Geology Reference
In-Depth Information
Figure 9.7.
Pan with large overhanging sidewall, Yarwondutta Rock, near Minnipa, northwestern Eyre
Peninsula, South Australia.
9.1.4
Differentiation of major types
Whether initiated on exposed surfaces or at the weathering front, the original saucer-shaped
depressions are further developed and several morphological types evolve on exposed surfaces.
Because water is in contact longer with the floors and lower sidewalls of the depressions than with
the upper sidewalls, weathering effectively causes the basin to be extended vertically into the rock
mass and, depending on whether or not there was undermining of the surface layer, a pit or a flask-
shaped depression is formed.
Where water acts on sensibly homogeneous granite, the availability of water (runoff), capacity
of the depression to hold water and the duration of wetting are all-important, and hemispherical
pits form as a result of reactions between water and the bedrock. The most common type of basin,
found in various climatic and lithological settings, is the flat-floored pan. Pans are circular or ellip-
tical in plan and their outlines are generally smooth and regular. Most are 1-2 m in diameter and
some 30-50 cm deep, though composite features are, of course, larger and also less regular in plan.
The sidewalls of many pans are overhanging by a matter of a few centimetres, though on
Ya rwondutta Rock, northwestern Eyre Peninsula, South Australia, the floor of one basin extends
about one metre beneath the edge of the protruding lip (Fig. 9.7). Pans are consistently developed
in flaggy rock characterised by discontinuous fractures that run essentially parallel to the rock
surface. Water readily penetrates along these joints so that lateral extension takes place more
rapidly than does vertical growth. This explains the large diameter/depth ratio of the pans. The floors
of the pans are frequently coincident with parting planes. This argument finds support at many
sites. At Lightburn Rocks, in the eastern Great Victoria Desert in South Australia, for example,
pans are well-developed on the crestal areas of flaggy or laminated rock, but pits occur on the gen-
tle, lower slopes of the flanks where massive rock is exposed.
The overhanging sidewalls of the pans are due to three effects, one structural, one biotic and the
other related to duration of wetting. First, in many areas, a superficial induration of oxides of iron
and manganese is developed, and this effectively cements the bedrock so that it withstands weath-
ering and erosional attack better even than does the fresh rock. Second, lichens do not colonise
surfaces which are frequently under water, but they are widely established on the upper slopes of
rock basins. Third, water persists longer in the floor of the depression and the lower sidewalls are,
for this reason, weathered back more rapidly than are the higher zones. This last factor is everywhere
