Geology Reference
In-Depth Information
4
Plains - the expected granite form
4.1
WEATHERING AND SURFACES OF LOW RELIEF
Granite in contact with water is readily and rapidly weathered. Two of the common rock-forming min-
erals found in granite, mica and feldspar, are readily, and, in geological terms, rapidly, attacked.
Shallow groundwaters are ubiquitous. Granites are typically well fractured, for in addition to orthog-
onal systems and sheet fractures, there are many random partings and numerous microfissures.
Granites include minerals with a well-developed cleavage. Many granite gneisses are foliated and
have mineral banding. Thus, there are various and varied avenues permitting the entry of water,
and with it, chemicals and biota. Granites are major components of the ancient shield lands which
are the nuclei of the continents. Such masses have been exposed to meteoric waters, and particu-
larly groundwaters, for hundreds of millions of years. For these reasons they have been weathered
and worn down to regions of subdued relief. Also, physical contrasts are introduced through the
development of a regolith, which is generally more susceptible to erosion than on fresh granite;
and many plains have been formed beneath and exposed as a result of the stripping of the regolith.
Plains are thus the expected and areally the most extensive granite landform. Despite the under-
standable interest evidenced in the positive relief features developed on granite, plains are by far
the most characteristic landform developed on granitic bedrock. Plains are indeed an essential com-
ponent of such well-known, and typically granitic features as Inselberglandschaften, for it is the con-
trast between the virtually featureless sweeping plains and the steep-sided residuals that endows
the landscapes with their dramatic aspect (Fig. 1.2e). The plains of inselberg landscapes vary both
morphologically and genetically. An early student of such features, Passarge (1904), classified the
plains of inselberg landscapes according to their origin and age, and named them after typical African
occurrences. The Banda type is, according to Passarge (1904), scoured by the wind and consists
of an assemblage of bedrock plains and depressions brought into conformity with degradational
elements by aeolian deposits. The Rovuma type is similar but was shaped during desert conditions
during the later Mesozoic, and was subsequently buried and exhumed. The Kordofan type includes
some aeolian elements but is shaped mostly by rivers, and the plains of Adamaua type are shaped
entirely by rivers and streams.
It is rare to have both the evidence and the confidence to interpret in such detail, and it is more
practical to classify plains according to their morphology and stratigraphic history.
4.2
PLAINS OF EPIGENE (SUBAERIAL) ORIGIN
Plains eroded in granite are well represented in tropical and subtropical lands, though they are by
no means restricted to them, and they are typical of the shield lands. Plains developed by epigene
or subaerial processes on granitic rocks vary in both extent and morphology. Pediments are a feature
