Geology Reference
In-Depth Information
may be extrapolated and applied to bornhardts in terms of perhaps the most plausible general
explanation of the form (Chapter 6). According to this two-stage concept, many bornhardts are ini-
tiated by differential weathering at the base of the regolith. At one stage they take the form of huge
masses or compartments of intrinsically fresh rock projecting into the base of the regolith (
Fig.
2.14b)
.
Such projections are unconfined for they are surrounded and covered by regolith, which
lacks strength. By analogy with Holzhausen's (1989) experimental work, compression would pro-
duce arcuate-upward stress trajectories, and eventually sheet fractures; though rapid application of
stress could conceivably produce faulting and upthrust (Fig. 2.14c) rather than sheet fractures.
If sheet structure is associated with lateral compressive stress, if it is indeed arching, then the resul-
tant domes are in some measure tectonic forms. As it is discussed later (Chapter 11), small-scale
tectonism finds expression in several aspects of granite bornhardts and platforms, for example. In
addition, some sheeting fractures are planes of dislocation (
Fig. 2.10).
In terms of compression, sheet
fractures associated with surfaces recently sculptured by glaciers or by rivers (as depicted for exam-
ple in
Fig. 2.6)
represent the release of stresses in parallelism with surfaces of least principal stress.
2.4
SUMMARY
Though any of several possible explanations of sheet jointing may be valid in particular areas, the
hypothesis offering the best general explanation is that involving lateral compression, induced by
horizontal stresses, either relic or modern, and the manifestation of stress patterns affecting brittle
rock within, say, a kilometre of the surface when vertical loading is decreased by erosion. In these
terms, sheet fractures and sheet structures are associated with tectonism. Such an explanation
accounts for many details of the field evidence, is consistent with measured stress conditions, and
offers a comprehensive view of the preservation of inselbergs and the sheet structure widely asso-
ciated with them.
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