Geology Reference
In-Depth Information
Where basins develop in granite that is sensibly isotropic, but where there is superficial mineral
induration, or biotic protection, flask-shaped basins or Opferkessel develop. Where the pits have
penetrated through the slab or sheet in which they have developed to the sheeting joint beneath,
water draining into it runs along the basal fracture so that the previously enclosed basin becomes
a throughway for running water. The base is widened, the water swirls around, creating shallow
9.1.5
Evacuation of debris
The evacuation of debris from basins has given rise to some discussion. It is to some extent a non-
question, for much of the granite sand remains in the basins, where, because it retains moisture
and attracts vegetation and hence organic acids, it probably enhances the rate of basin develop-
ment. Also, organic growths in the basins cause overflows of water rich in humic acids and also
are favourable to the growth of blue-green algae, which may affect the development of flutings or
Rillen on steep slopes (see Chapter 8).
Many rock basins, both in Australia and elsewhere, have been cleared of debris by humans. Not
only did Aboriginal people clean out the basins in order to make them more effective as water stor-
ages, but Afghan cameleers transporting stores to inland stations, and shepherds and stockmen,
have done the same. Slabs of rock (and later, sheets of corrugated iron) were placed over basins in
an attempt to reduce evaporation and contamination. Such anthropogenic cleansing may be not so
widespread as has been supposed, because physical and chemical agencies, i.e., solution, suspen-
sion, etc. have also been invoked to account for clean basins in humid areas where rock basins are
very common features.
Some workers give credence to the deflational work of wind in suitable environments, and even
in parts of humid western Europe, coarse fragments are concentrated by the evacuation of fines.
Dissolved salts are evacuated in solution. Also, fines are carried in suspension by turbulent flow,
and water flushing through the drainage network seems to offer an adequate and feasible mech-
anism to account for the transport of clays and fine sand derived from the weathering of the gran-
ite. The layering of sediments according to grain size within some basins suggests that turbulence
is not significant everywhere.
Sediment left behind in the basins shows concentrations of coarse debris in the centre of the
depressions and even small ripple marks; this in contrast with lower energy regimes where coarse
debris, fallen from the sides of the depression, is found at the edges of the basin deposit.
9.1.6
Rate of development
The evidence concerning the rate of growth is equivocal. Rock basins have developed on the flat-
tish upper surfaces of dislodged blocks in some areas suggesting that the forms evolve in a matter
of a few thousand of years. Basins are widely and well-developed in several polar regions (e.g.
Scandinavia, the Canadian Arctic) that were covered by ice until a few thousands of years ago. The
implication is that the basins have either survived glaciation or have developed since the exposure
of the surfaces on which they occur. The latter is not impossible. In the Bohemian Massif basins
have evidently developed at a rate of several centimetres each century, and, though the measure-
ments were not as precisely controlled as might be thought desirable, there is strong suggestion of
rapid growth. On the other hand, in the Iberian Peninsula and in the Snowy Mountains of New
South Wales, basins are absent from glaciated areas but are found on adjacent, unglaciated, sur-
faces. The state of the local glacier ice (hot, cold) is crucial, and the existence of preglacial, as well
as postglacial, basins cannot be ruled out.
On the other hand, Mistor Pan, which is a large and well-known rock basin on Dartmoor, south-
western England (Worth, 1953), was described in 1291 and again in 1609. Its depth was measured
in 1828, 1858, 1875 and 1929. All results were similar, the differences being as readily accountable
in terms of the difficulty in determining the upper limit of the depression as by any real increase in
depth. Thus, whereas in some regions, basin development has been rapid, elsewhere it is negligible.
