Geoscience Reference
In-Depth Information
filled with glacial and later sediment are an intriguing feature of many areas of Chalk
hills. Some of the present open valleys may have been cut under similar conditions,
then plugged with material, and then emptied again by later river action. It is now be-
lieved that much of the erosion of these valleys took place by meltwater flow active
under the Anglian ice sheets. Most of the buried valley floors slope southwards, indic-
ating drainage flow in that direction, but local reversals in valley slope do occur. These
northward-sloping valleys seem to result from situations where the rivers were forced
to flow up local gradients under the ice sheet, like water in an enclosed pipe.
FIG
242.
Cross-section showing buried valleys near Hitchin (Fig. 231,
c4
).
Some of the southeastern Chalk-slope valleys have eroded into the Upper Chalk,
but not cut deeply enough to expose hard band 4. Much of the erosion of these shallow
valleys is relatively young and has taken place into Anglian ice-laid material since it
was deposited about 400,000 years ago. The valleys often show beautiful branching
patterns of their tributaries, which probably result from the distinctive behaviour of the
muddy ice-laid material when the heads of the tributaries cutback into it. Good ex-
amples of these patterns are the valleys that flow from the north into the Lea trunk river
on the southern edge of Area 13 (
c5
).
Landscape D: Bishop's Stortford Clay hills
This Landscape occupies the southeastern corner of Area 13, where it is defined by the
occurrence of Early Tertiary sediments below the surface blanket. Up to 17 m of sandy
Woolwich and Reading Beds are overlain by some 65 m of London Clay, all deposited
in an arm of the sea during Early Tertiary times.
Above the bedrock, the surface blanket contains river sands and gravels which are
thought to have been deposited by an ancestor of the River Thames (Areas 10 and 11).
