Geoscience Reference
In-Depth Information
The special properties of the Chalk give its scenery the classic features seen in
most other Chalk bedrock areas in Southern England. Chalk is a remarkably homogen-
eous and fine-grained sediment, generally giving rise to hills that are smoothly rounded
and separated by distinct valleys. However, the lack of clear bedding or layering also
means that it has responded to the inevitable stresses within the Earth's crust by frac-
turing, where many other rock types would have responded by slipping along layers.
This fracturing makes much of the Chalk permeable to rainwater, so most Chalk val-
leys now are dry for much - if not all - of the year. Chalk valleys, therefore, were prob-
ably excavated mainly during the cold episodes of the Ice Age, when frozen ground
conditions sealed the bedrock and made it impermeable, and surface erosion could re-
move material in the short summers.
The northward-facing escarpment is the most striking feature of the scenery of the
South Downs, and corresponds directly to the position of the base of the Chalk layer.
The escarpment shows up very clearly, for example, in Figure 127, reflecting the con-
trast in erosional resistance of the Chalk compared with that of the underlying Early
Cretaceous Gault or Upper Greensand. Looking at the form of the line marking the
base of the Chalk in Figure 129, a number of deflections mark distinct departures from
a simple straight line. These are due to local folds that have deformed the Chalk layer.
Another feature of the escarpment is that it consists of large numbers of
coombes
or
hollows, usually less than 1 km across, which appear to have formed by local slope col-
lapse of the Chalk under Ice Age conditions of thawing and freezing. Other evidence
of the effects of this slope collapse is the jumbled deposits of Chalk mud and fragments
(known as
head
) resulting from flows of debris into the floors of many of the valleys.
