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.
[3] 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.
In the greater part of the basin the surface rock is Eocene London Clay, flanked at the margins by older deposits such as the Reading Beds.
In large areas towards the western end the London Clay is overlain by rather younger deposits of the Bagshot Beds etc., forming sandy heaths.
Vertical movements of this block have affected both deposition and structure, with the result that many of the Mesozoic rocks underlying the neighbouring Weald are not present, or are very much thinner.
On top of the Thanet Formation lies the Lambeth Group, which was deposited in a coastal plain and/or adjacent shallow sea water during the late Thanetian and early Ypresian ages.
The main headstream within the London Basin proper is the Kennet, which flows along the axis from the Marlborough area, joining the Thames at Reading.
[8] During the Pleistocene ice age the lower Thames is believed to have been diverted well to the south of its earlier course closer to the main axis of the basin, probably by the Anglian Stage.
Besides the whole Greater London Urban Area of over 9.8 million people,[9] the basin contains most of the M4 Corridor including Newbury, Reading, Bracknell, Wokingham, Finchampstead, Maidenhead and Slough; the urbanised area to the south-west of London (Camberley, Farnborough, Aldershot and Guildford); north and south Thames-side (Thurrock, Tilbury, Basildon, Southend-on-Sea, Northfleet, Gravesend etc.