Between Dorking and Leatherhead the Mole cuts a steep-sided valley through the North Downs, creating a high river cliff on the western flank of Box Hill.
In a survey in 1958, the geologist C.C. Fagg identified 25 active swallow holes between Dorking and Mickleham; most were only a few centimetres in diameter and were located in the vertical banks of the river below the water line [REF].