Geology Reference
In-Depth Information
(c)
Figure 8.10.
(c) Scratches formed by needle-like leaves moved by the wind scraping against algal coating of
rock surface, Keith district, South East of South Australia.
downslope side of boulders is due not so much to development as to exposure, for there grus is
evacuated downslope, whereas on the upslope side detritus tends to accumulate. Moreover, excep-
tions are understandable in terms of this working hypothesis. For example, at several sites on
northwestern Eyre Peninsula, the major flares occur on the northwestern aspect of the residuals,
but in every instance they are developed along major joints; the relative readiness with which
water can infiltrate such zones more than compensates for the marked desiccation of the regolith
on the exposed northern side of the hills.
Finally, the two-stage, or etch, hypothesis also offers an explanation for the exceptional devel-
opment of flared slopes in the shield lands of southern Australia, for the region has been compar-
atively stable for long periods, allowing time for deep weathering and formation of markedly
concave, even overhanging weathering fronts and, thus, basal slopes.
On the other hand, incipient flared slopes are not ubiquitously developed in the piedmont zone
(Twidale, 1967). There are many excavations where flared slopes, though anticipated, are either
absent or poorly developed, at least, not in the vertical range of the exposure. Thus, at Elkington
Rock, the upper 5 m of the dome exposed by excavation show only a very faint concavity on its
eastern flank (Fig. 6.19a). The flare may be developed at greater depth. Alternatively, however,
there may not have been enough runoff from the very small (10 m diameter) platform that was
naturally exposed and that is the crest of the dome, or major fractures may be necessary to allow
penetration of water sufficient to induce the weathering involved in the formation of a flare.
8.1.3
Changes after exposure
After exposure, flared slopes are modified in various ways. Scaling, some possibly inherited from
the weathering front (see Chapter 3), causes minor wearing back of the slope. Flutings (see below)
extend from near the former ground level and score the concave rock wall. In places separation of
flow has caused small plunge pools, and algae are associated with local inversions in the form of
channel floors that now stand out as ribs (see below). Triangular divides or lobes, some of them
coated with algae, are frequently developed between the lower extremities of flutings (
Figs 8.10a
and b). They may be caused by weathering by moisture contained in the soil lapping up against the
base of the slope and particularly in the small recesses formed by the flutings. In places the algal
