Geology Reference
In-Depth Information
(b)
Figure 11.19.
(b) Chaos of thin slabs on Minnipa Hill, near Wudinna, northwestern Eyre Peninsula, South
Australia, formed on 19 January 1999.
Figure 11.20.
Horizontal wedge
in situ
, Ucontitchie Hill, northwestern Eyre Peninsula, South Australia.
Angular buckles have been attributed to insolation. But the Sun's heat can be discounted as the
cause of the A-tents and the other forms considered here, because insolation argues heating and
cooling, and hence a reversible event, whereas the A-tent represents an irreversible change in the
volume of the granite involved. Displacement by such heating and cooling is conceivable, but per-
manent deformation is not. If insolation were responsible, A-tents ought to occur preferentially on
sunny aspects, such as crests and western slopes, but no such distribution has been noted. The
ephemeral though intense heat generated in forest, scrub or bushfires undoubtedly causes flaking
and spalling of exposed surfaces, but the bedrock surfaces on which A-tents are formed lack vege-
tation so that there is no fuel for fires. Also, the flakes produced by such heating are typically thin-
ner and less regular than the slabs involved in the thicker A-tents. Furthermore, there is some
