BOLSOVER, urban district, Derbyshire, England, 51 m. E. of Chesterfield, on branch lines of the L.M.S. and L.N.E. rail ways. Pop. (1931) 11,811. It lies at a considerable height on a sharp slope above a stream tributary to the river Rother. The castle round which the town grew up was founded shortly after the Conquest by William Peveril, but the existing building, a fine castellated residence, was erected on its site in 1613. The town itself was fortified, and traces of early works remain. The church of St. Mary is of Norman and later date; it contains some inter esting early stone-carving, and monuments to the family of Cavendish, who acquired the castle in the 16th century. Coal mining and quarrying are carried on in the neighbourhood.