neither by general argument nor by field evidence. Water
is, if anything, relatively more important in arid and semi-arid lands than elsewhere and, because
of its concentration in the piedmont, achieves results that are more pronounced than in other cli-
matic contexts.
One important result of the scarp-foot weathering and erosion manifested in the several land-
forms just described is that the piedmont angle, or nick, becomes pronounced. The feature is basi-
cally a structural form and in granitic terrains is roughly coincident with the margins of massive
compartments. But weathering and surface lowering due to volume decrease, as suggested by
Ruxton (1958), or preferential erosion of the zones of altered rock, cause the structural feature to
become emphasised. Truly angular nicks are particularly commonplace in granitic rocks where
vertical or near-vertical fractures are exploited and exposed by weathering and erosion.

8.6 FLUTINGS OR GROOVES

8.6.1 Description
Many steep slopes, bordering boulders as well as hills, and including flared slopes, are scored
by grooves, and are said to be fluted ( Fig. 8.17 a lso 1.2g). These grooves are variously known
as Silikatkarren, Pseudokarren, Granitkarren, regueros, canales, estrías, zlobki, cannelures,
Riefelungen and Kannelierungen, and so on, as well as flutings (Schmidt-Thomé, 1943; Bulow,
1942; Carlé, 1941). Most grooves are U-shaped in cross-section (cf. Rundkarren formed in lime-
stones), others are more open, while many others are broad, shallow and flat-floored. Their width
and depth is measured in centimetres (commonly of the order of 20-30 cm) though some a few
metres deep have been noted. They are found not only on the exposed outer walls of boulders, blocks
and hills, but also on overhanging slopes (Fig. 8.17c) and on the interior walls of tafoni (Wilhelmy,