Geology Reference
In-Depth Information
(c)
Figure 6.8. (c) Remnant of planation surface in granitic terrain of southern Greenland (Oen Ing Soen).
argument cannot be sustained. Bor nhardts are spectacularl y developed in coastal and inshore
zones in such areas as the Baia Guanabara on which stands the city of Rio de Janeiro, Brazil (see
Fig. 6.6b),
in northwestern Spain (see
Fig. 6.5b),
and on several sectors of the coasts of southern
Australia and of Galicia, NW Spain (
Fig. 6.1a)
,
but they are also well represented in the continen-
tal interiors that have not been touched by the sea for scores, even hundreds, of millions of years,
if ever. For example, there is no e vidence that the Yilgarn Craton of Western Australia, a region
rich in bornhardts, has been inundated by the oceans during the past 1,000Ma.
In some areas marine processes have been responsible for the differential erosion that has pro-
duced the inselbergs, and for the asymmetry of some examples (e.g. Fig. 6.1a), but there is no evi-
dence to suggest that marine forces alone are capab le of producing inselber gs in general, or
bornhardts in particular. It is merely that waves and other marine agencies (including biota) have
exploited weaknesses in the bedrock exposed in the coastal zone, leaving resistant masses in relief.
Explanations of bor nhardts based on climate ha ve long been popular . Agassiz (1865) inter-
preted the granite boulders of the Rio de Janeiro area as glacial erratics and a similar construction
was placed on boulders in Costa Rica. He took the g ranitic domes of the same area of southeast-
ern Brazil to be huge roches moutonnées. Le Conte (1873) reached the same conclusion with
respect to the domes of the Yosemite in central Califor nia, USA. In reality, there is no e vidence
that the Rio area has been glaciated during the relevant time period, and though the Yosemite has
been affected by Pleistocene glaciers and some of the domes (e.g. Gilber t Dome) ha ve been
markedly modified by them, they appear to be structural forms which predate the appearance of
the ice. The domes of the Sierra Nevada, California, and of northern Norway, occur in glacial or
glaciated regions, but, as with marine environments, their climatic context appears to be inciden-
tal. The capacity of glaciers to erode appears to depend on the physical condition of the basal ice
(cold, warm, frozen, wet-based). Glaciers are capable of bulldozing (Kleman, 1994) pre-existing
regoliths and of exposing the weathering front. Humid-climate glaciers like those of France and
Spain have not only stripped the regolith but have also destroyed any minor etch features formed
at the weathering front. But there is no reason to suppose that glaciers or ice sheets are inherently
capable of the dif ferential erosion required to produce domes from homo geneous bedrock.
Similarly, though freeze-thaw activity can prepare the rock for transpor t, and though solifluxion
may achieve the evacuation of some detritus, the mechanism cannot account for the dif ferential
erosion of a homogeneous rock mass to produce bornhardts.
The exposure and unimpeded view of inselbergs in arid or semi-arid lands have a considerable
psychological impact, and have persuaded several workers that there is a genetic link between land-
form and desert climates. Passarge (1895), for example, urged that some of the inselbergs of West
Africa are the result of wind erosion of the intervening plains during the Mesozoic, and, early this
