Geology Reference
In-Depth Information
Figure 7.21.
Suggested mode of development of all slopes topography.
7.4
DISCUSSION
Bornhardts, nubbins and castle koppies are genetically related forms, the two last-named
being derived from the marginal subsurface weathering of domed forms. Nubbins and castle
koppies are the reduced remnants of bornhardts. Elements of the two forms are developed in the
same hill at some sites. Thus, in the southern face of Blackingstone Rock, on eastern Dartmoor,
southwestern England, huge quadrangular blocks are exposed, and the form is castellated, but
the northern side is dominated by massive convex-upward slabs of rock that give a domical
shape to the hill (Simmons, 1964 and see Fig. 2.12). According to Holmes (1918), Mt Kobe, in
Mozambique, also displays contrasted morphology on opposed aspects, and some of the residu-
als at the Devil's Marbles, Northern Territory, embrace elements of nubbins and koppies (cf. Figs
7.7b
and 7.9).
The basic form, the bornhardt, is a structural feature developed on masses that are compact by
virtue of their being in stress. They are characterised by, and owe their domical shape to, the devel-
opment of sheet structure as a result of shortening. Nubbins and castle koppies, on the other hand,
though strongly influenced by structure and also found in multicycle landscapes, are in some
measure morphogenetic features. Nubbins are best developed in humid, tropical areas as a result
of the superficial disintegration of the outer shells (sheet structure). Castle koppies, on the other
hand, are due to lateral or marginal weathering, also in the subsurface, and under various climatic
conditions (humid, tropical, arid or semi-arid, arctic or subarctic).
Weathering eventually reduces both nubbins and koppies, though the latter especially are very
durable. What remain are small domes which are frequently scarcely more than low, convex-
upward platforms with a scatter of blocks and boulders. They are nevertheless genetically related
to bornhardts. The dome structure is the starting point of an evolutionary sequence that can follow
varied paths. All three prominent inselberg forms are initiated in the subsurface.
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