Geology Reference
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Figure 2.2.
(a) Pseudobedding at Roughtor, Bodmin Moor, southwest England (Geological Survey Museum
UK). (b) Pseudobedding in monzonite, the Yosemite, Sierra Nevada, California.
less than that are termed spalls or flakes). Sheet fractures have been observed at depths of 100 m
or more in some quarries, though elsewhere they fade with depth. Indeed, in some areas with well-
developed orthogonal fracture systems, as in some of the residuals found on northwestern Eyre
Peninsula, and in the silicic volcanics of the Gawler Ranges, in the arid interior of South Australia,
sheet fractures appear to be superficial (Campbell and Twidale, 1991). On the other hand, it is evi-
dent in some deep mines and other excavations that sheet fractures extend to great depths. Some
sheet fractures take the form of simple arcuate partings. Others consist of several separate fractures
arranged en echelon and together forming an arcuate parting ( Fig. 2.3a). It is frequently claimed that
the thickness of sheet structure increases systematically with depth, but there are many exceptions.
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