Page 83 - Fundamentals of Geomorphology

66 INTRODUCING LANDFORMS AND LANDSCAPES

creep (finer material) or talus creep (coarser mate- and streams. Notice that water flows over and
rial). Frost creep occurs when the expansion and through landscapes in unconcentrated and concentrated
contraction is brought about by freezing and thaw- forms.
ing (pp. 77–8).Terracettes frequently occur on steep
grassy slopes. Soil creep may produce them, although Unconcentrated flow
shallowlandslidesmaybeanimportantfactorintheir
formation. Rainsplash results from raindrops striking rock and soil
5 Fall is the downward movement of rock, or occa- surfaces. An impacting raindrop compresses and spreads
sionally soil, through the air. Soil may topple from sideways. The spreading causes a shear on the rock or
cohesive soil bodies, as in riverbanks. Rock-falls soil that may detach particles from the surface, usu-
are more common, especially in landscapes with ally particles less than 20 micrometres in diameter. If
steep, towering rock slopes and cliffs (Figure 3.6d). entrained by water from the original raindrop, the par-
Water and ice may also fall as waterfalls and icefalls. ticles may rebound from the surface and travel in a
Debris falls and earth falls, also called debris parabolic curve, usually no more than a metre or so.
and earth topples, occur, for example, along river Rainsplash releases particles for entrainment and sub-
banks. sequent transport by unconcentrated surface flow, which
6 Subsidence occurs in two ways: cavity collapse and by itself may lack the power to dislodge and lift attached
settlement. First, in cavity collapse, rock or soil particles.
plummets into underground cavities, as in karst Unconcentrated surface flow (overland flow) occurs
terrain (p. 198), in lava tubes, or in mining areas. In as inter-rill flow. Inter-rill flow is variously termed
settlement, the ground surface is lowered progres- sheet flow, sheet wash, and slope wash. It involves a
sively by compaction, often because of groundwater thin layer of moving water together with strands of
withdrawal or earthquake vibrations. deeperandfaster-flowingwaterthatdivergeandconverge
around surface bulges causing erosion by soil detach-
ment (largely the result of impacting raindrops) and
Gravity tectonics sediment transfer. Overland flow is produced by two
Mass movements may occur on geological scales. Large mechanisms:
rock bodies slide or spread under the influence of grav-
ity to produce such large-scale features as thrusts and 1 Hortonian overland flow occurs when the rate at
nappes. Most of the huge nappes in the European which rain is falling exceeds the rate at which it
Alps and other intercontinental orogens are probably can percolate into the soil (the infiltration rate).
the product of massive gravity slides. Tectonic denuda- Hortonian overland flow is more common on bare
tion is a term that describes the unloading of mountains rock surfaces, and in deserts, where soils tend to be
by gravity sliding and spreading. The slides are slow, thin, bedrock outcrops common, vegetation scanty,
being only about 100 m/yr under optimal conditions and rainfall rates high. It can contribute large vol-
(that is, over such layers as salt that offer little frictional umes of water to streamflow and cover large parts of
resistance). an arid drainage basin, and is the basis of the ‘partial
area model’ of streamflow generation.
2 Saturation overland flow or seepage flow occurs
where the groundwater table sits at the ground
FLUVIAL PROCESSES
surface. Some of the water feeding saturation over-
land flow is flow that has entered the hillside upslope
Flowing water
and moved laterally through the soil as throughflow;
Figure 3.7 is a cartoon of the chief hydrological pro- this is called return flow. Rain falling directly on the
cesses that influence the geomorphology of hillslopes hillslope may feed saturation overland flow.