Split and cracked blocks and slabs - Landforms and Geology of Granite Terrains - page 261

(c)

Figure 11.6. (c) On Dartmoor, southwestern England.

of all parted blocks, for, though it could conceivably have operated on Dartmoor, southwestern
England, and in the Serra do Gerez, Galicia, NW Spain, for instance, during the Pleistocene, it can
safely be ruled out for the low altitude tropical examples of parted blocks.
On the other hand, and as it has been mentioned, many granite domes and platforms, even those
in the comparatively stable cratonic areas, have been joggled, for small fault scarps, some arranged
in horst and graben patterns ( Fig. 11.1) , are commonplace, and A-tents and areas of chaotically
disposed slabs occur in association with some of the displaced blocks (see below). Thus, while
some dislodged blocks may be due to the action of ground ice, others can be attributed to seismic
shaking. Most, however, remain enigmatic.

11.3 DISLOCATED SLABS

Some granite forms consist of slabs and blocks that have been split, cracked and displaced. They
are all developed in intrinsically fresh bedrock. By far the most common member of this suite is
the A-tent. Others are overlapping slabs, displaced slabs, and vertical and horizontal wedges. The
blisters named by Blank (1951) appear to be related to A-tents, but their crests are not fractured.

11.3.1 A-tents
A-tents consist of pairs of slabs, frequently roughly rectangular in shape, each touching the adja-
cent rock surface at their outer extremities, but standing a few centimetres above the general sur-
face level where they are in contact in a fracture zone known as the roof line (Fig. 11.7a). They
delineate triangular cavities, as do ridge tents: hence their name. In some areas one or both of the
constituent slabs are split by a fracture disposed obliquely to the main crestal fracture.
A-tents, pop-ups in North America (Rutty and Cruden, 1993; Wallach, Arsalan, McFall
et al ., 1993) and chapitaux by French workers, are well-known in Australia (Twidale, Campbell and
Vidal Romaní, 1993; Twidale and Bourne, 2000), being abundantly developed, for example, on
inselbergs on Eyre Peninsula, South Australia, the Kulgera Hills, Northern Territory, and the Yilgarn
and Pilbara blocks, Western Australia (Plug and Plug, 1997 and see Fig. 11.7b) . They occur in
many parts of northern Africa (Boissonnas, 1974). They are reported from Labrador, central Texas
and Georgia, from the Rupununni savannas of Guyana and the Sabah district of east Malaysia.