Geology Reference
In-Depth Information
(a)
Figure 1.13.
(a) Drainage patterns in granitic terrains: straight, fracture-controlled valley near Kylie Lake,
southern Zimbabwe.
are zones of weakness which can readily be penetrated by meteoric waters. The bedrock adjacent
to the fracture zones is weathered and is thus susceptible to erosion by streams, which become the
prominent members of the drainage network by natural selection; for their competitors draining
unweathered granite do not extend or incise as rapidly as do those which run along fractures.
In granite as in other lithological and structural environments, anomalous drainage patterns are
developed in places, i.e. river and stream patterns do not conform to lines of structural weakness in
the country rock, but rather run transversely to the structural grain, local or regional. As in other
places recourse must be made to diversion, antecedence, superimposition, inheritance, stream per-
sistence and valley impression in explanation of such transverse drainage.
Thus, the Vaal River is anomalous with respect to bedrock structure where it flows over the
northern edge of the Vredefort Dome (King, 1942), including part of the granite core, in the Free
