Page 110 - Sedimentology and Stratigraphy

Denudation and Landscape Evolution 97

saturate the regolith and promote slumping on breakdown, whereas mafic rocks such as basalts are
slopes. Repeated freezing and thawing of the regolith rapidly weathered and eroded. Large amounts of clay
may also lead to creep downslope. Wind ablation is minerals are generated by the denudation of terrains
important because of the sparse vegetation cover in such as volcanic arcs, which are composed mainly of
subarctic areas. basaltic to andesitic rocks. Under extreme chemical
weathering of silicate rocks deep lateritic soils develop:
laterites are red soils composed mainly of iron oxides
Temperate regions
and aluminium oxides.
In temperate climates both physical and chemical Limestone bedrock is primarily weathered by disso-
weathering processes tend to be subdued. Erosion lution, and the pattern of denudation is therefore
is generally more vigorous under wetter climates, dominated by development of karst scenery
but on the other hand, vegetation, which is usually (Fig. 6.10). Solution related to joints and fractures in
denser in humid climates, tends to stabilise the surface the rock leads to the formation of deep, steep-sided
and can reduce erosion. The rate of denudation of canyons on the surface and cave systems under-
limestone terrains is strongly climate-controlled, for ground. Little clastic detritus is generated from the
in humid temperate or tropical regions the rate of denudation of limestone terrains: conglomerates of
denudation is ten times higher than in arid subtropi- limestone clasts may form near the site of erosion,
cal and subarctic regions (Einsele 2000). but most of the material is in solution, with sand-
sized detritus largely absent.
A characteristic scenery also forms where the bed-
6.6.3 Bedrock lithology and denudation rock is poorly lithified: badland terrains (Fig. 6.11)
form by the deep erosion of weakly consolidated sand-
The type of bedrock is a fundamental control on the stones and mudstones as large amounts of detritus are
rates and patterns of denudation. The main factor is carried away.
the rate at which weathering processes break down
the rock to make material available for erosion. The
greatest variability is seen in humid climates where 6.6.4 Soils and denudation
chemical weathering processes are dominant because
different lithologies are broken down, and hence Soil development has an important role in weathering
eroded, at widely different rates. The proportions of processes. First, water is retained in soils and hence
the rock-forming silicate minerals (Fig. 6.6) are the the thickness of the soil profile influences how much
main factor: quartz-rich rocks are least susceptible to water is available: if the soil profile is too thin it does

Fig. 6.10 Erosion by solution in beds of
limestone results in a karst landscape.