Plains – the expected granite form - Landforms and Geology of Granite Terrains - page 68

Such mantled pediments are cut in granite, but the intrinsically fresh bedrock is masked or
mantled by a veneer that consists largely of weathered material in situ . In places, as, for example,
bordering the channels of ephemeral streams, the bedrock is masked by a thin layer of stratified
alluvium, but the mantle consists overwhelmingly of grus. Such pediments have been described
from areas of pronounced winter rains, such as the Mediterranean region and Eyre Peninsula,
South Australia; in areas of continental and monsoonal climate such as Korea and in Japan, and
in areas that experience only episodic rains - desert and semi-desert regions like the Sahara, the
American West and Southwest, central Australia, and so on.

- Rock pediments or platforms are planate rock surfaces. McGee (1897) recorded his astonishment

on encountering these forms in Arizona late last century. They were different from anything pre-
viously recorded:

“At first sight the Sonoran district appears to be one of half-buried mountains, with broad allu-
vial plains rising far up their flanks, and so strong is this impression on one fresh from humid
lands that he finds it difficult to trust his senses when he perceives that much of the valley-plain
area is not alluvium but planed rock similar or identical with that constituting the mountains …
During the first expedition … it was noted with surprise that the horse-shoes beat on planed
granite or schist or other hard rocks in traversing plains 3 or 5 miles from mountains rising
sharply from the same plains without intervening foothills …” (McGee, 1897, pp. 90-91).

Rock pediments in granite are gently inclined and are typically dimpled and grooved. Many
carry remnants of a regolith ( Fig. 4.6) . Some fringe uplands, others stand in isolation as the
just-exposed crests of emerging domes or as the reduced remnants of once high masses (Fig.
4.7), though some would not classify the latter as pediments because they lack a backing scarp
and hence a piedmont angle.
-How pediments form or may have formed has given rise to considerable debate. For many writ-
ers, pediments developed on crystalline rocks are, in broad view, a consequence of scarp reces-
sion. Thus, Howard (1942, p. 134), who worked in crystalline terrains in the American
Southwest, wrote that “… the development of the pediment depends on the recession of the
base of the slope …”, and Pugh (1956, p. 28), who investigated granite landforms in Nigeria,
concluded that “… a mountain mass with a well-developed upper surface will shrink slowly by
scarp retreat, with the development of bounding pediments .”

But this view is open to serious questioning for, regardless of the precise nature of the forma-
tive processes, there is compelling evidence to suggest that though in many cases backwearing

Figure 4.6. Granite platform with patches of a thin regolith, between Corrobinnie Hill and Peella Rock,

northern Eyre Peninsula, South Australia.