32 / Basics of Environmental Science
Ice accumulating in a pre-existing hollow will erode the sides to the open-sided, approximately
circular shape of a cirque (also known as a ‘corrie’ or ‘cwm’). Where a relatively narrow glacier
flows into the sea the trough it excavates may later form a fjord, known in Scotland as a ‘sea loch’.
Some fjords are more than 1200 m deep. In latitudes higher than about 50°, ice has been the major
geomorphological (‘landscape-forming’) agency.
Soil will tend to move slowly downslope by ‘soil creep’, caused by the expansion and contraction of material
due to repeated wetting and drying, or ‘solifluction’, where the soil is lubricated by rain water (formerly the
term ‘solifluction’ was applied only to periglacial environments where the ground is frozen for part of the
year, but it is now used more widely and is recognized as an important process in some tropical areas). The
rate of soil creep has been measured in the English Pennines as between 0.5 and 2.0 mm at the surface and
0.25 to 1.0 mm in the uppermost 10 cm (SMALL, 1970, p. 224). If the soil is deep and the underlying rock
extensively weathered, large masses may slip suddenly and move rapidly as ‘earth flows’. The collapse of
coal tips at the Welsh village of Aberfan in 1966 was of this type (in this case known strictly as a ‘flowslide’).
The tips had been built over springs. Tip material absorbed the water, greatly increasing its weight but
simultaneously lubricating it until it lost its inertia catastrophically (SMALL, 1970, p. 29–34). Earthquakes
can break the bonds holding soil particles together, resulting in earth flows of dry material.
There are several ways in which masses of rock and earth can move downslope (HOLMES, 1965, p.
481). All such movements alter the shape of slopes, generally smoothing and reducing them. Figure
2.6 shows the stages by which this happens:
(1) material from the free face is detached
and falls to form a scree which buries a
convex lower slope; (2) further falls cause
the free face to retreat until it disappears
altogether, leaving a slope that grades
smoothly to the level of the higher ground;
(3) the slope itself then erodes further. It can
also happen that accumulated water increases
the weight of a mass of weathered material
until it shears away and slides down a
concavely curved shear plane between it and
the adjacent material. Because of the curved
slope, the sliding layers are tilted backwards
as they descend, so the toe of the slide is
tipped upwards, forming a barrier behind
which further debris will be held. This is a
‘rotational slide’, examples of which can be
seen in several places along the south coast
of England and in the Isle of Wight. Most
failures are quite complex and involve more
than one mechanism.
Geomorphology, the study of landforms and
the processes by which they are produced and
change, began with the work of an American
geologist, William Morris Davis (1850–
1934), of Harvard University. He proposed
that landscapes evolve through a ‘cycle of
erosion’ (the ‘Davisian cycle’). This begins Figure 2.6 Slope development