Surface runoff
The Kettles are two pools that are the end result of catastrophic subsidence in 1179: the Permian rocks underlying the area include substantial thicknesses of evaporites—gypsum and gypsiferous mudstone—and subsidence resulting from subterranean dissolution of these beds is a frequent occurrence, though the scale of the Hell Kettles subsidence is exceptional. Surface runoff and seepage from calcareous springs has created the two pools, one of which is the only body of spring-fed open water in County Durham: there were originally four subsidence depressions, but one filled in and two are now linked in a pond aptly named 'Double Kettle'.