Between Dorking and Leatherhead the Mole cuts a steep-sided valley through the North Downs, creating a river cliff on the western flank of Box Hill. The bedrock is permeable chalk and the water table lies permanently below the level of the riverbed, allowing water to drain out of the river through swallow holes in the bed and banks. The amount of water lost from the river is significant and in very hot summers the channel can become dry between Mickleham and Thorncroft Manor; this was recorded most recently in 1949 and 1976. At Leatherhead, the river leaves the chalk and flows across impermeable London Clay. At this point, the water table rises enough for the water to flow back into the main river channel.