The Mole valley is the valley linked to the river Mole. The River Mole is a tributary of the River Thames. It rises in West Sussex near Gatwick Airport and flows north west through Surrey for 80 km (50 miles) to the Thames near Hampton Court Palace. Much of the catchment area lies on impermeable rock (including Wealden Clay and London Clay), meaning that the river level responds rapidly to heavy rainfall. The Domesday Book lists twenty mills on the river in 1086. So it couls well be that the name of the river Mole comes from the Latin mola or mill, as indicated by the Oxford Dictionary of English Place Names.