Geoscience Reference
In-Depth Information
Within the Chalk hills, a complex network of dry valleys or 'coombes' seems to
have formed during glacial times, when the subsurface was permanently frozen to a
depth of several metres. The ice sealed the fractures that normally make the Chalk such
a permeable bedrock, resulting in poor surface drainage, slope collapse and erosive sur-
face streams. The dry valleys may be divided into two types:
edge valleys
that cut into
or across the Chalk edge itself, and
dip-slopevalleys
that dissect the Chalk down-slope,
away from the edge. Edge valleys show the greatest variability in size and shape, ran-
ging from shallow depressions to deeply incised cuts. They are generally aligned ap-
proximately perpendicular to the Chalk edge, and their morphology is controlled prin-
cipally by the width of the escarpment. Complex, multi-branching systems tend to de-
velop where the zone is broad, while straight, simple coombes tend to be found where
the scarp is high, straight and steep. Dip-slope coombes are typically longer than those
on the scarp face and often form well-developed, branching valley networks. Near the
edge, the dip-slope valleys are shallow and gentle, and the resulting landscape smooth
and rounded. Further down dip, as the streams on frozen ground increased in volume
and power, the valleys became deep coombes with steep sides. Both classes of dry val-
ley have a broadly U-shaped cross-section.
FIG
193.
The Manger (Fig. 190,
c2
), a dry valley in the Chalk scarp at Uffington. (Copy-
right Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
