Geology Reference
In-Depth Information
(c)
Figure 5.12.
(c) The Kennedy Rock, Matopos Hills, Zimbabwe (Lister, 1987).
crust, whereas structural forms are due to the exploitation of passive crustal features. Fault scarps
and grabens are tectonic forms, in contrast with fault-line scarps and fault-line valleys which are
due to the preferential weathering and erosion of zones of weakness, namely fault zones.
That the word structural has two different meanings is confusing. In addition, the definitions
are oversimple and incomplete. Though some well-known granite forms are tectonic in the terms
outlined above, others are more complex, and are due to crustal activities which, however, are ini-
tially suppressed by lithostatic loading, and only find expression as a result of erosion of some, at
least, of the superincumbent load. Yet, others are structural in the older sense of being due to the
exploitation of bedrock weaknesses, many of which, however, are due to past magmatic, thermal
or tectonic events, a point made most forcibly by Lagasquie (1978). Thus, A-tents (see Chapter 11)
are associated with the release of compressive stress. Weathering and erosion play no part in their
development. They can be regarded as tectonic forms.
