This site consists of the two largest subsidence depressions in the Mendips, formed by the underlying limestones having been dissolved by subterranean waters causing the surface rocks to collapse into the void. Clear evidence of their being collapsed structures rather than erosional or solutional features, is gained from the fact that the surface rocks at both localities are insoluble, being marls at Devil's Punchbowl and a series of limestones and clays which have been impregnated by silica at Wurt Pit.