Geoscience Reference
In-Depth Information
that much of the scenery of this Landscape has been formed since the departure of the
Anglian ice, some 400,000 years ago. In this erosional work, the main agents have been
the rivers and their tributaries: reaches of the Nene tend to have a wide valley- often
over 1 km across - and are typically fringed by river terraces, representing episodes in
the evolution of the river during the varied climatic fluctuations since the Anglian cold
episode.
Landscape B: Clay-Greensand-Gault belt
This belt extends diagonally across Area 13, containing Milton Keynes, Bedford (
b5
),
Huntingdon (
b2
) and Cambridge (
b3
). It is underlain by mudstones of Late Juras-
sic age (e.g. Oxford, Ampthill and Kimmeridge Clays), as well as sandstones (Lower
Greensand) and mudstones of Early Cretaceous age (Gault). Because the local topo-
graphy is complicated by these bedrock variations and by the patchy presence of large
amounts of Anglian ice-laid sediment, it is worth selecting some parts of this Land-
scape for more detailed treatment. I shall first outline some of the features of the Great
Ouse and Cam drainage system, and then examine more closely the patterns of hill
slopes in the three sub-areas (I, II and III) marked in Figure 231, from northeast to
southwest along the Clay-Greensand-Gault belt.
The Great Ouse is the largest river of the Area, and it is interesting to examine
information on its flow rate. The Great Ouse has a mean flow of only 10.4 m
3
/s at Bed-
ford, where it is draining an upstream catchment of 1,460 km
2
. Further downstream at
Brownshill Staunch (about 8 km from St Ives,
b6
) and just before the Great Ouse be-
comes tidal, its mean flow rate is 14.4 m
3
/s from a catchment of 3,030 km
2
, reflecting
its drainage of much of Area 13 and some of Area 9. For comparison, the mean flow in
a downstream reach of the largest river of Southern England, the River Severn, is 106
m
3
/s from a wetter catchment of 9,900 km
2
.
