Split and cracked blocks and slabs - Landforms and Geology of Granite Terrains - page 280

(c)

Figure 11.24. (c) On rock platform, Corrobinnie Hill, northern Eyre Peninsula, South Australia.

mode both near the upper end of the range. Some of them are slightly curved or convex upward in
respect to the surface of the host rock. On boulders on Tcharkuldu Hill, on Eyre Peninsula, South
Australia and at The Granites, near Mt Magnet, Western Australia, some of the thin plates have
been either worn away or have fallen, so that as many as three layers, each with polygonal cracking
developed, are exposed. The narrow hairline cracks of the deeper layer stand in marked contrast
with the wider, less sharply defined, fractures of the outer skin.
The granitic rocks in which the most clearly defined and prolific polygonal cracking occurs are
characteristically equigranular and medium grained, though examples on fine grained aplitic
rocks have also been noted. Thus, the granite at Tcharkuldu Hill, where the cracking is most abun-
dantly developed, is of this type. Conversely, where the granite is porphyritic, as, for instance, at
Wudinna Hill, no polygonal cracking has been observed. Such features are formed at the weather-
ing front. Commonly, though not universally, polygonal cracking is associated with heavy surfi-
cial indurations of iron oxide, manganese oxide, and possibly silica. This association is obvious at
such notable granitic polygonal cracking sites as Tcharkuldu Hill, The Granites, near Mt Magnet,
and Augrabies, in southern Namibia. The advanced weathering of the fractures, which define the
polygonal plates, causes the latter to be reduced in area. Some may be eliminated. Broad flats
develop between the surviving plates, which take the form of miniature mesas or mogotes
(Twidale, Bourne and Vidal Romaní, 1999). Some slopes, like one on King Rocks, near Hyden in
Western Australia, carry many such mogotes ( Fig. 11.25).

11.4.2 Previous interpretations
Though widely distributed and frequently noted, polygonal cracking has not until recently
received the attention it deserves in the geomorphological literature, though it has been attributed
to several agencies and mechanisms. Johnson (1927) thought that cracking is due to insolation and
chemical weathering. The shells are only thin and within the range of daily and secular tempera-
ture changes. But if insolation (or forest fires) has any role in the formation of the cracks, they
ought to be preferentially developed on the sunny (in the southern hemisphere, the northern