Geology Reference
In-Depth Information
Rocks, near Hyden, in Western Australia, has a capacity of almost 7 million litres (1.5 million
gallons). The various types have been described from many parts of the world. Basins are widely
distributed and are characteristic of granite outcrops of all shapes and sizes.
9.1.2
Nomenclature
Rock basins have been referred to by several names. Though first mentioned in the scientific
literature in the late Eighteenth Century, the forms had actually been perceived, measured and
discussed in general terms for many centuries. They were described from Dartmoor in south-
western England, where they were known as rock basons, in 1291, more than seven centuries ago
(Worth, 1953).
Rock basins of various types have been referred to also as rock holes, weather pits, pot-holes,
water eyes, cauldrons, cisterns, granite pits, and, in Australia, night-wells, and gnammas. In Idaho
they are known as bath tubs, and other local names have doubtless also been applied. They are the
tanques and vasques rocheuses of the French literature, and the Verwitterungsnäpfe, Opferkessel,
Baumverfallspingen, Felsschüssel and Dellen of German workers. They have also been called
kociolki in Poland, pias, marmitas, cassolas and pilancones in Spain, caldeiros, poços, and oriçangas
in Brazil, and araceenhorst in Surinam.
9.1.3
Origin
For some years, about two centuries ago, rock basins were attributed to human agencies, and in
Britain, for example, were thought to be related to Druidical ceremony. Red staining of the water
held in the basins and due to red algal growths, or of the basin rims (due to the release of iron dur-
ing the alteration of biotite and other minerals) suggested blood to the ancients. Such an anthro-
pogenic origin can be dismissed as a general explanation if only because many basins were initiated
beneath the natural land surface (Twidale and Bourne, 1975), at the weathering front (see below).
Water, with solution, hydration and hydrolysis the major processes at work, is responsible for
differential weathering at the weathering front and also for growth of the basins after exposure
(Smith, 1941 and see
Fig. 9.3a)
.
The granite is rotted and disintegrates, leaving a residue of gritty
sand, with some larger fragments (Fig. 9.3b). Evorsion, and mechanical abrasion by granite sand
carried by swirling water, however, contribute to the formation both of armchair-shaped hollows
located on the flanks of residuals, and of the cylindrical forms.
It is difficult to ascertain with certainty why specific basins are located where they are, for the
rock that provides the evidence has been weathered and evacuated. Any condition that can produce a
depression, however minor, is enough, for accumulated water alters the rock with which it comes
into contact and so enlarges the original hollow. Granite is not compositionally homogeneous.
Xenoliths are commonplace and frequently induce preferential weathering as do clusters of
phenocrysts (
Fig. 9.3c).
Thus, in the Linares district of Andalusia, and at several sites in Galicia
and Girona, northwestern and northeastern Spain respectively, small masses of biotite-rich or
otherwise melanocratic rock have been exploited to produce small, angular basins comparable in
size and shape to the xenoliths that are still intact. Similarly, the essential minerals in granite are
not everywhere evenly distributed. It is not uncommon to find bands or discrete masses of feldspar
or biotite, both of which minerals, being susceptible to moisture attack, could be preferentially
weathered to form an initial hollow that could later develop into a basin.
Gravitational loading
(Fig. 9.4),
i.e., the pressure exerted by the rock itself when the load con-
centration effect is produced (minimum thickness calculated to be of the order of 300 m), could
cause local strain, lattice distortion and disequilibrium, and fragmentation of crystals. In contact
with moisture, either at the surface, or, more probably, initially within the zone of groundwater, the
strained sites or linear zones would be weathered, inducing the development of small depressions.
Similarly, tectonic strains may be concentrated in pressure points (like those induced by the inser-
tion of rock bolts) as a result of uneven contacts between blocks beneath the surface, as for example
