Geology Reference
In-Depth Information
Figure 4.8.
Conical residual or coolie hat, 2 m high and weathered in granite, on Houlderoo pediment (part
mantled, part rock), southern piedmont of the Gawler Ranges, South Australia.
In addition to this evidence suggestive of the stability of bounding scarps, many of the insel-
massive, others are well-fractured and pervious, yet all are fringed by pediments, despite their
generating little overland flow. The regolith consists largely of grus
in situ
. For this reason any sug-
gestion that the pediments adjacent to granitic inselbergs are comparable to those described from
some sedimentary terrains and due to lateral corrasion by divaricating streams must be rejected.
The field evidence suggests that mantled and rock pediments are end members of a continuum
(Bryan, 1922, 1936). The mantled forms carry a veneer of weathered rock smoothed by wash and rill
work. The rock pediments are exposed areas of the weathering front and are due to mantle-controlled
planation. They are mantled pediments from which the regolith has been stripped. Evidence for
this conclusion derives from the nature of the mantle exposed on and adjacent to the platforms;
from the corestones still
in situ
and embedded in the grus at some sites and scattered over the ped-
iments as boulders in others; from the physical continuity of the platforms and such features as
flared slopes that are demonstrably a particular form of weathering front; and from the pitted,
grooved and dimpled morphology of the platforms, these minor features being initiated at the weath-
ering front beneath the regolith (Chapters 8 and 9). The relative rate of lowering of the weathering
front and the mantle surface determines the thickness of the regolith. Where the former has out-
paced the latter, the mantle is thick, but where the converse has occurred, the front is exposed as a
rock pediment.
The mantled and rock forms are genetically related. Mantled pediments are surfaces of trans-
portation, i.e. surfaces shaped by erosion and deposition (redistribution) by wash and by rills, but
in contrast with the pediments of sedimentary regions the mantle of debris developed on many
granitic pediments is discontinuous: running water in the form of wash and rills plus a few minor
streams has planed the surface of the regolith to give the smooth, mantled surfaces, and has also
scoured the mantle to expose the bedrock in rock platforms.
Though there are a few exceptions, pediments are essentially of limited extent. Some rock pedi-
ments occur in isolation, but most granite pediments are fringing forms and extend at most only a
few kilometres from the mountain front. Pediments are particularly well-developed on granitic
rocks. There are several possible reasons for this. First, pediments are well-developed in weak
rocks, and granite is particularly vulnerable to moisture attack; in any granitic massif located at or
near the land surface there are large compartments of weathered material vulnerable to planation.
Second, granite typically weathers to sand or grus, which is readily transported by rills and streams
