Geography Reference
In-Depth Information
The cycle of erosion
William Morris Davis envisaged a cycle of erosion that
was initiated by uplift of the land surface. This was
followed initially by rapid incision by rivers and later by
valley-fl oor widening. The overall pattern was of curved
(convexo-concave) slopes declining in angle and eventually
terminating in a landscape of low relief known as a peneplain
(until land uplift rejuvenated the landscape and the cycle
began again). The essentials of his model are shown in
Figure 6(A). Effects that could not be fi tted into the so-called
'normal cycle', such as the landforms produced by glaciation,
were viewed as 'climatic accidents', though different cycles
of erosion were later proposed for regions with climates that
differed from the largely temperate and fl uvially dominated
regions of the USA and Europe where the original model
was developed. One of these alternative models, which
was thought more appropriate for the semi-arid regions
of southern Africa, advocated the parallel retreat of slopes
rather than Davisian slope decline (Figure 6(B)). According
to this model, steep slopes and extensive remnants of the
initial land surface remained late into the cycle.
While they did focus the thoughts of geomorphologists on
the idea of systematic change over time, such 'cycles' were
also a constraint. These theoretical models were too simple
in relation the complex evolution of real landscapes. In
particular, landscapes do not stay stable long enough for the
completion of the full cycle because of both the tectonic forces
controlling uplift and the environmental changes affecting
Earth-surface processes. Modern ideas on landscape
evolution give much greater attention to how landscapes
react to changing conditions, to rates of landscape change in
the past, and to the response of the landscape to likely future
environmental change.
