Geoscience Reference
In-Depth Information
movement during the latest stages of the Variscan mountain building. The soft sedi-
ment fill of the basin made it more susceptible to erosion than the surrounding older,
harder rocks, allowing the Bristol Channel to form as it is now.
East of the Basin, the Bristol Channel narrows and becomes an estuary, until it
ceases to be tidal near Gloucester. This northeasterly reach, largely in Area 8, is char-
acterised by the change in fold and fault trends referred to above, from east-west in the
south, to northeast-southwest, and then north-south in the north. It seems likely that the
change in direction of the estuary and the river has resulted from erosional topography
controlled by the presence of these structures.
Further north, the eastern edge of the Severn-Avon catchment is formed by the
watershed of the Cotswold Hills. We shall see in Area 9 how the drainage divide here
is controlled by various resistant layers in the Jurassic bedrock, which slope regionally
and very gently to the southeast. In this Region, the gentle tilting movements appear
to have been influenced partly by uplift of the Atlantic margin in the west and partly
by the lowering of the North Sea to the east. Area 9 also contains evidence of pos-
sible late-stage uplift of the Cotswolds in response to erosional unloading of the Severn
Valley. The River Severn itself flows generally southerly through Area 9, much of it
near parallel to the north-south trending Malvern Line to the west. In contrast, in its
upper reaches, it flows generally easterly and southeasterly, from headwaters that are
less than 20 km from the sea at Cardigan Bay. In our consideration of Area 9, we shall
review evidence that this highest section of the Severn began as the headwaters of an
ancestor of the River Thames and flowed eastwards all the way across Southern Eng-
land to the ancestral North Sea.
FIG
153.
Map of western England and Wales showing the main Variscan fold trends and
the Variscan Front. The surface cover of younger rocks (from Permian New Red Sand-
stone to Early Cretaceous age) is also shown, along with the catchment area of the
Severn-Avon river system.
