London and the Thames Valley Region - Southern England - page 202

FIG 191. The Goring Gap (Fig. 190, c1 ), looking north (upstream) along the Thames
from near Pangbourne. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
In the west of Area 10, the Wiltshire Clay Vale occurs between the dip slope of
the Cotswolds to the west and the Marlborough Downs to the east. To the south the
Vale extends past Trowbridge and Fro me (Area 8) to join with the clays of Blackmoor
Vale and the Vale of Wardour, described in Area 4.
The Wiltshire Clay Vale is primarily underlain by the Late Jurassic Oxford Clay,
which gives the area a subdued, undulating topography and heavy, wet soils. In the
west, substantial outcrops of rather older sandy bedrock (called Kellaways Sand ) pro-
duce rich, free-draining soils that are highly valued as arable farmland. The centre of
the Vale is dominated by the wide, level floodplain of the Bristol Avon, which, des-
pite modern agricultural development, still retains traces of an ancient pattern of flood
meadows. Surface sediments include river deposits of fine-grained alluvium and ter-
race gravels. The eastern margin of the Vale is marked by a resistant band of Late Jur-
assic Corallian Limestone and Early Cretaceous Greensand - equivalent to the Mid-
vale Ridge of Oxfordshire - which is backed by the Late Cretaceous Chalk scarp of the
Marlborough Downs.
The market towns and villages of the area are particularly distinctive. A number
of valuable building stones are available locally, from the warm, yellow Middle Juras-
sic Cotswold Stone in the northwest to the rough, brown Late Jurassic Corallian Rag
from the east. Ancient towns such as Malmesbury, Chippenham, Melksham and Trow-
bridge, all of which became wealthy due to the thriving post-medieval wool trade, have