Geoscience Reference
In-Depth Information
construction material. The area is attractive and offers fine, commanding views over
the clay vales to the north and south.
To the south of the Midvale Ridge is the Vale of White Horse. As with much of
Area 10, the rocks here dip gently to the southeast, exposing Late Jurassic Kimme-
ridge Clays along the northern sides of the Vale, followed by Early Cretaceous Gault
Clay a little to the south. These rock types produce typical low-lying clay-vale scenery,
though this is punctuated in places by prominent-weathering outcrops of Portland lime-
stone. The southern margin of the Vale, immediately north of the Chalk scarp of the
Lambourn Downs, is underlain by Early Cretaceous Upper Greensand (mudstones and
sandstones), which supports rich orchards in the vicinity of Harwell (
b5
).
The Vale of White Horse drains generally eastwards to join the Thames at Abing-
don. Like that of the Thames, this floodplain offers fertile arable farmland and frequent
gravel deposits, upon which many of the area's towns were first settled. Buildings in
the Vales of the Upper Thames are often brick-built, reflecting the widespread use of
local clay as a building material. This contrasts with the Wiltshire Vale to the west (see
page 248), where local limestones were extensively used as building stones.
The City of Oxford (Fig. 192) probably started to grow because of the strength
of the local farming economy and its role as a transport hub, located where the River
Thames cuts through the Midvale Ridge. Its ancient heart rests on a tongue of Quatern-
ary gravels that have formed terraces where the Thames and its tributary the Cherwell
run parallel to each other before joining.
It is interesting to compare landscape features of Oxford with those of England's
other ancient university town, Cambridge (see Area 13). Oxford's large eastern and
southern extension from Headington to Cowley is relatively new, and reflects major in-
dustrial development in the early twentieth century that largely passed Cambridge by.
There is also a considerable difference in the scale of the development of the ancient
parts of the two cities, perhaps due to the importance of the river link with London in
the case of Oxford. However, it also seems to reflect the difference in scale of the rivers
and the terraces that have provided the frameworks for growth. The Thames some kilo-
metres north of Oxford has a mean flow rate of 15.4 m
3
/s, draining 1,609 km
2
of catch-
ment (Fig. 188), whereas the Cam in Cambridge has a much smaller mean flow rate of
only 2.8 m
3
/s from 762 km
2
of catchment (see Chapter 8, Fig. 230). So the altogether
smaller size of the Cam, both in flow rate and in the size of its catchment, has strongly
influenced the size and number of branch water courses constructed, and the scale of
the terrace and bedrock hill topography.
