Geology Reference
In-Depth Information
Figure 9.17.
Reservoir at Dumonte Rock, near Wudinna, Eyre Peninsula, South Australia, showing gutters
draining naturally exposed surface and continuing along exposed weathering front (below
X-X). Some even converge in the natural subsurface.
become shallow and wide, as a result of the dispersion of flow between soil fragments, and disap-
pear only one or two metres away from the edge of the outcrop. Others, however, extend several
metres at least along the sloping contact between fresh and weathered rock. After exposure the
channels are enlarged and otherwise modified, but some of them at least originate at the weather-
ing front and are therefore intrinsically of etch character.
9.5
ROCK LEVEES
At a few sites gutters are bordered by raised rims, standing a few centimetres above the adjacent
slope, and known as rock levees, or rimmed gutters (
Fig. 9.18a),
after their morphological and
locational similarity to alluvial forms, which are, however, developed on an altogether larger scale.
Such forms occur on Domboshawa (Whitlow and Shakesby, 1988), a granitic bornhardt near
Harare, Zimbabwe, where the slopes are scored by broad flat-floored valleys drained by narrow
channels or gutters many of which are bordered by rock levees. The rims or levees have been
attributed to the precipitation of a protective patina of iron oxides from waters spilling out from the
channels, for which there is local evidence around some basins (see below) but which does not
occur consistently on the raised rims. They have been attributed to protection by a patina of opal-
ine silica. The absence of lichens (such as
Heppia spp.
), due to moist conditions adjacent to the
channels, has also been noted, and it has been suggested that this has a conservative function inso-
far as on the adjacent flat divides lichens and algae actively weather the granite and render it
susceptible to erosion by wash.
An alternative explanation is suggested by observations on Domboshawa. The flattish floors
adjacent to the levees are pitted and only sparsely colonised by lichens, suggesting that these areas
may once have been covered by a thin regolith (
Fig. 9.18b).
It is envisaged (
Fig. 9.19)
that bare rock
was exposed in the areas adjacent to the gutters, the edge of the regolith having fallen and been
washed into the channels. The exposed rock surfaces were comparatively dry, and hence stable.
The regolith, however, retained moisture, so that the weathering of the underlying bedrock con-
tinued. It was lowered, leaving the near-channel zones in relief as rims or levees. Siliceous and
