Geology Reference
In-Depth Information
though volume decrease of the re golith consequent on alteration and subsurf ace evacuation in
solution and by flushing may also have contributed.
6.6 ANTIQUITY AND INSELBERG LANDSCAPES
The quintessential and classical inselberg landscapes are developed on the shield lands of Africa,
Australia and India; and for good reason, for it is in such ancient continental nucleii that there has
been time for long-continued weathering, resulting in the eradication at the weathering front of all
but the most durab le resistant compartments. Even these, however, have been reduced to minor
proportions. There has also been time for the stripping, partial or complete, of the regolith and for
the exposure, again partial or complete, of the residual remnants as inselber gs or massifs, which
despite being areally subordinate to the plains, are so prominent and e ye-catching that they, and
not the plains, have given their name to the landscape as a whole. Though high rate of activity can
compensate for time, inselbergs and inselberg landscapes reflect great age and in particular long
periods of subsurface weathering.
6.7 SUMMARY
Bornhardts are domes in both the structural and the topographic senses of the term. They form in a
variety of ways, but most are Härtlinge, monadnocks de résistance or de dureté, which have formed
in two major stages. This interpretation accounts for the field characteristics. Though initiated by
differential structurally-controlled subsurface weathering, the stripping of the regolith and expo-
sure of the weathering front is in places due to ice sheets and glaciers, else where to wind-driven
waves, though mostly to river action. Thus, they are convergent in the sense that similar for ms
have evolved in different ways.
Many appear to reflect e xploitation b y shallo w g roundwaters of v ariations in fracture
density, though the effects of mineralogical variations have probably been underestimated. Many
of the structural weaknesses exploited at the weathering front have their origins in magmatic, ther-
mal and tectonic events of the distant past. F or this reason it is suggested that, though it is con-
venient to consider such etch for ms as ha ving developed in tw o-stages, in reality g ranite
bornhardts are multi-stage rather than two-stage in origin. Moreo ver, deep-seated crustal events
continue to influence landform development in granitic terrains (Blès, 1986).
It has long been accepted that inselbergs and bornhardts are climatic forms; the particular cli-
matic connection has varied from time to time and from writer to writer, though savanna or desert
environments have found most favour. But groundwaters are ubiquitous, so that in ter ms of two-
stage development the first of the requirements necessary for bornhardt evolution obtains all over
the continents. Cool climate processes cause exploitation of steeply inclined fractures and produce
angular forms, and warm humid conditions permit soil formation, the formation of vegetated debris
slopes and a softening of the piedmont angle, in mark ed contrast with desert bornhardts. But the
basic forms are similar, regardless of climatic setting.
Inselbergs, on the other hand, are particularly well represented in stable, shield regions, whether
arid or humid , w arm or cool: such for ms ha ve been repor ted from F ennoscandia a nd
Newfoundland as well as Angola and Namibia, from the selvas of Nigeria (Jeje, 1973) and the mon-
soon lands of peninsular India, as w ell as the mediter ranean lands of souther n and southwestern
Australia. Many were initiated in warm humid conditions though the agent responsible for expo-
sure has varied. But the common and cr ucial factor in inselberg development is not climate but
time. The shield lands are relatively stable and have long been exposed to weathering, and in par-
ticular subsurface weathering. There has been time for e ven the most resistant of rocks to be
reduced to small size; most ha ve been substantiall y weathered, and onl y the cores of e ven the
toughest compartments of resistant rock survive as inselbergs surrounded by extensive plains, or as
inselberg landscapes (Brook, 1978).
