Minor forms developed on steep slopes - Landforms and Geology of Granite Terrains - page 187

Flared slopes and other forms of basal fretting are merely a special case of scarp-foot weathering
and erosion inducing the development of the piedmont angle. Its especially pronounced develop-
ment in arid and semi-arid lands in part reflects the increased relative significance of moisture in
such climatic regions. It may also be due to the greater likelihood of views of such sharp breaks of
slope being uninterrupted by vegetation, for though the moisture responsible for scarp-foot weather-
ing encourages vegetation growth in arid regions, it is localised, whereas in humid areas woods and
forests commonly conceal the true nature of the scarp-foot zone.

8.4 ROCK PLATFORMS (SEE ALSO DISCUSSION OF ROCK PEDIMENTS

IN CHAPTER 4)

8.4.1 Description
Flared slopes extend laterally into gently sloping bedrock surfaces called rock platforms, and for
this reason are considered here as well as in Chapter 4. At some sites a depression intervenes
between hillslope and platform. Platforms vary in width between a few centimetres at the base of
some boulders, and a few metres and several hundreds of metres, as, for instance, at Peella Rock
and Corrobinnie Hill in South Australia (Figs 4.6 and 8.15a) , and The Humps and Varley Township
Hill in Western Australia (Fig. 8.15b), and Ampidianambilahy, Andringitra Massif, Madagascar
(Vidal Romaní, Ramanohison and Rabenandrasana, 1997). They vary in extent from a few square
metres to several square kilometres. They are most characteristically developed bordering residual
hills, but they are also found in isolation, unrelated to any upland mass, though whether this has
resulted from the elimination of a former inselberg or whether there never was an upland and the
platforms are merely crestal exposures of very large-radius concealed or incipient domes, is a
moot point. In detail, some platforms are dimpled, due to the development of shallow, saucer-
shaped depressions, and scored by gutters that, together, form a rudimentary drainage network.
Small medas, blocks, boulders and patches of grus remain on some platforms.

8.4.2 Origin
Platforms are erosional in the sense that they cut across jointing and other rock structures. Like
flared bedrock slopes, they have been shown to extend beneath the present regolith and, also like
flares, they are regarded as etch forms, or exposed parts of the weathering front. Indeed they are
merely lateral extensions of flared slopes. They are especially well-developed in wet sites such as
the scarp-foot zone, topographic lows such as the Corrobinnie Depression, South Australia, and
bordering ancient watercourses or lakes, for instance, in the southwest of Western Australia. The
grus and boulders found on some platforms are remnants of the regolith that formerly covered the
planate forms and that has now been partially removed.

8.5 SCARP-FOOT DEPRESSIONS

8.5.1 Description
Shallow valleys and moats or linear depressions (Clayton, 1956), also known as Bergfuss-
niederungen and dépressions de piedmont, are found around the bases of some inselbergs in arid
and semi-arid (especially savanna) landscapes. The valleys are integrated with the local drainage
system, but the depressions are enclosed. Such depressions have been reported from West Africa,
from the Sudan, where they are known as fules, and from the Egyptian Desert. They occur in
central and southern Australia, and in the Mojave Desert of southern California.
Most of the moats are just a few metres across, but some attain widths of several scores of
metres, and that around Gebel Harhagit, in Egypt (Dumanowski, 1960), is more than a kilometre wide
in places. Some in central Australia, developed around granite nubbins, are of similar dimensions