London and the Thames Valley Region - Southern England - page 191

FIG 182. Block diagram, looking westwards, to show how the Chalk outcrop pattern at
the surface results from erosion of the fold pattern in the bedrock.
Later, in the Early Cretaceous, sea level rose substantially, flooding the previously
river-dominated Wessex Basin and depositing the shallow marine sands of the Lower
Greensand, which are often seen to infill ancient river valleys cut earlier into the
Wealden bedrock. Later still, by the mid Cretaceous, the deeper marine Gault Clay and
Upper Greensand were deposited (Fig. 181). By the Late Cretaceous, rising sea levels
progressively inundated most of northwest Europe. The Thames Valley Region was
submerged beneath a sea several hundred metres deep, and pure white chalk was de-
posited over much of Southern England.
During Early Tertiary times, the whole of Southern England was subjected to
crustal convergence, causing further movements along faults and local folding of the
bedrock. This occurred at roughly the same time as the major convergence that caused
the tectonic-plate-related mountain building of the Alps and Pyrenees to the south.
Some old faults relating to the lowering of the Wessex Basin relative to the London
area were reactivated, but this time in the opposite direction: the Wessex Basin was
uplifted and the London Platform subsided. The uplifted Wessex Basin was further
compressed to form the Weald uplift or anticline, while in the London region a broad,
eastward-opening downfold (syncline) developed, creating the impressive arcuate bed-
rock pattern that is clearly picked out by the Chalk today (Figs 180 and 182).