Page 111 - Sedimentology and Stratigraphy

98 Continents: Sources of Sediment

Fig. 6.11 Badlands scenery formed by
the erosion of weak mudrock beds.

not retain sufficient water for chemical weathering washed away by rainfall or floods. The effects of
reactions in the bedrock to be effective, but if it is too wind action on the regolith are also reduced where
thick it is able to store and lose water through plant a vegetation cover binds fine detritus into soil. A
evapotranspiration, hence reducing the availability of sparse plant cover in cold or arid regions leaves the
water for weathering reactions. Second, biochemical regolith exposed to erosion by water and wind. In
reactions in soils create acids, collectively known as deserts overland flow following storms may be very
humic acids, which increase rates of solution of car- infrequent but in the absence of much plant life a
bonate bedrock. Third, soils are host to plants and lot of loose debris may be washed away in a single
animals, which also play a role in breaking down flash flood.
bedrock, especially roots that can penetrate deep The nature of the vegetation colonising the land
into the rock and widen fractures. Although many surface has changed considerably through geological
soil processes may enhance weathering, soil develop- time. Four stages in the development of land plants
ment can inhibit erosion by hosting a vegetation are significant in terms of sedimentological processes
cover that protects the bedrock. (Fig. 6.12) (Schumm 1968).
1 Pre-Silurian: there was no land vegetation at this
time so it can be assumed that the denudation rates of
6.6.5 Vegetation and denudation continental areas were generally higher than they are
today.
The types of vegetation and the coverage they have 2 Silurian to mid-Cretaceous: the main plant groups
over the land surface are determined by the climate were ferns, conifers and lycopods with relatively sim-
regime, which is in turn influenced by the latitude ple roots systems with a limited binding effect on the
and altitude. A dense vegetation cover is very effective regolith.
at protecting the bedrock and its overlying regolith 3 Mid-Cretaceous to mid-Cenozoic: angiosperms
from erosion by rain impact and overland flow of (flowering plants) became important and had more
water. Even steep mountain slopes can be effectively complex root systems that were more effective at
stabilised by plants. In tropical regions destruction of binding the soil.
the vegetation cover by natural events such as fires or 4 Mid-Cenozoic to present: the evolution of grasses
anthropogenic activity (man-made effects) such as meant that there was now a widespread plant type
logging can have a catastrophic effect on erosion: the which covered large areas of land surface with a
bedrock beneath the plant roots may be very deeply dense fibrous root system that very effectively binds
weathered and the regolith susceptible to being soil.