London Platform

The London Basin is an elongated, roughly triangular sedimentary basin approximately 250 kilometres (160 mi) long which underlies London and a large area of south east England, south eastern East Anglia and the adjacent North Sea. The basin formed as a result of compressional tectonics related to the Alpine orogeny during the Palaeogene period and was mainly active between 40 and 60 million years ago.

The generally accepted boundaries are the Late Cretaceous Chalk Group's escarpments of the Chilterns and Marlborough Downs to the north and the North Downs and Berkshire Downs to the south. To the south lie the Weald and Salisbury Plain and to the north is the Vale of Aylesbury. The approximate western limit is in the Marlborough area of Wiltshire. The eastern end merges with the North Sea Basin, extending on land along the north Kent coast to Reculver and up the east coast of Essex and into Suffolk, where it is overlain by 'Crag' deposits which cover much of eastern Suffolk and Norfolk and are better considered as part of the North Sea Basin.

The axis of the basin runs west-east from Marlborough and Newbury (Berkshire) to Chertsey (Surrey) before swinging slightly north of east through Westminster, passing midway between Chelmsford and Southend-on-Sea (Essex) to the east coast between the estuaries of the Crouch and the Blackwater. Though north of the current mouth of the Thames, this line is well to the south of the centre-line of the basin which is asymmetric, its southern limb dipping more steeply than the northern.

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