Exploration of a gull cave in the Claverton Gorge near Bath has revealed an extensive network of rift passages. Uranium series dating of a speleothem from the cave shows that gravity sliding began at least 350 ka ago, which means that the River Avon valley was already well developed by that time. Solution features within the cave pre-date the mass movement and suggest that groundwater once flowed to a proto- River Avon which lay to the north-west, before the present topography was established. River capture by the proto- Avon of former Thames headstreams created the Claverton Gorge and led to overdeepening of the Avon valley with the associated foundering of rock strata in the Bath region.