Geoscience Reference
In-Depth Information
ection) the bedrock has been tilted downwards - relative to its original position - by
about 700 m, so a quick calculation shows that the overall amount of the tilt is less than
1 degree.
In latest Tertiary times, further downward movement taking place in the southern
North Sea caused the eastern edge of East Anglia to sink slightly once again, depositing
a thin veneer of sandstones and limestones. The general term
Crag
has been given to
these deposits.
Surface blanket and more recent history
The surface blanket of softer material that rests upon the bedrock contains evidence for
episodes that have occurred during the Quaternary (the last 2.6 million years at most).
There is debate about the number, the timing and the areas covered by the ice-sheet
invasions associated with this time period, but it is generally agreed that the most ex-
tensive glaciation was due to the advance of the Anglian Ice Sheet about 450,000 years
ago (see Chapter 2, Figs 13 and 14). At this greatest extent, ice covered the whole Re-
gion except for the area of the higher Chalk hills southwest of Luton.
Both before and after this Anglian cold episode, East Anglia lacked ice-sheet cov-
er and was subjected to very varied but predominantly cold conditions. There is evid-
ence that the ground was often frozen, and that seasonal melting created conditions of
widespread river erosion and deposition, along with highly active slumping of slopes
and melt-induced movements of the soils.
Much more recently, the worldwide sea-level rise that has followed the melting of
the last ice sheet (see Chapter 2, Fig. 13) has been responsible for much of the sedi-
mentation in the very flat areas of the Fens and the Broads, as well as widespread sed-
imentation in most of the main river valleys. Local features that are the results of these
episodes will be picked out in the Area discussions that follow.
East Anglia's flatness
Although a plot of the bedrock succession, generalised for the whole of the East Angli-
an Region (Fig. 225), shows some 800 m of bedrock, it must not be thought that layers
accumulated to this total thickness at any one place. Indeed, as described above, the
succession shows that gentle movements of the Earth's surface caused the thickest ac-
cumulations of each of the main layers to move through time from the northwest to the
southeast (Fig. 224), so that the layers rest on the ancient London Platform foundation
rather like overlapping plates in a plate rack.
The presence of the London Platform foundation only a few hundred metres be-
low the East Anglian landscape, and its relatively uniform cover by sediments since, is
