Between Dorking and Leatherhead the Mole cuts a steep-sided valley (the Mole Gap) though the North Downs, carving a high river cliff on the western flank of Box Hill and a smaller cliff at Ham Bank in Norbury Park.
Prior to the last Ice Age, the River Thames followed a more northerly route to the North Sea, from Reading via Marlow, Chorleywood, St Albans, Hertford and along the present Suffolk-Essex border [REF].