Geoscience Reference
In-Depth Information
Drainage towards the Wash on the east coast
In the northeast of Area 9, some 20 km of the headwaters of the River Nene flow
eastwards from a drainage divide near Watford Gap towards Northampton in Area 13.
In this westernmost part of the catchment, the Nene and its small tributaries are very
gently incised - at most a few tens of metres - and the most clearly defined hills are
capped by Northampton Sandstone. The area of the drainage divide is marked by flat
hill tops covered with ice-laid deposits, often associated with river sediments thought
to have been deposited by rivers flowing from the ice sheets. These ancient river de-
posits are very much concentrated along the Nene Valley, where a buried channel has
been identified, demonstrating the early existence of an ancestral Nene. As in the case
of the Avon Valley discussed above, the ice sheet involved may have been the Anglian,
but there is some uncertainty about this. Whichever cold episode was responsible, most
of the present gentle landscape topography has been created by river erosion since the
ice melted.
The headwaters of the Great Ouse flow eastwards from near Brackley to Bucking-
ham, on the edge of Area 9, and then further on towards the northern edge of Milton
Keynes. This part of the catchment lacks important scarps and slopes, though it has
cut gently into the Middle Jurassic Oolite. There is a surface layer of ice-laid mater-
ial over much of the higher ground here, and this may have discouraged more wide-
spread down-cutting. The River Tove, a tributary from the west and north, has cut into
the Early Jurassic mudstones, but steep slopes are again not a feature. Upstream from
Buckingham, well-marked meanders are a striking feature of the river scenery, though
with a small wavelength to match the low average flow rates. As with the Nene, it is
still uncertain which cold episode of the Ice Age resulted in the deposition of the ice-
laid material, but it appears likely that the valley and hill pattern has been eroded since
the ice sheet melted. River terrace deposits in the immediate Buckingham area and in
the Tove tributary suggest considerable variability in the river evolution histories.
Drainage towards the Thames and the southeast coast
These tributaries drain generally southwards down the slope of the tilted bedrock lay-
ers. The headwaters of the western tributaries have cut downwards into the Early Jur-
assic mudstones and then, in the downstream direction, cut across the more resistant
Middle Jurassic Oolite layer before flowing over the Late Jurassic mudstones. These
produce distinctive valley wall slopes in the Windrush (Oxford-Cheltenham road),
Evenlode (Oxford-Worcester railway) and Cherwell valleys, where the Great Oolite
layer particularly tends to cap the valley wall slopes. These rivers all have low mean
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