Geology Reference
In-Depth Information
Figure 8.24.
Inverted Rillen on southern, flared sidewall of Turtle Rock, near Wudinna, Eyre Peninsula,
South Australia.
Environmental changes could be invoked to account for the absence of algae in the later flows, but
not all ribs are associated with rock basins. Indeed some on Yarwondutta Rock serve very small
catchments. It may be that, on leaving the confined gutters of the upper slopes, the small streams
spread on to the upper parts of the flared overhangs. Algae continue to grow on the central moister
areas, but the wash lateral to these causes erosion of the flaked or laminated surface rock, creating
depressions and leaving the central area in relief (Twidale and Campbell, 1986).
But other explanations are possible. For example, the colonisation of local rises by algae can be
seen as effects rather than causes. It can be suggested that the ribs are due to the contrast in rate of
weathering between wet areas and those that are alternatively wet and dry. The surface of the resid-
uals is flaked or scaled, inherited either from the weathering front (Chapter 3), or developed after
exposure by insolation or by water-related weathering. The linear vertical zones downslope from
the gutters scored on the upper surface of the inselbergs are moist because of the water flowing
from above, and retained in the interstices between flakes. Summer storms maintain this supply in
these narrow sectors. To either side there are zones which are drier in summer (in southern
Australia) because the supply of moisture drawn up between flakes by capillarity from the soil at
the base of the hill disappears as the surface soil is desiccated. Summer storms replenish the sup-
ply here also, but the soils are well below field capacity and the meteoric waters percolate to the
water table at depth. Alternations of wetting and drying cause the flakes to fall away, so that those
sectors of the sidewall are worn back, leaving the more stable zones below the gutters in relief
as upstanding narrow ribs. Such wetting and drying of basal slopes could also explain some of the
basal fretting referred to earlier
(
Chapter 8.2)
.
REFERENCES
Alexander, F.E.S. 1959. Observations on tropical weathering: a study of the movement of iron, aluminium
and silica in weathering rocks at Singapore.
Quarterly Journal of the Geological Society of London
115:
123-142.
Anderson, A.L. 1931. Geology and mineral resources of eastern Cassia County, Idaho. Idaho Bureau of Mines
and Geology Bulletin 14.
Branner, J.C. 1913. The fluting and pitting of granites in the tropics.
Proceedings of the American
Philosophical Society
52: 163-174.
