Page 193 - Fundamentals of Geomorphology

176 PROCESS AND FORM

Geotechnical Lithology Solution
properties rates

Climate
Landslide
rates
Slope
hydrology

Slope Creep/solifluction
gradients and wash rates Soil depth

Rateofsoil
Landform production

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.
Elevation
(m) Gently sloping plateau From this simulation and another in which solution
50 Initial is the sole process, Kirkby makes a number of deduc-
hillslope
tions that appear to correspond to features in actual
40,000
40 20,000
100,000
200,000
Steep landscapes. First, geotechnical properties of rock, in
20,000
bluff
2,000
2,000
4,000
4,000
30 1,000 particular the rate of decline towards the threshold gra-
10,000
10,000
1,000
dient of landslides, are more important than solution
20
Hard rock band in determining slope form. Only on slopes of low gra-
10 dient and after long times (200,000 years and more)
do solutional properties play a dominant role in influ-
0
0 40 80 120 160 200 encing slope form. Second, gradient steepening and
soil thinning over ‘resistant’ strata are strictly associ-
Distance from divide (m)
ated with the current location of an outcrop, though
Figure 7.7 Simulation of hillslope change for an initial resistant beds, by maintaining locally steep gradients,
gently sloping plateau ending in a steep bluff with tend to hold the less resistant beds close to the landslide
a band of hard rock dipping at 10 into the hillside. threshold and so increase gradients everywhere. Third,
◦
Time is in years. gradients close to landslide threshold gradients com-
Source: Adapted from Kirkby (1985) monly outlive landslide activity by many thousands of
years and, because of this, may play a dominant role in
(not shown on the diagram) involves initial thicken- determining regional relief in a tectonically stable area.
ing on the plateau and thinning by landslides on the Fourth, soils are generally thin under active landsliding
scarp. Soil distribution is uneven owing to the local- and wash; thick soils tend to indicate the predomi-
ized nature of landslides. Once the slope stabilizes, nance of solution and creep or solifluction processes.