Geoscience Reference
In-Depth Information
CHAPTER 7
London and the Thames Valley Region
GENERAL INTRODUCTION
T HE LONDON AND THAMES VALLEY REGION includes the western watershed
between the Severn and Thames valleys in Gloucestershire and Wiltshire, one of the
main drainage divides in Southern England. It also extends eastwards, through Oxford-
shire, Berkshire and Buckinghamshire to London, Essex, Kent and the sea. Travelling
from west to east, I have divided it into Areas 10, 11 and 12 (Figs 179 and 180).
I shall start by examining the bedrock geology, and how it relates to the general
river-valley and hill pattern of the Region (Fig. 181).
The central feature of this Region is the London Basin, bounded to the north and
south by hills of Late Cretaceous Chalk. The folding of the Chalk has formed the Lon-
don Basin downfold to the north and the Weald uplift to the south (Fig. 182). This is a
classic example of how gentle folding of the bedrock can influence landscapes and to-
pography on a Regional scale.
Earlier chapters, particularly Chapters 5 and 6, have shown how important the Var-
iscan mountain building has been in the landscape evolution of Southern England. This
mountain-building episode culminated about 300 million years ago and was followed
by New Red Sandstone sedimentation during the Permian and Triassic. Southern Eng-
land then entered a period of relative stability, although gentle regional tilting did oc-
cur, along with important variations in sediment accumulation and minor faulting of the
crust.
One important effect of the faulting has been to produce differences in the bedrock
successions to the north and south of the London Basin in layers of sediment of Early
Cretaceous age or older. Most of this faulting was normal (see Chapter 3, Fig. 33),
caused by stretching of the crust that fractured it into discrete blocks, some rising and
some subsiding. The stretching was a local response to larger movements of western
Europe, linked to the opening of the Atlantic Ocean to the west and the sinking of the
North Sea area to the east. In the Thames Valley area, the result of this new pattern was
that a relatively stable London crustal block (often called a platform ) became separated
by fracturing from a subsiding Wessex Basin to the south (Figs 183 and 184). The Wes-
sex Basin is now represented near the surface by the thick succession of Early Creta-
ceous bedrock in the Weald area to the south of the North Downs.
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