Geology Reference
In-Depth Information
mobilization of particles during fires when sediment
wedges that have accumulated behind vegetation col-
lapse, as well as mobilization by bioturbation and by small
landslides.
of the ground in winter. Generalizations about the rates
of soil creep in other climatic zones are unforthcom-
ing owing to the paucity of data. In mediterranean,
semi-arid, and savannah climates, creep is probably
far less important than surface wash as a denuder of
the landscape and probably contributes significantly
to slope retreat only where soils are wet, as in sub-
stantially curved concavities or in seepage zones. Such
studies as have been made in tropical sites indicate
a rate of around 4-5 mm/year. Solifluction, which
includes frost creep caused by heaving and gelifluction,
occurs 10-100 times more rapidly than soil creep and
affects material down to about 50 cm, typical rates
falling within the range 10-100 mm/year. Wet condi-
tions and silty soils favour solifluction: clays are too
cohesive, and sands drain too readily. Solifluction is
highly seasonal, most of it occurring during the sum-
mer months. The rate of surface wash, which comprises
rainsplash and surface flow, is determined very much by
the degree of vegetation cover, and its relation to cli-
mate is not clear. The range is 0.002-0.2 mm/year. It
is an especially important denudational agent in semi-
arid and (probably) arid environments, and makes a
significant contribution to denudation in tropical rain-
forests. Solution (leaching) probably removes as much
material from drainage basins as all other processes
combined. Rates are not so well documented as for other
geomorphic processes, but typical values, expressed as
surface-lowering rates, are as follows: in temperate cli-
mates on siliceous rocks, 2-100 mm/millennium, and
on limestones 2-500 mm/millennium. In other cli-
mates, data are fragmentary, but often fall in the range
2-20 mm/millennium and show little clear relation-
ship with temperature or rainfall. On slopes where
landslides are active, the removal rates are very high
irrespective of climate, running at between 500 and
5,000 mm/millennium.
Mass wasting
Rapid and intermittent hillslope transport processes
involve
mass wasting
- creep, flow, slide, heave, fall,
subsidence (p. 63-6).
Bioturbation
Geomorphologists have until recently tended to dismiss
the effects of animals and plants on hillslope processes,
this despite the early attribution of soil creep to the action
of soil animals and plant roots (Davis 1898). However,
animals and plants make use of the soil for food and
for shelter and, in doing so, affect it in multifarious
ways. For instance, the uprooting of trees may break up
bedrock and transport soil downslope. Since the mid-
1980s, the importance of
bioturbation
- the churning
and stirring of soil by organisms - to sediment trans-
port and soil production on hillslopes has come to the
fore. Andre Lehre (1987) found that biogenic creep is
more important than inorganic creep. Another study
concluded that bioturbated areas on Alpine slopes in
the Rocky Mountains of Colorado, USA, have sediment
movement rates increased by one or two orders of mag-
nitude compared with areas not subject to significant
bioturbation (Caine 1986). A review in 2003 concluded
that bioturbation is undeniably a key geomorphic factor
in many landscapes (Gabet
et al
. 2003), a fact strongly
supported by William E. Dietrich and J. Taylor Perron
(2006).
Climate and hillslope processes
Extensive field measurements since about 1960 show
that hillslope processes appear to vary considerably with
climate (Young 1974; Saunders and Young 1983; Young
and Saunders 1986). Soil creep in temperate maritime
climates shifts about 0.5-2.0 mm/year of material in
the upper 20-25 cm of regolith; in temperate conti-
nental climates rates run in places a little higher at
2-15 mm/year, probably owing to more severe freezing
Transport-limited and supply-limited
processes
It is common to draw a distinction between hill-
slope processes limited by the transporting capac-
ity of sediment and hillslope processes limited by
the supply of transportable material (Kirkby 1971).
