Geoscience Reference
In-Depth Information
GENERAL INTRODUCTION
I
HAVE LINKED AREAS
8
AND
9 together to form the Severn Valley Region (Fig.
150). This is because the Bristol Channel and the Severn and Avon valleys form one of
the most important landscape features of this part of Southern England. Another dom-
inant feature of this Region is the Cotswold Hills, extending from Bath northeastwards
through Banbury.
The generalised bedrock succession for this Region (Fig. 151) contains a major
unconformity or time gap. At this gap, the scale used to represent the thickness of the
rock successions shown in Figure 151 has been changed, because the Carboniferous and
older bedrock generally involves much greater thicknesses than the younger bedrock.
This provides evidence for a slowing in the rate of sediment accumulation in this Region
towards the end of the Carboniferous, which suggests that Earth movements were more
active in earlier times, when the raising and lowering of the Earth's surface resulted in
vigorous production of sediment and its trapping in subsiding basins.
The time gap in the succession can be mapped across the Region and is known as
the
Variscan Unconformity.
Figure 151 shows this gap spanning the time between 305
and 299 million years ago, but this is misleading because 305 million years is the young-
est age found in the underlying layers, while 299 million years is the oldest age found
in the overlying layers. In most localities in the Region the time gap is much longer in
duration.
The Variscan Unconformity is spectacularly exposed along the coastline at Wood-
hill Bay, Portishead, some 10 km west of central Bristol (Fig. 152).
