district, administrative county of West Sussex, historic county of Sussex, southeastern England. It occupies an area in central Sussex. Mid Sussex extends from The Weald, a sandstone area of hilly grasslands and woodlands in the north, across undulating grasslands of the Ouse and Adur river valleys in the centre, to the steep chalk scarps of the north-facing South Downs in the extreme south, inland from the English Channel. Haywards Heath, the district seat, and Burgess Hill, both commuter stops on the Brighton-to-London railway, are residential centres with a light-industrial base. The town (parish) of East Grinstead in The Weald is an older market centre and suburban community that also has a light-industrial base. Mixed farming (including dairying and market gardening) occurs throughout the district. Area 129 square miles (333 square km). Pop. (2001) 127,383.