Geoscience Reference
In-Depth Information
FIG
150.
The Severn Valley Region (Areas 8 and 9).
The lower part of the unconformity here consists of Devonian rocks of the Lower Old
Red Sandstone, deposited as more-or-less flat beds about 400 million years ago. These
beds were tilted and then eroded to form a flat surface during the Variscan mountain
building, before being covered by layers of Triassic dolomitic conglomerate about 270
million years ago. These rocks provide a vivid record of the Variscan mountain build-
ing in this Region: the Devonian strata were tilted by folding, moved upwards and then
eroded, before being covered by sediment derived, at least partly, from Variscan moun-
tains that still existed nearby.
The Severn Valley Region contains an unusual diversity of scenery and bedrock
because it straddles an important boundary represented by the Variscan Unconformity,
dividing the older and newer bedrock successions of England and Wales (Fig. 153).
Carboniferous and older bedrock geology underlies most of the Southwest, Wales,
northwestern England and some parts of the English West Midlands. New Red Sand-
stone and younger bedrock dominates most of the rest of Southern England and the
eastern part of northern England. The map in Figure 153 shows the trend of Variscan
folds generated during the plate convergence that caused the Variscan mountain build-
ing. South of the line marked 'Variscan Front' the main fold trends run east-west, rep-
resenting convergence in a roughly north-south direction. North of the Variscan Front,
in Areas 8, 9 and 10, folds of similar age trend generally north-south and result from a
different pattern of movement.
