Inselbergs and bornhardts - Landforms and Geology of Granite Terrains - page 138

(c)
Figure 6.25. (c) Distribution of residual hills in relation to fractures, Traba Massif, Galicia, nor thwestern
Spain (Rodríguez, 1994).
6.4.6 Coexistence of forms associated with compression/shearing
Sheet fractures are everywhere associated with bornhardts. As was suggested in Chapter 2, there
are grounds for interpreting them as due to cr ustal compression. Also, A-tents and other for ms
associated with the release of compressi ve stress (Ritchot, 1975) (Chapter 11) are widely devel-
oped on bor nhardts. Thus, the coexistence of bor nhardts, sheet str ucture and an assemb lage of
minor forms is consistent with the hypothesis in terms of which bornhardts are an expression of
compressive stress, either directly applied or related to shear.
6.4.7 Topographic settings
Some bornhardts occur in plains settings, and, as they include some of the most spectacular exam-
ples known, they are given greater significance than they perhaps deserve. If bornhardts were the
last remnants surviving after scarp retreat and pedimentation, the residuals ought to be found only
in plains contexts, and they ought also to be restricted to major divides. They are not. Many resid-
uals stand on divides, but others occur in valley floors or in valley-side slopes ( Figs 6.8a and 6.26) .
Many occur in upland settings - in the Umgeni valley (Valley of a Thousand Hills) in Natal; in the
Yosemite and Domeland, both in the Sier ra Nevada of California; in the Rio de Janeiro r egion,
Brazil; in the Kamiesberge of Namaqualand; in the Rocky Mountains of Colorado (Lester, 1938;
Herrmann, 1957); and so on. These residuals surely argue against the suggestion that the forms are
inherently Fernlinge, or the last remnants surviving after long-distance scarp retreat.
6.4.8 Occurrence in multicyclic landscapes
Bornhardts characteristically occur in multicyclic landscapes (Fig. 6.8). The link between born-
hardts and multic yclic landscapes is tw ofold. First deep erosion is implied b y the e xposure of
granitic rocks, w hatever their origin, and deep dif ferential subsurf ace w eathering ar guably
requires a period or periods of standstill and baselevelling, such as is evidenced by palaeosurfaces
of low relief. It is significant that most Zimbabwan inselbergs stand on a high plain located lower
than the prominent African surface (Lister, 1987) ( Fig. 6.27) , and that in Kenya most are located on