Geology Reference
In-Depth Information
(b)
Figure 6.21. (b) Stepped nor thwestern slope at Yarwondutta Rock, nor thwestern Eyre P eninsula, South
Australia.
(a)
(b)
Figure 6.22. Sections showing possible mechanisms of slope lowering: (a) etching out of scarp-foot zone and
(b) lowering of the whole piedmont plain.
- There has been some backwearing of the bounding slopes, not by river action for the residual
is of limited extent and does not generate streams. But there is a substantial runoff in the form
of wash (hence the reservoir shown in
Fig. 6.20)
and weathering by moisture in the scarp-foot
zone has caused the bedrock slope to recede. This backwearing amounts to only a few metres,
which is of quite a dif ferent order of magnitude from the scores, e ven hundreds of kilometres
demanded by inselberg landscapes in many parts of the world and by the scarp retreat hypothesis.
- The residual has g rown episodically in relief amplitude in time. This explanation meets the
objection, due to King (1949, 1968) to the two-stage hypothesis of bornhardt development that
the maximum depth of weathering recorded in a given region is commonly much less than the
height above the plains of the highest inselber gs in that region. The implication was that some
inselbergs are too high to have been initiated in the subsurface, but only a single cycle of weath-
ering and erosion, not multicyclic or multiphase development, was considered (Twidale, 1982).
Stepped inselbergs indicate episodic exposure. The cause of the implied alternations of weather-
ing and erosion (tectonic, climatic, changes in erodibility due to weathering) remains speculative.
Phases of stripping and exposure can be identified in the sedimentary records of adjacent basins.
Stepped inselbergs are not peculiar to Eyre Peninsula or even to Australia. On such bornhardts as
Wudinna Hill and Ucontitchie Hill on Eyre Peninsula, and Kokerbin Hill and Hyden Rock in the
southwest of Western Australia, linear subhorizontal zones of flared slopes and associated plat-
forms, steepened slopes, breaks of slope, and tafoni occur at v arious levels, so that the residuals
have a stepped appearance (
Fig. 6.23)
.
Moreover, inselbergs with distinct bevels and steps occur
in many parts of the w orld, and are for e xample clearly illustrated from Angola in the superb
sketches of Jessen (1963) (e.g.
Fig. 6.24).
