Geology Reference
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Figure 6.29. Diagrammatic sections through v arious inselbergs on nor thwestern Eyre P eninsula, South
Australia, showing suggested relationship between profiles and strain.
Figure 6.30. The Humps is a g roup of g ranite gneiss domes located on the Old (lateritised) Plateau, in
the southern Yilgarn of Western Australia. It and its sur rounding plains are of the order of
60-100Ma.
Bornhardt massifs like the Everard Ranges, nor thern South Australia, stand above plains on
which silcrete developed and which are therefore probab ly at least of Earl y Cainozoic age. The
crests of some of the bor nhardts of northwestern and northern Eyre Peninsula, South Australia,
may be of Mesozoic age, and this f inds support in the putative Jurassic-Early Cretaceous age of
the summit surface of the adjacent Gawler Ranges. Again, evidence has been adduced to suggest
that some inselbergs preserved on the Yilgarn Block, in the southwest of Western Australia, are of
late Mesozoic and earliest Tertiary age (Fig. 6.30), and some residuals on Wilsons Promontory,
southern Victoria, are Early Cretaceous forms (Hill, Ollier and Joyce, 1995).
This is not to suggest that all inselber gs are ancient for ms. On the contrar y, many are clearly
youthful, having been only recently exposed. But others are old in that they have been exposed to
the elements for scores of millions of y ears, and they argue against scar p retreat. On the other
hand, they are compatible with the two-stage hypothesis. Their survival poses problems. But it is
not enough to state that they exist, therefore they must be possible. Their survival requires expla-
nation. Several factors may be invoked. Once in positive relief, the hills shed water; they are mas-
sive with few open fractures so that most water runs off the surface; they are intrinsically dry sites,
and as “ water is your sore decayer ” (Shakespeare, Hamlet, V, i, 160-161 In Hibbard (Ed.), 1994),
the bedrock is altered and eroded, but only slowly. Bornhardts, like other ancient uplands, are thus
best explained in terms of reinforcement or positive feedback mechanisms.
 
 
 
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