Geology Reference
In-Depth Information
(a)
(b)
Figure 11.7.
(a) Section of A-tent. (b) A-tent on Kokerbin Hill, Western Australia.
A-tents typically occur on midslope (Fig. 11.8a). They are up to 6 m long and up to almost 8 m
wide, but most are smaller, means of 2 m long and 1 m wide being more typical. All A-tents have
a crestal fracture and most are also fractured at their lateral terminations; but in some the slab
extends with no apparent break into the adjacent hillslope. Most of the A-tents that have been stud-
ied are 10-15 cm high and involve slabs about 10 cm thick, but one on Wudinna Hill, northwest-
ern Eyre Peninsula, South Australia, has slabs 580 mm thick standing 820 mm above the floor
(
Fig. 11.8b;
also X in
Fig.11.9)
,
and one on Carappee Hill, on northern Eyre Peninsula, comprises
slabs only 13 mm thick (Fig. 11.8c). A beam of granite some 15 m long, just over 2 m wide at its
broadest point, 79 cm thick maximum, standing up to 15 cm higher than the adjacent slope
(
Fig. 11.10a)
and located on Wudinna Hill, was formerly construed as a vertical wedge, caused by
squeezing between adjacent slabs in a direction normal to the length of the beam. It was first inter-
preted (Jennings and Twidale, 1971) as due to NE-SW compression and direct contact squeezing
by the slabs on either side. But the sides of the wedge are not (now?) in contact with the adjacent
walls, and the feature is more plausibly interpreted as an elongate A-tent due to compression from
NW and SE, in line with other A-tents on the hill and in the vicinity. Many small triangular wedges
occur along fractures on a disturbed sector of the adjacent slope (W in Fig. 11.9). In most areas the
crests of A-tents are aligned (
Figs 11.11a
and b), but in others their orientation varies, in some
instances being aligned normal to each other at adjacent sites (Fig. 11.11c). The terminal and
crestal fractures typical of the angular forms are not present in blisters, formed by arched shells or
slabs, typically 1-2 cm thick. There is a continuum of forms between thin sheets that sound hol-
low when tapped, through arched slabs which lack crestal cracks, arches with incomplete or dis-
continuous cracks, and A-tents. Most angular A-tents occur on midslope sites, whereas arches are
