The geomorphology of the south-central Mendip Hills

Abstract

The Hills, consisting mainly of cavernous Carboniferous Limestone, form a gently rolling plateau surface at 800 to 850 ft. (245–260 m.) O. D. Several monadnocks of Old Red Sandstone stand up to 200 ft. (60 m.) higher. The steep south flank of the plateau extends nearly down to sea-level and is basically a Triassic feature exhumed from beneath soft Mesozoic strata. The plateau, on the other hand, is thought to be largely a subaerial erosion surface of late Pliocene date. Six erosion benches are recognised on the south flank between 670 ft. (205 m.) and 70 ft. (21 m.) O.D. They reflect the intermittently falling Pleistocene sea-level, and may be correlated with Zenner's eustatic levels from the Calabrian to the Main Monastirian. A more irregular bench in the east is a structural feature developed at the base of the Lower Lias Clay. A system of dry valleys similar in pattern to normal river systems is entrenched into the plateau. The original drainage pattern on the plateau surface was directed southwest or south, probably superimposed from a Miocene dome of Chalk. Entrenchment at the start of the present erosion cycle soon brought adjustment to the Palaeozoic structure. The Cheddar proto-system, closest to the Bristol Channel, had base-level advantage and beheaded most of the others. The valleys debouch through gorges cut in the south flank. The size of the gorges varies according to the size of their fossil catchments. Changes of gradient in the thalwegs are due to a succession of knickpoints receding from the south flank baselevels. Valleys and gorges were incised by surface streams; the upper parts in the late Pliocene and early Pleistocene before drainage had gone underground, the lower parts when streams returned to the surface on Ice Age permafrost. Underground drainage gravitated toward the lowest limestone outcrops. The two main springs are located at the mouths of the two main gorges, Cheddar and Ebbor. Ancient water tables traced in the dry springhead caves can be correlated with the erosion benches. Most of the hundreds of closed depressions or dolines on the limestone plateau were created by solution working down from the surface. Also on the plateau are eighteen internally draining karst basins up to 50 ft. (15 m.) deep and 300 acres (1.2 km. 2) in area. They developed at various stages of the Pleistocene by desiccation of headwater valleys. The older examples have a complex history and supported lakes on permafrost during glacial periods. Most show one or more overspill channels.