Geology Reference
In-Depth Information
But they are dissected by streams working toward their new baselevel. Valley-in-valley forms are
first developed (Fig. 4.15). These comprise valley-side facets separated by breaks of slope, and
corresponding graded stream sectors separated by nick points in the shape of waterfalls or rapids.
In granitic terrains such breaks of slope may develop for structural reasons. Thus, particularly
massive blocks may form local baselevels to which stream sectors are graded. Similarly, and espe-
cially in areas of gneissic rocks, particularly massive bands of rock may give rise to stepped relief
on the valley-side slopes, as, for instance, in the Rooifontein Valley of central Namaqualand,
where many such local baselevels can be related to structural factors. In some areas, however, a
similar morphology is developed on the opposite side of the valley (Fig. 4.16), suggesting that the
forms may be cyclic, depending on the distribution of the local (structural) baselevel. In like fash-
ion, if waterfalls are cyclic rather than structural, they ought to be developed on all rivers in a given
region, whereas structurally determined features tend to be random or isolated.
Valley-side facets tend to extend laterally and nick points inland so that, in time, a new surface
of low relief comes to replace the former plain. Remnants of the latter, and indeed of even earlier
Figure 4.15.
Valley-side facets (X and Y) in granite at Reedy Creek, western Murray Plains, South Australia.
By correlation with the nearby Murray valley, the upper valley is of Pliocene age (Twidale,
Lindsay and Bourne, 1978).
Figure 4.16.
Valley-side facets or steps, stripped of regolith, and probably held up by resistant bands in the
gneiss, Rooiberg, northern Namaqualand, Cape Province, South Africa.
