Geology Reference
In-Depth Information
9
Minor forms developed on gentle slopes
Some bornhardts and large residual boulders have gently sloping upper surfaces. Some of the cre-
stal bevels of bornhardts were platforms before the plains around them were lowered. They are
commonly dimpled, due to the development of rock basins, and grooved by clefts and runnels. In
addition, though more rarely, pedestals, rock doughnuts and rock levees are developed.
9.1
ROCK BASINS
9.1.1
Description
Rock basins are depressions formed in solid bedrock (Twidale and Corbin, 1963). Morpho-
logically they vary in detail, but most are oval, elliptical or circular in plan. Some, strongly influ-
enced by jointing, are angular in form, and others, resulting from the coalescence of two or more
individuals, are irregularly lobate in plan. All are temporary water storages. Water accumulated
from rainfall and runoff is lost either through evaporation, or use by animals (including humans),
or by underground seepage. Alternatively, if supply exceeds storage and losses, the water in the
basin overflows, cutting an outlet so that the depression eventually becomes part of an integrated
drainage system.
Several morphological types have been recognised:
- Pits are hemispherical, are developed on gentle slopes and are isolated. Outlets are rare
(Fig.
9.1a).
Pits with overhanging sidewalls are called flasks.
-Pans are also developed on gentle slopes, and are comparatively shallow and flat floored (Figs
1.2f, 9.1b and c). Outlets are few, though some pans are linked by gutters or runnels, and thus
form part of an integrated drainage system (
Fig. 9.2a).
-A mchair-shaped hollows are asymmetrical in section normal to the contours. They have high
backwalls on the upslope side, but are open downslope (Figs 9.1d and e), where there is fre-
quently a distinct outlet in the form of a runnel or gutter. Armchair-shaped hollows are typical
of the moderately steep (20-30°) slopes that lead down from the flattish crestal areas to the
steep bounding slopes and the plains (Fig. 9.2b).
- Cylindrical hollows vary in plan shape, though they are approximately circular or oval, and are
rectangular in vertical section, so that they are appropriately referred to as being of cylindrical
or terete form (Fig. 9.1f). The bedrock cylinder opens into a sheet fracture so that there is in
some instances a through drainage, and the shallow spiral scallops discernible on some side-
walls reflect the swirl of waters rushing down the opening during and after heavy rains; more
commonly the basins are choked by detritus and vegetation, so that the bedrock morphology is
only revealed by excavation and the removal of the fill.
Rock basins vary in size as well as shape. The modal diameter is of the order of a metre and of
depth perhaps 0.5 m. Most hold a few litres of water, but some are much larger. One on King
