Geoscience Reference
In-Depth Information
FIG
234.
Former courses of the River Cam.
Red arrows mark former river pathways
1) 300, 000 years ago (upper terrace level). Cold
2) 150, 000 years ago (middle terrace level). Cold, then warm.
3) 30, 000 years ago (lower terrace level). Cold.
The River Cam and its upstream tributary, the Rhee, flow some 50 km northeast
to join the Great Ouse a few kilometres south of Ely, running parallel to (and draining)
the northwestern face of the Chalk hills. During the general lowering by erosion of the
landscape, this face of the Chalk hills must have migrated southeastwards, down the
direction of the gentle tilt. This will not have been a steady process and, particularly
during the Anglian glaciation, the presence of ice (and meltwater) will have influen-
ced the pattern of erosion by the rivers. An example of this is the way the Rhee-Cam
river flows generally on Early Cretaceous (Gault) bedrock, except for some areas near
Cambridge where it crosses the earliest Late Cretaceous (lower Chalk) that now ex-
tends across its valley to the Harlton (
b10
) and Madingley (
b11
) ridges. In this area,
interglacial (Ipswichian) gravels have provided evidence that valley erosion was active
just south of the Harlton ridge some 120,000 years ago. The width of the valley of the
Rhee and Cam here is evidence for the erosional effectiveness in the past of what now
