Geology Reference
In-Depth Information
(d)
(e)
Figure 3.5.
(d) Showing overall effect where exposed (the darker zone) by the removal of soil from the base
of the rock, at Murphys Haystacks, northwestern Eyre Peninsula, South Australia. (e) Valley on
Domboshawa, near Harare, Zimbabwe, showing pitting extending some 2 m up valley sidewalls.
on Domboshawa, a granitic bornhardt near Harare in central Zimbabwe (Fig. 3.5e), are pitted to
a depth of about two metres. Some areas of pitted surface have clearly been eliminated by recent
wash, but a detrital fill of significant thickness was formerly present.
Elsewhere, the course of weathering can be deduced from an examination of weathered mantles
or regoliths. Regoliths develop from the surface downwards. The weathering front, the interface
between the regolith and the intrinsically fresh rock, may be diffuse, but in granite is commonly
sharp and takes the form of either a distinct plane or a narrow zone of transition. This abrupt
change reflects the crystallinity of the rock and also the susceptibility of feldspar and mica to
water attack. The fresh rock is impermeable, but once water has penetrated along crystal cleav-
ages, or microfissures, alteration takes place within the body of the rock. The permeability of the
