Geography Reference
In-Depth Information
the amount of sediment. In a slide there is little water within the
moving material itself although water may have helped overcome
friction to initiate the event.
There are also slow mass movements. As a whole unit, a mass of
rock or soil moves slowly downslope. The slow mass movement of
soil is called soil creep and typically operates at 1 to 5 millimetres
per year. The movements are caused by expansion or contraction
heaving (e.g. wetting and drying of soil or freeze-thaw action).
Other movements are usually caused by biological activity which
mixes the soil in all directions. When this occurs on sloping ground
then gravity will result in more downhill movement than uphill
and there is a gradual transport of material. Depending on the
environment, one of the processes of creep may be more domi-
nant. In cold, upland areas freeze-thaw is probably the most
important, whereas in tropical forests biological mixing may
dominate.
Box 3.2 Plough creep
Humans can accelerate soil creep through ploughing and this is often
known as tillage erosion. Each time the soil is turned over by the plough
the soil moves. If the ploughing is directly up or downslope then there
tends to be an overall downhill movement of soil when the soil settles
back in the period after the ploughing. Where the ploughing direction
follows the contour and cuts across the slope then material is moved
either down or upslope depending on the direction of the plough.
Where the plough turns soil downhill then this will result in an overall
movement approximately a thousand times greater than soil creep.
Contour ploughing in both directions as the plough moves one way
across the slope and then the other still produces movement a hundred
times greater than natural creep. Over the last few hundred years
tillage erosion may have been responsible for more soil movement in
many areas than natural soil creep has over the last 10,000 years.
Water moves particles in what are called 'wash processes'. Rain-
splash, rainwash and rillwash are the most important of these wash
processes. The impact of raindrops can detach material which then
jumps into the air. The splash can cause the sediment to move either
up or downslope but because of gravity there is an overall downslope
