Page 164 - Fundamentals of Geomorphology

SMALL-SCALE TECTONIC AND STRUCTURAL LANDFORMS 147

envisages a two-stage process of deep weathering and and earlier Mesozoic regolith. They were then exposed
stripping, similar to the two-stage process envisaged in during the Early Cenozoic era as the rejuvenated Salt
the formation of granite boulders. It assumes that the River and its tributaries stripped the regolith. If the two-
fracture density of a granite massif has high and low com- stage theory of bornhardt formation should be accepted,
partments. In the first stage, etching acts more readily on thenthedevelopmentofnubbinsandkoppiesfromborn-
the highly fractured compartment, tending to leave the hardts is explained by different patterns of subsurface
less-fractured compartment dry and resistant to erosion. weathering. Nubbins form through the decay of the outer
In the second stage, the grus in the more weathered, few shells of sheet structures in warm and humid cli-
densely fractured compartment is eroded. This theory mates, such as northern Australia (Figure 5.29a). Koppies
appears to apply to the bornhardts in or near the valley probably form by the subsurface weathering of granite
of the Salt River, south of Kellerberrin, Western Australia domes whose crests are exposed at the surface as plat-
(Twidale et al. 1999).These bornhardts started as subsur- forms (Figure 5.29b). However, inselbergs and associated
face bedrock rises bulging into the base of a Cretaceous landforms in the central Namib Desert, Namibia, show

a
() Time 1 () Time 1
b

Time 2

Time 2

Time 3

Time 4 Time 3

Time 5

Figure 5.29 Formation of (a) nubbins and (b) castle koppies from bornhardts.
Source: After Twidale and Campbell (1993, 243, 244)