Other granitic residuals and uplands - Landforms and Geology of Granite Terrains - page 156

Figure 7.8. Suggested development of castle koppies.

action in the scarp-foot zone during the gradual exposure of the residuals could produce the cliffed
bounding slopes. In the central Serra de Gêrez, in the northwestern Iberian Peninsula, the flanks
of the castle koppies or borrageiros, which in glacial times were small nunataks, have been steep-
ened as a result of the preglacial regolith having been bulldozed away by local glaciers (Vidal
Romaní, Brum, Zézere, Rodrigues et al., 1990). Minor etch forms such as rock basins were elim-
inated at the same time though they survive in areas bordering the glaciers, where the regolith
remains essentially intact. Glaciers also evacuated scree debris accumulated during interglacial
nival episodes. Both of these mechanisms contributed to the exposure and shaping of the koppies
in coherent rock. Similar chronologies can be reconstructed to account for the castellated land-
scapes of many of the Hercynian uplands of western Europe, for instance in such regions as the
Massif Central, as well as Alpine fold belts like the Pyrenees.
On the other hand, cold climate processes can hardly be invoked in explanation of the well-known
koppies of Zimbabwe (Fig. 1.2d), or those of mid-latitude deserts such as the Sahara ( Fig. 7.5c).
The location of some koppies on the crests of large-radius domes ( Fig. 7.7) suggests that they are
the last remnants of massive sheet structures, the marginal areas of which have been worn away.
Only the crestal zones are preserved, suggesting that there has been strong marginal attack, most
likely by subsurface moisture ( Fig. 7.8). Several conditions are especially conducive to pro-
nounced marginal attack. In arid and semi-arid lands there is a tendency to steep slopes due to the
contrast between active weathering in the moist subsurface and the stability of the dry, and hence
stable, exposed surfaces. Whereas nubbins are initiated wholly in the subsurface, koppies may
have evolved where the crests of dome structures are exposed and therefore relatively stable. Local
wet sites are also conducive to intense marginal weathering, as, for instance, at the Devil's Marbles,
Northern Territory ( Fig. 7.9) . Long periods of land-scape stability such as are associated with the
various land surfaces in such interior sites as Zimbabwe, a country rich in koppies, allow even
modestly aggressive weathering processes to have marked effects. All that is necessary is that the
upper part of the residual be either exposed or located in the drier, near-surface zones of the
regolith, while lower parts are affected by moisture in the deeper regolith. Also, vertical or near-
vertical foliation and steeply inclined fractures not only allow moisture to penetrate into the rock,
but also impose a measure of structural control on the form of the weathering front.

7.1.3 Large conical forms or medas
The medas of the granitic uplands of western Iberian Peninsula are conical in shape ( Fig. 7.10a),
and similar, if isolated, forms are found elsewhere ( Fig. 7.10b) . Other, miniature forms (up to 4 m
high) are developed in granite at Houlderoo Rocks, in the southern piedmont of the Gawler
Ranges, South Australia (Twidale, Mueller and Campbell, 1988 and see Fig. 4.8). The latter have
evolved in a wet site, the piedmont zone of a major massif, where there is evidence of subsurface
weathering to the depth of a few metres in the Late Cainozoic. For example, there are flared basal