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