Inselbergs and bornhardts - Landforms and Geology of Granite Terrains - page 125

(b)
Figure 6.13. (b) Domical hill de veloped on g ranite stock, easter n Musg rave Ranges, nor thern South
Australia.
6.3.4 Variations in fracture density
Many of the explanations of bornhardts based on lithological variation seem to have local validity.
In many other instances, however, the granite underlying the plains appears to be mineralogically
similar to, if not identical with, that of w hich the residuals are composed. The Hiltaba Granite
Suite of nor thwestern Eyre Peninsula, South Australia, displays several textural variations (e.g.
equigranular, porphyritic), but bornhardt inselbergs are common to all of the plutons, and w hat-
ever the local petrolo gy the same g ranite that is e xposed in the residuals underlies the adjacent
plains. The reason for contrasts in w eathering and erosion in these, and in man y other, areas
appears to be variations in fracture density between juxtaposed compartments of rock.
Many writers have alluded to variations in fracture density as a signif icant factor in landform
development on granitic rocks. Thus, Le Conte (1873, p. 327) noted that the massive domes of the
Yosemite region of the Sier ra Nevada, California, “ consist of hard material, little affected by
joints ” and, more specifically, Mennell (1904), writing of the Matopos of Zimbabwe, stated:
“… the influence of the divisional plane of the rocks must not be overlooked, and it is
to the variations in the number and character of the joints that the varied scenic aspects of the
Matopos may be traced. Where stretches of comparatively level country occur, it will generally be
found that the joints are numerous and irregular in direction, so that the rock readily breaks up and
presents a large surface to the agencies of disintegration. In such cases the superior hardness of
particular bands avails them little, as they are unable to show a solid front to the disrupting forces.
On the other hand, joints are often entirely absent over considerable areas, and the tendency of the
rock then is to weather into smooth rounded surfaces with a very large radius of curvature.
Probably the actual outlines of the hills or ridges and the general direction of most of the Matopo
valleys are determined by widely spaced master joints, which have been exploited by erosional
agencies .” (Mennell, 1904, p. 74).
6.3.5 Differential subsurface weathering and the two-stage concept
A different perspective was introduced into the bornhardt debate with the suggestion that subsurface
weathering is involved and that, like many boulders (Chapter 5) , many, perhaps most, bornhardts