Geology Reference
In-Depth Information
Davis (1899, 1909) himself regarded peneplains as products of what he called normal erosion,
that is, river work in temperate humid conditions. Davis (1909) cited parts of the western Great Plains
in Montana, and of Siberia as examples, but most of the peneplains he and later workers recog-
nised were, in fact, palaeoplains preserved on upland crests.
Planation surfaces morphologically similar to the peneplain adduced in the theoretical model
are developed on granitic rocks in the southwest of Western Australia (
Fig. 4.1a
)
and on central
and northern Eyre Peninsula, South Australia. Similar features are found also in southern Africa,
for instance, in Northern Province, in Western Cape Province, in parts of central Namibia, and in
central Brazil.
Such granitic peneplains are underlain partly by fresh rock, though mostly by weathered mate-
rial. Some, however, differ in detail from the model deduced by Davis (1899, 1909). In many of
the areas cited rivers are intermittent or seasonal in their flow, and are braided in pattern. The
peneplains of northern and central Eyre Peninsula are developed in weathered, permeable granites
(Bourne, Twidale and Smith, 1974). They also carry a discontinuous but extensive carapace of cal-
crete developed during the later Pleistocene (most of the constituent lime was carried on the wind
from the extensive dune calcarenites of the west coast). Thus, rainwater readily permeates into the
subsurface. For this reason, there are few surface streams, save during and immediately following
rains. But in general terms the examples cited conform to Davis' model. Again, the residuals that
stand above peneplains are not everywhere Davisian monadnocks that rise gently from the sur-
rounding planate surfaces; on the contrary, in many places inselbergs, pediments and peneplains
coexist (
Fig. 4.2).
Peneplains are not restricted to temperate lands. Indeed, they are not confined to any conven-
tionally defined climatic region. Like pediments, they are characteristically developed on weak
rocks, typically argillaceous sediments or weathered crystalline rocks. It is difficult to determine
whether slope decline or backwearing has been involved in their development. On general grounds
it can be argued that in areas of weak rocks downwearing ought to have been dominant, but there
may have been local or ephemeral complications, such as aridity and the development of a gibber
veneer, or the development of a duricrust, in either case implying a caprock and scarp retreat. The
end result was the lowering of the land surface, but there is no means of demonstrating past slope
behaviour in the field.
Surface waters infiltrate into the regolith where they not only continue the processes of weath-
ering but also transport some of the products of alteration (Davis, 1963). After heavy rains, fines
may be flushed through the system, but more significantly, salts are taken out in solution. At pres-
ent, and on average, of the order of 4 billion metric tonnes of carbonates, silica and sulphates, etc.
(a)
Figure 4.2.
(a) Ucontitchie Hill and surrounding mantled pediment, Eyre Peninsula, South Australia.
