Page 190 - Fundamentals of Geomorphology

HILLSLOPES 173

mobilization of particles during fires when sediment of the ground in winter. Generalizations about the rates
wedges that have accumulated behind vegetation col- of soil creep in other climatic zones are unforthcom-
lapse,aswellasmobilizationbybioturbationandbysmall ing owing to the paucity of data. In mediterranean,
landslides. semi-arid, and savannah climates, creep is probably
far less important than surface wash as a denuder of
Mass wasting the landscape and probably contributes significantly
to slope retreat only where soils are wet, as in sub-
Rapid and intermittent hillslope transport processes stantially curved concavities or in seepage zones. Such
involve mass wasting – creep, flow, slide, heave, fall, studies as have been made in tropical sites indicate
subsidence (p. 63–6). a rate of around 4–5 mm/year. Solifluction, which
includes frost creep caused by heaving and gelifluction,
Bioturbation occurs 10–100 times more rapidly than soil creep and

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