Along a narrow swath from Nottingham through to Hartlepool, broad shallow depressions up to 100m in diameter and, more rarely, scarp-edged subsidence hollows are observed. These topographical features coincide with the sub-crop of the Permian strata beneath the Quaternary deposits and are attributed to the dissolution of sub-surface gypsum. Boreholes (<150 m deep) prove the existence of several layers of gypsum within the Permian geological succession.