Granitic bornhardts: their morphology, characteristics and origins

Author : C.R. Twidale
Publication : Bulletin of the Geological Society of Malaysia
Page : 237-255
Volume Number : 42

Bulletin of the Geological Society of Malaysia, Volume 42, Dec. 1998, pp. 237 - 255


Granitic bornhardts: their morphology, characteristics and origins


Department of Geology and Geophysics, University of Adelaide, Adelaide, South Australia 5005


Abstract: Bornhardts are bald domical hills either standing in isolation as inselbergs ("island mountains"), or forming components of massifs. Though especially well represented in granitic terrains, they are also developed in other lithologies. Their plan form is determined by systems of steeply inclined fractures, while their profiles are associated with convex upward fractures. They meet the adjacent plains or valleys in a piedmont angle or nick. They are climatically azonal. They occur in various topographic settings and consistently in multi cyclic landscapes. Epigene forms as old as Cretaceous are known, and exhumed forms ranging from Late Pleistocene to late Archaean in age have been recognised.

Bornhardts have been explained in various ways: as literal and littoral inselbergs, as climatic (savannah) forms, as minor horsts, and so on. Some are upstanding because they are shaped in rock that is compositionally different from, and implicitly more resistant to, weathering and erosion than that which underlies the adjacent plains and valleys. Some are exposed stocks. Such explanations have local validity, but two hypotheses have been suggested as general explanations.

First, bornhardt inselbergs can be construed as remnants of circumdenudation following scarp retreat but much field evidence is not accounted for and some is incompatible with several of the deducible consequences of the hypothesis. Second, bornhardts have been interpreted as two-stage forms, which originated at the weathering front as resistant masses due either to low fracture density or to rock type, and which became landforms as a result of the stripping of the regolith and exposure of weathering front topography. This two-stage model satisfies much of the field evidence and the isolated residuals of shield inselberg landscapes are the last remnants of resistant compartments which have survived long-continued subsurface weathering. According to this hypothesis, topography is consequent upon, and subsequent to, the formation of sheet fractures which are associated with compressional stress. Nubbins (block- and boulder-strewn domes) and castle koppies (small blocky hills) are derived from the weathering of bornhardts.