Page 188 - Fundamentals of Geomorphology

HILLSLOPES 171

modellers to represent the spatial variability of the pro- profile’. This defines the location of the gently sloping
cesses, whereas in the past they could model them only land – in valley bottoms or in uplands. In combination,
as point processes. An enormous literature describes the these characteristics define the following landforms:
use of DEMs to produce both primary and secondary
attributes; an equally large literature also considers how • Plains with a predominance of gently sloping land
best to incorporate primary and secondary attributes into combined with low relief.
spatialmodels that simulate physical processes influenced • Plains with some features of considerable relief.This
and controlled by the nature of topography (e.g. Wilson group may be subdivided by the position of the gen-
and Gallant 2000). tly sloping land into three types – plains with hills,
Slope and aspect are two of the most important topo- mountains, and tablelands.
graphic attributes. Slope is a plane tangent to the terrain • Hills with gently sloping land and low-to-moderate
surface represented by the DEM at any given point. It relief.
has two components: (1) gradient, which is the maxi- • Mountains with little gently sloping land and high
mum rate of change of altitude and expressed in degrees local relief.
or per cent; and (2) aspect, the compass direction of
the maximum rate of change (the orientation of the line There are many such schemes, all with their good and bad
of steepest descent expressed in degrees and converted points. Modern research in this field combines terrain
to a compass bearing). Because slope allows gravity to attributes to create some form of regional topographic
induce the flow of water and other materials, it lies at classification (e.g. Giles 1998; Giles and Franklin 1996).
the core of many geomorphological process models. For
instance, slope and flowpath (i.e. slope steepness and
length) are parameters in the dimensionless Universal HILLSLOPE PROCESSES
SoilLossEquation(USLE),whichisdesignedtoquantify
sheet and rill erosion by water (p. 178). Gravity, flowing water, and temperature changes are the
main forces behind hillslope processes, with the action
Landform classification of animals and plants being important in some situa-
tions. Weathering on hillslopes, as elsewhere, includes
The toposphere contains a stupendous array of land- the in situ conversion of bedrock into regolith and the
forms. Unfortunately, landforms are notoriously diffi- subsequent chemical and mechanical transformation
cult to classify quantitatively. Geomorphologists make of regolith. Several hillslope processes serve to trans-
a fundamental distinction between erosional landforms port regolith and other weathering products. They range
(sculptured by the action of wind, water, and ice) and from slow and continual processes to rapid and inter-
depositional landforms (built by sediment accumula- mittent processes. Slow and continual processes fall into
tion). They also recognize basic differences between three categories: leaching, soil creep, and rainsplash and
landforms in terrestrial, shallow marine, and deep marine sheet wash.
environments, each of which fosters a distinct suite of
geomorphic processes. However, many landform classi-
fications use topographic form, and ignore geomorphic Transport processes
process. For example, one scheme for large-scale land- Leaching
form classification uses three chief topographic charac-
teristics (Hammond 1954). The first characteristic is the Leaching involves the removal of weathered products in
relativeamountofgentlyslopingland(landwithlessthan solution through the rock and the soil. Solution is an
an 8 per cent slope).The second characteristic is the local efficacious process in hillslope denudation. It does not
relief (the difference between highest and lowest eleva- alwaysleadtosurfacelowering,atleastatfirst,becausethe
tion inanarea).The third characteristic is the ‘generalized volume of rock and soil may stay the same. Solution takes