The regional setting

Abstract

For the purpose of the present discussion, southeastern England will be understood to embrace East Anglia, the London Basin, the Weald anticline and the Hampshire Basin. However, because of the immediate practical importance of subsidence still apparently continuing, it will focus most attention upon the Thames Estuary and on London. The whole region is one in which the formations exposed at the surface are no older than Jurassic (about 160 million years (Ma)). They continue upwards through Cretaceous (Gault, Greensand, Chalk) and Tertiary to Pleistocene and Recent deposits. The Mesozoic rocks rest upon a platform of much older rocks, ranging from Carboniferous (under Kent) through Devonian under London itself to lower Palaeozoic. Figure 1 shows the formations penetrated by representative boreholes through the Mesozoic cover. The platform of Devonian lies 300 m or less below surface under London (Sherlock 1962), but the upper boundary of the ancient rock descends to 1500 m or more under the Weald (Gallois 1965). Not enough information is available to enable the thickness and structure of these old buried rocks to be worked out, but it is certain that they had been extensively folded by disturbances that culminated at perhaps 390—420 Ma (‘Caledonian’) and 280 Ma (‘Hercynian’). For long periods of geological time, what is now the infrastructure of southeastern England formed part of a great mobile belt extending, according to current ideas, longitudinally across a continental mass which included Eurasia and America.