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movements affecting the bedrock layers by folding and tilting them, so that some parts
were raised (to be later removed by erosion) and other parts were lowered (Fig. 10, A
and B). In the millions of years since the sediment layers were laid down, they have
been buried, compacted, deformed by various processes, and finally uplifted to form
part of the landscape that we know today (deformation processes are treated more fully
in Chapter 3). The Chalk layer has been moved and folded as a result of mild com-
pression or convergence, to form a downfold or syncline between the Chilterns and the
North Downs, and an upfold or anticline between the North and South Downs (Fig.
10, C). Later, the central part of the anticline was eroded away to produce the bedrock
pattern that we recognise today (Fig. 10, D). The vein-like river valleys visible on the
elevated Chalk hills of Figure 9 are evidence of this continuing erosion.
FIG 10. Deposition and folding of the Chalk.
LANDSCAPE MODIFICATION BY RIVERS
Weathering of landscape surfaces and the production of soils by the action of rainwater,
air and organisms are important factors in shaping landscapes. These processes affect
the bedrock when it is very close to the surface, and most of them weaken the material
that they work on. This is particularly so when tough silicate rock minerals are altered
to soft clay minerals, which are then easily eroded. Freezing and thawing also works
to weaken the bedrock as water in cracks freezes and expands, breaking the rocks into
fragments.
Whilst weathering is a widespread and general process, most of the other import-
ant landscape processes involve the formation of discrete features that we shall call
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