Geology Reference
In-Depth Information
Dry granite is stable and resistant whereas granite in contact with water is rapidly altered. Thus,
blocks exposed in linear zones tend to become upstanding and to form local baselevels for upstream
sectors of rivers crossing the barriers. In this way a stepped topography evolves, dependent in the
first instance on the varied behaviour of granite to weathering. Wahrhaftig (1965) discusses vari-
ous possible reasons for the upstanding outcrops becoming upstanding in the first instance, but
fracture density is almost certainly the cause - the ridges are based on outcrops of massive blocks.
An alternative explanation of stepped topography can be suggested, using the scarp-foot weath-
ering and phased or episodic exposure responsible for stepped inselbergs (see Chapters 6 and 8).
4.6
EXHUMED PLAINS
As well as being important components of modern scenery granitic plains have also been recog-
nised in unconformity, in the stratigraphic column. Some have been exhumed to form part of the
contemporary landscape. Thus, as an example of the still-buried types, in the south area of Cape
Town, South Africa, the Table Mountain Sandstone rests on a planate surface cut in Precambrian
granite (Toit, du, 1937). The unconformity is well exposed in coastal sections and road cuttings,
and is indeed followed over considerable sectors by the main road linking Cape Town with the
Cape of Good Hope (Fig. 4.18a). An unconformity of quite remarkable smoothness and eroded in
considerable measure in granite is exposed in the Grand Canyon in the western USA (Sharp, 1940).
A Precambrian peneplain developed in crystalline rocks and partially exhumed from beneath
(a)
(b)
Figure 4.18.
(a) Unconformity between Table Mountain Sandstone and granite, south of Cape Town, South
Africa. (b) Exhumed Precambrian surface of low relief inland from Kap Ingersoll, north
Greenland (Cowie, 1961).
