Geology Reference
In-Depth Information
(b)
(a)
Figure 8.20.
(a) Pitted floor of runnel on eastern slope of Ucontitchie Hill, northwestern Eyre Peninsula,
South Australia. (b) Potholes along grooves on Pic Boby, Andringitra Massif, Madagascar.
retaining moisture, they contribute materially to channel development. Lichens are capable of both
mechanical and chemical weathering, but it is surely more likely that they colonise zones that are
already moist rather than growing in linear patterns in the first place. For this reason, most workers
assign to lichens, mosses and other organic agencies a contributory rather than dominating or initiat-
ing role. Moreover, in some circumstances organic (algal) slime has a protective function. Rills rich in
organic matter initially increase the rate of weathering and give rise to intensely pitted channel floors.
Some channels are merely continuations of the gutters draining upper slopes, but some are con-
fined to the steep slopes. Pot-holes indicate that scouring and abrasion are active (Fig. 8.20b), but the
very steepness, and, in places, the overhanging character of some of the bounding walls of the gran-
ite residuals preclude simple abrasion as a significant factor because the flows become separated
from the bedrock surface (Vidal Romaní, Ramanohison and Rabenandrasana, 1997; Petit, 1971).
In these circumstances a number of other factors may be cited. Separation of flow may result in
or lead to collapse and impact in linear zones - the so-called water-curtain effect. Scouring could
cause increased surface roughness and induce further perturbation, air entrainment and turbulence,
and hence increased erosion in linear zones running down steep bedrock surfaces. The field evi-
dence suggests that, important as free flows are in evolution of these grooves, trickles of water are
also significant. Such trickles, some deriving from patches of soil, others from sheeting joints, per-
sist long after rain has ceased, even on overhanging slopes to which they adhere by surface tension.
These are seepages rather than flows. The mechanical action of running water cannot be invoked
in explanation of the grooves. Besides wetting the surfaces over which they flow and encouraging
algal and other growths, such seepages and trickles transport salts and minerals in solution and are
essential to the formation of speleothems (see Chapter 10).
Some have suggested that where high velocity flow develops, cavitation may occur, with high
pressure waves and high speed water jets causing local rock shattering; and this could conveniently
account for the pot-holes and other depressions found along some grooves, as, for example, on
