Geology Reference
In-Depth Information
Many corestones display pitting (Twidale and Bourne, 1976) (Fig. 3.5), but this probably devel-
oped subsequent to the formation of the corestones, when lowering of the land surface brought
them into the near surface zone of intense weathering associated with abundance of moisture and
biota. Some corestones are set in a mass of grus. Such weathering is known as granular disinte-
gration. Some corestones are surrounded by layer upon layer of thin (1-5 mm) discontinuous
flakes, slivers or laminae, wrapped around the corestones, like the leaves of a book (Fig. 3.1a).
At other sites the concentric layers are thicker (10-30 cm) and look like the leaves of an onion:
hence the name onion-skin weathering, which is also referred to as spalling (Figs 3.1b and c,
and 5.10). Finally, in some localities corestones have evidently been formed through the separa-
tion, by fracturing, of tetrahedral masses, each with a concave inner face, at each corner of the
joint block. Some of the roughly ovoid core masses have flat ends and look like barrels (see
Chapter 6, section 3.3).
Whatever the type of marginal weathering, however, almost all observers agree that the transi-
tion from the fresh rock preserved in the corestone to the friable, altered marginal areas - the
weathering front - is remarkably abrupt. This sharp change is almost certainly due to the physical
character of granite which is of very low porosity (Mabbutt, 1961) and permeability when fresh
and cohesive, but which becomes much more permeable once it is even slightly weathered.
5.6
CAUSES OF PERIPHERAL WEATHERING
Some workers have invoked insolation as the cause of disintegration and spalling, but corestones set
in grus are commonly found scores of metres beneath the land surface, far beyond the effects of diur-
nal, annual or even secular temperature changes. Heating and cooling cannot explain such weather-
ing deep beneath the surface (Farmin, 1937). Surface flaking (
Fig. 5.14)
,
which is such a notable
feature of granitic outcrops in arid and semi-arid lands, may, however, be of this origin, though it can
also be construed as having been initiated at the weathering front and then been exposed by the strip-
ping of all but the basal flakes of the regolith (Chapter 3). The common occurrence of concentrations
of iron oxides in the laminae marginal to corestones (Fig. 3.6 pp. 57) strengthens this suggestion.
Several workers have suggested that the concentric structure observed in the marginal areas of
many granite joint blocks is due to the release of pressure consequent upon erosional offloading
Figure 5.14.
Surface flaking on granite platform, western Pilbara, Western Australia.
