At the largest scale London lies within the bowl of the London Basin, with most of the built-up area lying on the Tertiary and younger sediments, and only a small part of south London (Sutton, Banstead and Croydon) lying on the chalk backslope of the North Downs. The centre of the basin is dominated by the modern valley of the Thames, which forms a level corridor running from west to east. The modern floodplain is around half a mile wide to the west of Greater London, expanding to two miles wide to the east. This is bordered by slightly higher and older terraces often extending several miles from the floodplain, for example in Hounslow and Southwark. Other significant river valleys include those of the Colne, Crane, Brent, Lea (with a floodplain more than a mile wide in places), Wandle and Ravensbourne, which run north and south towards the Thames.