Geology Reference
In-Depth Information
Figure 2.6.
Sheeting in granite in parallel with recently formed erosional surfaces in a deep river valley in the
Pindo, Galicia; note contrast with fracture pattern and orientation exposed on upper hillslope.
this fact as evidence that the force of expansion in the rock has been dissipated by slight move-
ments along the joint planes”. In some areas, orthogonal joints predate sheet structure, but even
where sheet fractures cannot be shown to be younger than the orthogonal system there are usu-
ally many other potential slippage planes in rocks and crystals. Strain could be taken up by
grain boundary sliding, and along crystal cleavages, for example.
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The association of sheet structure with bornhardts is irrational if the former are interpreted as
a consequence of offloading without the application of compressional stress. Sheet structure is
supposed to be a manifestation of radial expansion, whereas the field evidence, both in gross
and in detail, suggests that the residuals are masses of rock in compression (below, and Chapter
6). This is indicated by the condition of the joints within the bornhardts, which are tight, and
commonly take the form of discontinuous hairline cracks, and by the presence of A-tents and
wedges, which are clearly associated with compressional stress (Chapter 11). Many bornhardts
persist as long term landforms because the rocks of which they are constructed are in com-
pression: fractures are few and tight, water cannot readily penetrate the mass, and weathering
and hence erosion are slow. If the bornhardts were indeed developed on masses of granite that
were decompressed and relaxed, their joints would be open. The rock masses would not survive
weathering and erosion and would not, therefore, persist as the residuals that are such promi-
nent, and in places dramatic, landscape features.
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Although it is conceivable that the parallelism between sheet jointing and land surface need not
be perfect, it is difficult in terms of the offloading hypothesis to explain inverse relationships
such as have been observed in the Yosemite Valley in the vicinity of Tenaya Lake, in Joshua Tree
National Monument in southern California and at Quarry Hill, near Wudinna, on northwestern
Eyre Peninsula, South Australia (
Figs 2.7a and b)
(Twidale, 1964, 1973). Such relationships are
probably the result of deep erosion and inversion of relief or of exposure of lenticular sheets
distorted by faulting. If sheet fractures predated and determined the shaping of the land sur-
face, stress contrasts alone could have produced topographic inversion through the preferential
weathering of antiformal crests, leaving the synforms upstanding (Figs 2.7 (c, d, e and f ) and 2.8).
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Many local and detailed lines of evidence argue against the offloading hypothesis. For instance,
there is an inconsistency between the age of erosional features said to be the cause of sheeting
