Geology Reference
In-Depth Information
(b)
Figure 4.2.
(b) Map of Ucontitchie area.
are transported annually in streams alone (Livingstone, 1963). This figure takes no account of the
immense volumes of salts in solution that are transported to the oceans in groundwaters nor those
that are carried in internal drainage systems. The volume of such dissolved solids varies greatly
from region to region, from some 15.5 metric tonnes per square kilometre in Australia to 47 in
Europe. What is clear is that vast quantities of dissolved minerals are carried in rivers and ground-
waters, and that such subsurface erosion could materially contribute to surface lowering following
volume decrease and compaction in the regolith.
4.2.2
Pediments
By definition, pediments are gently inclined, cut bedrock surfaces located in the piedmont zone
(Figs 4.2 and
4.3)
.
Though many pediments carry a regolith veneer, the surface form reflects the
slope of the bedrock surface. This is the essential difference between pediments and alluvial fans,
for the form of the latter is a consequence of deposition, and the inclination of the surface is a
function of the gradient of the streams responsible for the transportation and deposition of the
debris. On granitic rocks the slope of pediments varies between 0.5° and 7°, but they are typically
inclined at 0.5°-2.5° with respect to the horizontal. Most are gently concave upward, many are rec-
tilinear, and a few are convex. But whatever their geometry, pediments meet the backing escarpment
in an abrupt break of slope called the piedmont angle or pediment nick. Three types of pediment
have been recognised: those with a cover of allochthonous debris, those with a mantle derived pri-
marily from the weathering of bedrock
in situ
, and rock pediments or platforms which are essen-
tially devoid of a cover of unconsolidated material. Of these, the first type is essentially restricted
to sedimentary terrains. Such covered pediments are rarely formed in granite. Some are devel-
oped, as near Usakos, in central Namibia
(Fig. 4.4),
but they are unusual. On the other hand, man-
tled and rock pediments (the latter also known as platforms - see Chapter 8) are well-developed
on granitic rocks.
