Geology Reference
In-Depth Information
Geotechnical
properties
Solution
rates
Lithology
Climate
Landslide
rates
Slope
hydrology
Slope
gradients
Creep/solifluction
and wash rates
Soil depth
Rateofsoil
production
Landform
Figure 7.6
Components and linkages in Kirkby's model of hillslope evolution.
Source:
Adapted from Kirkby (1985)
thick soils form everywhere except over the hard band.
From this simulation and another in which solution
is the sole process, Kirkby makes a number of deduc-
tions that appear to correspond to features in actual
landscapes. First, geotechnical properties of rock, in
particular the rate of decline towards the threshold gra-
dient of landslides, are more important than solution
in determining slope form. Only on slopes of low gra-
dient and after long times (200,000 years and more)
do solutional properties play a dominant role in influ-
encing slope form. Second, gradient steepening and
soil thinning over 'resistant' strata are strictly associ-
ated with the current location of an outcrop, though
resistant beds, by maintaining locally steep gradients,
tend to hold the less resistant beds close to the landslide
threshold and so increase gradients everywhere. Third,
gradients close to landslide threshold gradients com-
monly outlive landslide activity by many thousands of
years and, because of this, may play a dominant role in
determining regional relief in a tectonically stable area.
Fourth, soils are generally thin under active landsliding
and wash; thick soils tend to indicate the predomi-
nance of solution and creep or solifluction processes.
Elevation
(m)
Gently sloping plateau
50
Initial
hillslope
40
Steep
bluff
30
20
Hard rock band
10
0
0
40
80
Distance from divide (m)
120
160
200
Figure 7.7
Simulation of hillslope change for an initial
gently sloping plateau ending in a steep bluff with
a band of hard rock dipping at 10
◦
into the hillside.
Time is in years.
Source:
Adapted from Kirkby (1985)
(not shown on the diagram) involves initial thicken-
ing on the plateau and thinning by landslides on the
scarp. Soil distribution is uneven owing to the local-
ized nature of landslides. Once the slope stabilizes,
