Geology Reference
In-Depth Information
Pediplains and panplains
that chemical weathering reduces the mass of weath-
ered material, but only on rocks especially vulnerable
to solution (such as limestones) were chemical processes
thought to have an overriding influence on landscape
evolution. However, it now seems that forms of chemical
weathering are important in the evolution of landscapes.
Groundwater sapping
, for instance, shapes the features
of some drainage basins (e.g. Howard
et al
. 1988). And
the solute load in three catchments in Australia com-
prised more than 80 per cent of the total load, except in
one case where it comprised 54 per cent (Ollier and Pain
1996, 216). What makes these figures startling is that
igneous rocks underlay the catchments. Information of
this kind is making some geomorphologists suspect that
chemical weathering plays a starring role in the evolution
of nearly all landscapes.
In tropical and subtropical environments, chemical
weathering produces a thick regolith that erosion then
strips (Thomas 1989a, 1989b, 1994). This process is
called
etchplanation
. It creates an
etched plain
or
etch-
plain
. The etchplain is largely a production of chemical
weathering. In places where the regolith is deeper, weakly
acid water lowers the weathering front, in the same way
that an acid-soaked sponge would etch a metal surface.
Some researchers contend that surface erosion lowers
the land surface at the same rate that chemical etching
lowers the weathering front (Figure 15.2). This is the
theory of
double planation
. It envisages land surfaces
of low relief being maintained during prolonged, slow
uplift by the continuous lowering of double planation
surfaces - the
wash surface
and the
basal weathering
surface
(Büdel 1957, 1982; Thomas 1965). A rival view,
depicted schematically in Figure 15.3, is that a period
of deep chemical weathering precedes a phase of regolith
stripping (e.g. Linton 1955; Ollier 1959, 1960; Hill
et al
.
1995; see Twidale 2002 for an excellent review).
Whatever the details of the etching process, it is very
effective in creating landforms, even in regions lying
beyond the present tropics. The Scottish Highlands
experienced a major uplift in the Early Tertiary. After
50 million years, the terrain evolved by dynamic etching
with deep weathering of varied geology under a warm
to temperate humid climate (Hall 1991). This etching
led to a progressive differentiation of relief features, with
the evolution of basins, valleys, scarps, and inselbergs.
Penck's model of slope retreat was adopted by Lester
Charles King, who, in another model of landscape evo-
lution, proposed that slope retreat produces pediments
and that, where enough pediments form, a
pediplain
results (King 1953, 1967, 1983). King envisaged 'cycles
of pedimentation'. Each cycle starts with a sudden
burst of cymatogenic diastrophism and passes into a
period of diastrophic quiescence, during which sub-
aerial processes reduce the relief to a pediplain. However,
cymatogeny and pediplanation are interconnected. As a
continent is denuded, so the eroded sediment is deposited
offshore. With some sediment removed, the continental
margins rise. At the same time, the weight of sediment in
offshore regions causes depression. The concurrent uplift
and depression institutes the development of a major
scarp near the coast that cuts back inland. As the scarp
retreats, leaving a pediplain in its wake, it further unloads
the continent and places an extra load of sediment off-
shore. Eventually, a fresh bout of uplift and depression
will produce a new scarp. Thus, because of the cyclical
relationship between continental unloading and the off-
shore loading, continental landscapes come to consist of a
huge staircase of erosion surfaces (pediplains), the oldest
steps of which occur well inland.
Another variation on slope retreat concerns the notion
of
unequal activity
espoused by Colin Hayter Crickmay
(1933, 1975). Davis's, Penck's, and King's models of land-
scape evolution assume that slope processes act evenly
on individual slopes. However, geomorphic agents act
unequally. For this reason, a slope may recede only
where a stream (or the sea) erodes its base. If this
should be so, then slope denudation is largely achieved
by the lateral corrasion of rivers (or marine erosion at
a cliff foot). This will mean that some parts of the
landscape will stay virtually untouched by slope reces-
sion. Some evidence supports this contention (p. 399).
Crickmay opined that lateral planation by rivers creates
panplains.
Etchplains
Traditional models of landscape evolution assumed
that mechanical erosion predominates. It was realized
