Geology Reference
In-Depth Information
Figure 6.16. Effect of repeated shearing of cubic b locks: (a) single b lock, (b) compar tment with se veral
blocks (Twidale, 1980).
also those due to variations in rock composition and texture. And Falconer (1911) long ago pro-
vided an admirably succinct summary of the two-stage hypothesis embracing this broader view:
“
A plane surface of granite and gneiss subjected to long-continued weathering at base level
would be decomposed to unequal depths, mainly according to the composition and texture of the
various rocks. When elevation and erosion ensued, the weathered crust would be removed, and an
irregular surface would be produced from which the more resistant rocks would project. Those
rocks which had offered the greatest resistance to chemical weathering beneath the surface would
upon exposure naturally assume that configuration of surface which afforded the least scope for
the activity of the agents of denudation. In this way would arise the characteristic domes and
turtlebacks which suffer further denudation only through insolation and exfoliation. Their general
elliptical outlines, which Merrill (1897) would ascribe very largely to the influence of crustal
stress and strain, are probably in great part due simply to the modification by weathering of orig-
inal phacolitic intrusions
.” (Falconer, 1911, p. 246). And again, referring to boulder-strewn gran-
ite hills in Hausaland, he wrote that within the walls of Kano:
“
Kogon Dutsi, the larger of two flat-topped hills of diorite, although deeply decomposed, still
preserves in its lower part detached boulders or cores of unweathered rock. If the subsequent ero-
sion had continued until the weathered material had been entirely removed, the flattened hill
would have been replaced by a typical kopje of loose boulders resting upon a smooth and rounded
surface of rock below
.” (Falconer, 1911, p. 247).
Thus, the etch or two-stage concept appears to offer a comprehensive explanation of bornhardts
and inselbergs. The exposure of the resistant compar tments has most commonl y involved river
erosion, and it ma y be that w here the materials being attack ed were weak, slope decline most
likely occurred. Where the plains betw een bornhardts were duricrusted, or protected b y a lag
accumulation, scarp retreat operated. But w here it occur red it is local, ephemeral, and not an
essential feature of the mechanism, merel y the means b y which plains have been lowered, inci-
dentally resulting in the exposure of the inselberg residuals. Whichever mechanism was operative,
however, the plains separating the residuals were lowered by rivers. But they exploited a structural
contrast and were not responsible for the formation of the residuals.
