The ridge of the North Downs is intersected by the valleys of a series of rivers: the Wey, Mole, Darent, Medway and Stour. These drain much of the Weald to the south. The western rivers are tributaries of the Thames; they have carved steep valleys through the chalk and provide natural corridor routes. In addition to existing rivers, the Downs are crossed by a number of wind gaps – prehistoric river valleys no longer occupied by rivers – including those at Farnham, Betchworth, Caterham, Lyminge and Hawkinge. Except for the river valleys and wind gaps, the crest of the escarpment is almost continuous along its length. The dip slope is dissected by many small dry valleys, and in the broad eastern part in Kent, by further river valleys such as that of the Little Stour.