Geology Reference
In-Depth Information
Such an explanation accounts for the varied results of A-tent development involving expansion,
and for the preferred location of angular A-tents and other forms described here on midslope
where stresses are greatest on an arched (convex) surface. Variations in crest orientation can be
explained in terms either of shear couples or of separate periods of stress from different directions.
Arched and angular forms are in these terms members of a continuum. Given that the near-surface
granite is subdivided into layers (it is divided by sheeting joints and flaggy partings), lateral pres-
sure may result in arching. Further compression would cause a crestal fracture to develop, and still
greater pressure, the marginal fractures. Alternatively, A-tents may evolve during a single com-
pressive event.
Calculations of the internal friction of slabs involved suggest that A-tents are precariously
stable under the influences of gravity and pore pressure, and that increases in horizontal stress
such as are generated in earthquakes and tremors could cause instability and buckling.
If the forms are related to a catastrophic event or events such as earthquakes or tremors, tri-
angular wedges can be explained in terms of a sudden, possibly short-term, increase in compressive
stress resulting in differential movement along sheeting planes, and frictional drag, fracture and
dislocation on the exposed sheeting plane, very much as slippage along bedding planes during
folding can produce slickensides, recrystallisation, etc. The resultant shaking would explain how
both friction and inertia have been overcome, as is implicit in slipped slabs. Increased horizontal
compression would also account for the buckling and expansion implicit in the formation of
A-tents. It is notable that on Lightburn Rocks where A-tents are well-developed, there are also many
recently developed fractures, including some vertical dislocations between juxtaposed slabs. Also,
the assemblage of forms discussed here is well-developed on northern Eyre Peninsula, in a zone
bounded by faults and known to be seismically active. The late Dr D.J. Sutton reported that a small
portable seismograph set up on Mt Wudinna for twelve hours in late 1975 recorded two seismic
events in that short period. The displaced slabs on Quarry Hill are due to blasting - an artificial
miniature earthquake.
11.3.7
Relationship of A-tents and pressure ridges
The A-tents in sandstone developed in Wyoming, and illustrated in Scott (1897), form an elongate
ridge, and the question arises as to the relationship between A-tents and pressure ridges developed
along fault traces such as those of the San Andreas Fault in California (Thomas, Wallach,
McMillan
et al
., 1993). In particular, are A-tents and the linear ridges described from southern
Ontario and from the bed of Lake Ontario comparable? In broad terms, both appear to be due to
tectonic pressures, but first, judging by illustrations of those formed on land in eastern Canada, the
ridges developed there are not hollow but consist of strata pushed up against each other and
upthrust, rather like miniature horst structures; and they develop along considerable lengths of the
fault trace, presumably wherever compressive stress is enough to rupture the strata. Second, the
linear ridges are due to direct pressure of one block against another, as are the vertical wedges
plotted on the flank of Wudinna Hill and noted in
Fig. 11.9.
In A-tents, on the other hand, the pres-
sures responsible for the buckling are transmitted and affect only pre-existing suitable situations,
namely a lamination of the near-surface rock. The difference can be illustrated in relation to the
rock beam or elongate A-tent illustrated in
Fig. 11.10.
It can be interpreted as either having been
squeezed up by direct pressure from the adjacent blocks (as have the wedges plotted on Fig. 11.9);
or, and this is the construction favoured here, raised because of pressure normal to the beam and
imparted at either end.
11.4
POLYGONAL CRACKING
11.4.1
Description
Isolated and randomly disposed groups of cracks occur on many granite surfaces. They have been
called hieroglyphs (
Fig. 11.23a)
.
Some, located on the convex swells of boulders, describe radial
