On top of the Thames Group is the Bracklesham Group, which consists of the Bagshot Formation, the Windlesham Formation and the Camberley Sand Formation. The Bracklesham Group is also composed of shallow marine (near-shore) and coastal facies rocks. It can be up to 140 metres thick and is of late Ypresian to Lutetian age. The basin is mainly drained by the River Thames, but does not coincide with the Thames drainage basin. The upper Thames cuts through the Chilterns via the Goring Gap, and consequently the Thames drains parts of the Cotswolds, Vale of White Horse and Vale of Aylesbury. 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. To the south rivers such as the Mole and Medway, draining from the Weald, cut through the North Downs into the basin; these are presumed to date from before the erosion of the Weald dome. 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. The north eastern part of the basin is now drained to the North Sea by rivers including the Crouch, Blackwater, Stour and Orwell. Much of the basin is urbanised. Besides the whole Greater London Urban Area of over 9.8 million people, 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.); north Kent towns including Chatham, Rochester, Gillingham, Sittingbourne, Faversham, Whitstable, Herne Bay and Canterbury; Essex towns including Brentwood, Chelmsford, Braintree, Colchester, Clacton-on-Sea and Harwich; and towns to the north and northwest of London including Bishops Stortford, Harlow, Hertford, Stevenage, Harpenden, Hatfield, Welwyn Garden City, Potters Bar, St Albans, Luton and High Wycombe.