Earth Sciences / 33when land is raised by tectonic movements. In a young landscape hills slope steeply and the slope of
river beds is irregular. As the landscape matures, hill slopes become gentle and river beds slope smoothly.
Eventually, the old landscape has eroded to a gently rolling peneplain (‘almost a plain’), a word Davis
coined. His idea was (posthumously) challenged, in 1924, by the German geologist Walther Penck
(1888–1923), who argued that once a slope has settled at an angle which is mechanically stable for the
material of which it is composed, it will maintain that angle. Erosion will wear away its face, but will
not make it more shallow, so the face will retreat but the angle will remain fairly constant, and the
steeper the slope the faster it will erode, because the slow-moving weathered material on a shallow
slope will protect the underlying surface. Thus, if a slope is steeper near the bottom than it is higher up,
the lower slope will erode faster than the upper slope and the structure will collapse. As the argument
developed, geomorphologists came to realize that a true understanding of ‘the slope problem’ can best
be gained from studies of low-latitude landscapes that have not been formed mainly by glacial action,
as were those on which the theories of Davis and Penck were largely based (SMALL, 1970, pp. 194–
224). Interest in the topic is not purely academic, for an understanding of how rock and soil behaves on
sloping ground is necessary for engineers calculating the risks of landslides, erosion, and flooding, and
devising schemes to minimize them. The matter is of major environmental importance.
Rivers provide the principal means by which particles eroded from surface rocks are transported from
the uplands to the lowlands and eventually to the sea. Rivers are also major landscape features in their
own right and, by cutting channels across the surface, important agents in the evolution of landscapes.
It is not only mineral particles they transport, of course. Water draining into a river from adjacent land
also contains organic matter and dissolved
plant nutrients, and rivers also carry those
substances we discharge into them as an
apparently convenient method of waste
disposal. They are also a major source of water
supplied for domestic and industrial use.
Water drains from higher to lower ground,
moving slowly as ground water between the
freely draining soil and an impermeable layer
of rock or clay, eventually emerging at the
surface as a spring, seeping from the ground,
or feeding directly into a river. The ‘water
table’ is the upper limit of the ground water,
below which the soil is fully saturated. These
terms have the same meanings in British and
North American usage, but confusion can
arise over ‘watershed’, which has two
different meanings. A drainage system
removes water from a particular area, and one
such area is separated from adjacent areas.
In Britain, the area from which water is
removed by a particular drainage system is
called a ‘catchment’ and in North America it
is called a ‘watershed’. One catchment is
separated from another by a ‘divide’, which
in Britain is sometimes known as a
‘watershed’, and within a catchment the
drainage system forms a pattern. Figure 2.7
Figure 2.7 Drainage patterns. A, subparallel;
B, dendritic; C, semi-denritic; D, trellised;
E, rectangular; F, radial