district, administrative county of East Sussex, historic county of Sussex, England. The district takes its name from that of the Weald (Weald, The), a region of forested ridges that lies between the chalk hills of the North and South Downs. Wealden is bordered on the north by Kent and on the south by the English Channel coast, where the borough of Eastbourne forms an urban enclave in what is otherwise still a rural district. In the northwest is Ashdown Forest, an area of natural woodland and heath. In Wealden district's southeast corner is Herstmonceux, which has a moated castle that was the site of the Royal Greenwich Observatory, formerly at Greenwich, from 1948 to 1990. Area 323 square miles (837 square km). Pop. (2001) 140,021.