Page 106 - Sedimentology and Stratigraphy

Erosion and Transport 93

form. Soil profiles become thicker through time as
bedrock is broken up and organic matter accumu-
lates, but a soil is also subject to erosion. Movement
under gravity and by the action of flowing water may

remove part or all of a soil profile. These erosion
processes may be acute on slopes and important on
flatter-lying ground where gullying may occur. The
soil becomes disaggregated and contributes detritus to
rivers. In temperate and humid tropical environments
most of the sediment carried in rivers is likely to have ! "
been part of a soil profile at some stage.
Continental depositional environments are also
sites of soil formation, especially the floodplains of

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rivers. These soils may become buried by overlying
layers of sediment and are preserved in the strati- !
graphic record as fossil soils (palaeosols: 9.7).

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6.5 EROSION AND TRANSPORT :

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Weathering is the in situ breakdown of bedrock and # $
erosion is the removal of regolith material. Loose
material on the land surface may be transported
downslope under gravity, it may be washed by
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water, blown away by wind, scoured by ice or
moved by a combination of these processes. Falls,

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slides and slumps are responsible for moving vast
quantities of material downslope in mountain areas
but they do not move detritus very far, only down to
the floor of the valleys. The transport of detritus over Fig. 6.8 Mechanisms of gravity-driven transport on slopes.
greater distances normally involves water, although Rock falls and slides do not necessarily include water,
whereas slumps, debris flows and turbidity currents all
ice and wind also play an important role in some
include water to increasing degrees.
environments (Chapters 7 & 8).
phy of the unit. If the rock breaks up during its move-
6.5.1 Erosion and transport under gravity ment it is a rock fall, which accumulates as a chaotic
mass of material at the base of the slope. These move-
On steep slopes in mountainous areas and along cliffs ments of material under gravity alone may be triggered
movements downslope under gravity are commonly by an earthquake, by undercutting at the base of the
the first stages in the erosion and transport of weath- slope, or by other mechanisms, such as waterlogging of
ered material. a potentially unstable slope by a heavy rainfall.
Movement downslope may also occur when the rego-
lith is lubricated by water and there is soil creep. This is
Downslope movement
a much slower process than falls and slides and may
There is a spectrum of processes of movement of not be perceptible unless a hillside is monitored over a
material downslope (Fig. 6.8). A landslide is a coher- number of years. A process that may be considered to
ent mass of bedrock that has moved downslope with- be intermediate between creep movement and slides is
out significantly breaking up in the process. Many slumping. Slumps are instantaneous events like
thousands of cubic metres of rock can be translated slides but the material is plastic due to saturation by
downhill retaining the internal structure and stratigra- water and it deforms during movement downslope.