Page 107 - Sedimentology and Stratigraphy

94 Continents: Sources of Sediment

With sufficient water a slump may break up into a from the surface and erode the regolith. The quantity
debris flow (4.5.1). of water involved and its carrying capacity depends
not only on the amount of rainfall but also the char-
acteristics of the surface: water runs faster down a
Scree and talus cones
steep slope, vegetation tends to reduce flow and trap
In mountain areas weathered detritus falls as grains, debris and a porous substrate results in infiltration of
pebbles and boulders down mountainsides to accumu- the surface water. Surface run-off is therefore most
late near the bottom of the slope. These accumulations effective at carrying detritus during flash-flood events
of scree are often reworked by water, ice and wind but on steep, impermeable slopes in sparsely vegetated
sometimes remain preserved as talus cones, i.e. con- arid regions. Vegetation cover and thicker, permeable
centrations of debris at the base of gullies (Fig. 6.9) soils in temperate and tropical climates tend to reduce
(Tanner & Hubert 1991). These deposits are charac- the transport capacity of surface run-off.
teristically made up of angular to very angular clasts Sheet wash becomes concentrated into rills and
because transport distances are very short, typically gullies that confine the flow and as these gullies coa-
only a few hundred metres, so there is little opportu- lesce into channels the headwaters of streams and
nity for the edges of the clasts to become abraded. A rivers are established. Rivers erode into regolith and
small amount of sorting and stratification may result bedrock as the turbulent flow scours at the floor and
from percolating water flushing smaller particles margins of the channel, weakening them until pieces
down through the pile of sediment, but generally fall off into the stream. Flow over soluble bedrock such
scree deposits are poorly sorted and crudely stratified. as limestone also gradually removes material in solu-
Bedding is therefore difficult to see in talus deposits tion. Eroded material may be carried away in the
but where it can be seen the layers are close to the stream flow as bedload, in suspension, or in solution;
angle of rest of loose aggregate material (around 308). the confluence of streams forms larger rivers, which
Talus deposits are distinct from alluvial fans (9.5) may feed alluvial fans, fluvial environments of deposi-
because water does not play a role in the transport tion, lakes or seas.
and deposition.
6.5.3 Erosion and transport by wind
6.5.2 Erosion and transport by water
Winds are the result of atmospheric pressure differ-
Erosionbywateronhillsidesisinitiallyasasheet wash, ences that are partly due to global temperature
i.e. unconfined surface run-off down a slope following distributions (8.1.1), and also local variations in pres-
rain. This overground flow may pick up loose debris sure due to the temperature of water masses that

Fig. 6.9 A scree slope or talus cone in a
mountain area with strong physical
weathering.