Geology Reference
In-Depth Information
5
Boulders as examples of two-stage forms
5.1
THE TWO-STAGE OR ETCHING MECHANISM
As explained in the previous chapter, many granite plains, both regional and local, are weathering
fronts that have been exposed as a result of the stripping of the regolith. They have evolved in two
stages and are of etch type. Many other landforms, large and small, and many of them developed
in granitic rocks, are of similar origin. Bornhardts and basins, for example, have developed as a
result of differential weathering at the weathering front. Some gutters and flutings are due to dif-
ferential weathering and erosion, again at the weathering front (see Chapters 6, 8 and 9).
Now and in the past, where meteoric waters infiltrate to the base of the regolith they exploit
a wide range of structural contrasts and weaknesses - mineral banding and foliation, veins,
sills and dikes, crystal boundaries and cleavage, as well as discontinuities of various origins and
patterns (Vidal Romaní, 1990 and see Fig. 5.1). Some essentially linear, but in detail irregular,
(a)
(b)
Figure 5.1.
Quartz veins in granite (a) in Namaqualand (Western Cape Province), South Africa; (b) in the
French Pyrenees.
