Geology Reference
In-Depth Information
(a)
(b)
Figure 13.1
(a) Flared notch, on granite slope in Namaqualand (Western Cape Province), South Africa indi-
cating former presence of soil cover. (b) Part of the upper slope of Domboshawa, a bornhardt
near Harare in central Zimbabwe. The basin and valley (on left) are occupied by detritus, but
that this fill was once much greater is indicated by the flared slopes (formed beneath a soil
cover) and by the notch etched into the base of the slope in the depression nearest the camera.
it is not suggested that bornhardts for instance are always tectonic forms: some are, yet others have
their origins in crustal events. Tectonic stresses have paved the way for later developments by, for
example, determining fracture spacing and condition.
Other stresses however have played their part and ought not to be neglected. Gravitational load-
ing and tectonic pressure, i.e. the pressure exerted by one mass on the one against which it is being
pressed, either through gravity or by crustal stress, may cause strain or distortion in mineral texture
and thus render the rocks more susceptible to weathering and hence erosion. Such loading may
explain on the one hand the location of basins and tafoni, and on the other aligned basins and
gutters at sites where no fracture is apparent.
