Geoscience Reference
In-Depth Information
been hardened by the addition of iron minerals (often now rust-coloured) formed in the
sands after they were deposited.
In Late Cretaceous times, a distinctive episode began in which the entire London
Platform became submerged and unusually uniform marine sedimentation extended
across it. The sediment deposited at this time was to become the particularly pure, fine-
grained, white limestone known as Chalk. This unique material, made largely of the
very small plates of floating microscopic plants, was deposited widely across the floor
of a shallow sea (perhaps 200 m in depth) that extended over most of the British Isles.
The compact and homogeneous nature of the Chalk has yielded the scenery of steep
escarpments, rounded hills and dry valleys that are typical of the Chalk hills of East
Anglia. The white Chalk has been an important local building material, particularly
when certain unusually hard bands have been quarried. It has also been an important
source of the generally black and very strong flint nodules that have grown within it, by
precipitation of silica dissolved in the groundwater. Flints have been quarried locally
from the Chalk, but more commonly collected from coastal beaches and river gravels
for use in buildings and as tools by early humans.
There is a time gap in the bedrock succession representing an episode in the lower
Cretaceous from which no deposits have been preserved. In the East Anglian Region,
this episode involved uplift and eventual abandonment of the Late Cretaceous Chalk
sea. Rather later, deposition began again in the London area, which started to subside
to form the London Basin or downfold. This Early Tertiary deposition culminated in
the deposition of the London Clay.
In mid-Tertiary times faulting and folding of the Earth's crust became quite wide-
spread across the southern parts of Southern England, although only gentle tilting of
bedrock layers occurred in East Anglia. By this time, some 25 million years ago, the
East Anglian Region was largely land and subject to widespread river erosion. Indeed,
this time saw an important change from general sinking of the East Anglian surface
as sediment accumulated, to general uplift and erosion of the surface due to tectonic
processes. This was the time when the main pattern of the present-day bedrock map
started to form.
In general terms the bedrock pattern consists of a simple series of slightly curved
belts (Fig. 226), each providing outcrops of one of the main layers of the bedrock suc-
cession. This map pattern is very simple compared with the pattern of other parts of
Britain, because it is the result of the erosion of a gently tilted succession that was
broadly uniform across the whole Region. The curvature of the zones is due to a change
in the tilt direction of the rocks, from easterly in the north of the Region, to southeast-
erly in the south of the Region, where the rocks slope towards the London downfold.
Over the extent of the whole Region (between 200 and 150 km depending on the dir-
