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A Theory of Bornhardts

1948; Wiley; Volume: 112; Issue: 1/3 Linguagem: Inglês

10.2307/1789160

1475-4959

Lester King,

African studies and sociopolitical issues

Bornhardts, or granite-gneiss inselbergs, have attracted attention from geomorphologists since the time of their description by Bornhardt him? self,1 and attempts to explain their form and occurrence have been many and varied. This is not surprising, for where they are well developed born? hardts often make the most striking features of the landscape (Plate 2). Where they are present in abundance, as in some of the eastern districts of Southern Rhodesia, they sometimes confer upon the landscape a truly awful and mysterious appearance as though the observer had descended upon another planet. Something of their atmosphere was caught by the novelist Rider Haggard. The writer's good fortune has led him to study many scores of bornhardts in the Union of South Africa, the Rhodesias and the East African territories, and the theory regarding their origin is now sufficiently advanced for publica? tion. To discuss in extenso the earlier theories of bornhardt (inselberg) development is beyond the scope of this paper. The writer has tested them all in the field, found them wanting in some respect, and has with their aid evolved the present, eclectic account. Briefly, no single thing, structure or process is responsible for the development of bornhardts. Bornhardts are due to the twin processes of pediplanation (scarp retreat and pedimentation) acting upon suitable rock types, following a geological history which involves stream rejuvenation. Each of these three essential conditions is examined in the sequel. Bornhardts, in whatever territory they appear, exhibit a homogeneity of form that is striking in the extreme. The family includes the high, domed inselbergs*' of Bornhardt, alone or in groups; the less perfect, more irregular masses sometimes found in ridges, in clumps or isolated; and lastly, the tors or koppies, which are generally smaller, and often a jumble of jointbounded granite blocks rather than true bornhardts. As might be expected, all gradations appear between the various members of the family, just as all gradations in stage of development are also possible in the planed landscape with which bornhardts and castle koppies are commonly associated. The features generally held in common by bornhardts may now be examined. (1) Bornhardts are developed only in rocks of plutonic origin?plutonic igneous or plutonic metamorphic. Examples are known in various granites, granodiorites, diorites, occasionally in norites and even charnockites, and in a wide variety of orthogneisses and hybrid rocks of plutonic admixture. The rocks must be intrinsically hard and moderately resistant to chemical weathering in order that the faces of the bornhardts shall stand steeply. Thus bornhardts are rarer proportionately in basic than in acid or intermediate rocks.