Geology Reference
In-Depth Information
Figure 2.5.
Weathering along sheeting plane (X) at Whites Quarry, near Minnipa, northwestern Eyre
Peninsula, South Australia.
Ollier and Pain, 1980) most were put in position at depths of a few kilometres. Decompression
achieved through the removal of superincumbent load is said to cause the development of radial
stress which is tensional and is relieved by the development of fractures tangential to the stress and
parallel to the land surface; these are, according to the proponents of this hypothesis, sheeting joints.
The fundamental premise of the hypothesis is that the form of the land surface in broad terms deter-
mines the geometry of the sheet jointing, for it is in relation to this that the radial stress develops.
Sheeting is a secondary feature formed after the development of the topography.
The general parallelism of sheeting joints and land surface can be taken as lending support to the
offloading hypothesis, though the interpretation can be reversed with equally satisfactory logic.
The formation of relatively thin slabs or sheets of rock close to the land surface and developed in
response to recent erosion has also been accepted as persuasive evidence of offloading. For exam-
ple, sheets of rock have been developed in cirque headwalls, in the floors of recently deglaciated
valleys, and in relation to older glacial forms (as for instance in northern Italy), and recently dis-
sected riverine valleys (as on the Atlantic coast of Galicia -
Fig. 2.6).
Though it is plausible and persuasive, the offloading theory in the sense outlined by Gilbert
(1904) and adopted by many later workers, namely, that pressure release is the sole cause of sheet
jointing, may be called to question on several grounds.
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Simple triaxial tests show that compression and decompression of essentially isotropic materi-
als do not cause fracturing save in special circumstances which are unlikely to be found in
nature; such fractures are unlikely to develop in the context of slow erosional unloading (Fell,
MacGregor and Stapledon, 1992). Even in anisotropic materials it appears that several cycles of
compression and decompression can be applied to unconfined specimens before, with increased
loads, the material ruptures in fatigue. Unloading appears to be mechanically incapable of pro-
ducing sheet fractures.
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It is difficult to understand why, if expansive stress developed during erosion, it has not been
accommodated along pre-existing lines of weakness. Sheeting is either absent or only poorly
developed in closely-jointed granites, and as White (1946, p. 5) stated “most geologists accept
