The development and origin of karst in the Upper Greensand Formation (Cretaceous) of south-west England

The Upper Greensand of south-west England can be divided on bulk lithology into two roughly equal parts, each 25 to 30 m thick. The lower part, the Foxmould Member, consists of weakly cemented glauconitic sandstones with low carbonate contents. The member weathers, largely by oxidation, to soft, loose, yellow and foxy brown sands. In contrast, the overlying Whitecliff Chert and Bindon Sandstone members consist of calcareous sandstones and sandy calcarenites with numerous chert-rich horizons. Dissolution, particularly during the warm humid climates of the Eocene and the periglacial climates of the late Pleistocene, has been the dominant weathering process in these two members. Karstic features observed on the east Devon and west Dorset outcrops include widespread pervasive dissolution that has locally reduced the in situ thickness of the Whitecliff Chert and Bindon Sandstone members to less than half their original thickness, along with deep solution pipes, and at one locality, caves. These discrete solution features occur beneath a thick capping of Chalk that is not markedly affected by dissolution. Over much of east Devon and west Dorset, the residual loose sands and chert blocks derived from the dissolution of the Upper Greensand were remobilised during the late Pleistocene to form extensive Head deposits.