Geoscience Reference
In-Depth Information
on years ago and the present day, river networks became established on the
high ground and the sea repeatedly advanced into and retreated from the Lon-
don Basin. Erosion continued and intensified, lowering the landscape gener-
ally and causing the Chalk scarps to retreat inwards towards the centre of the
basin.
D. Today, the landscape has been eroded to expose Early Cretaceous sediments
in the core of the Weald uplift, bounded to the north and south by the Chalk
scarps of the North and South Downs. Tertiary sediments are preserved within
the London Basin, while to the north, at its northern margin, another Chalk
scarp marks the Chiltern Hills.
Modification under Ice Age conditions
The Thames Valley Region was only rarely invaded by ice sheets during the Quatern-
ary. The most widespread invasion of the past few million years occurred during the
Anglian cold phase 450,000 years ago, when ice reached the London Basin and diver-
ted the course of the ancestral River Thames. The more local effects of the Anglian
glaciation on the London Basin are discussed further in the Area accounts.
Although most of the Region has not been covered by ice sheets, it has still been
severely affected by the periglacial (near-glacial) climate. For most of the time the land
was treeless tundra, with a permanently frozen subsurface layer of soil and bedrock.
At the surface, soil and bedrock were frozen in winter but thawed every summer, an
effect which, combined with the snow-fed spring meltwater, created fast-flowing and
highly erosive rivers. The 'dry valleys' of the Chalk were carved at this time: chalk is
naturally porous because it is highly fractured, but the frozen subsurface blocked the
fractures and prevented water from draining away through the chalk as it did during
warmer times. Instead the water flowed across the surface, carving valleys that are vis-
ible today.
Other cold-climate effects on the scenery were land-slumping and land-slipping,
accelerated by freeze-thaw cycles. These created characteristic indents in hill ranges
(see Areas 10 and 12) and concentrated Sarsen Stones in valley floors (see Area 10).
Glacial processes also modified the earlier, river-created landscape, for example in
south Essex in Area 12.
The River Thames was severely affected by the Ice Ages. Before the great Angli-
an glaciation the Thames entered the London basin from the west approximately where
it does now, but then flowed northeastwards along the Vale of St Albans, some 30 km
north of its present course. It then turned northwards in the Chelmsford area and flowed
across north Essex and Suffolk, eventually joining the Rhine somewhere in the area
now covered by the southernmost North Sea (see Area 11). Just under half a million
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