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