Sussex, historic county of southeastern England, covering a coastal area along the English Channel south of London. For administrative purposes, Sussex is divided into the administrative counties of East Sussex and West Sussex and the unitary authority of Brighton and Hove.
A ridge of chalk hills, the South Downs, runs across the county from east to west, reaching the sea in a line of imposing cliffs, notably at Beachy Head. The northern slopes of the Downs form an abrupt scarp line, where the chalk gives way to the heavy clays and sands of the Weald. To the south the Downs slope more gently toward the English Channel. South of Chichester a fertile coastal plain broadens out into the flat headland of Selsey Bill. Coastal erosion, especially around Selsey Bill, has produced continual changes in the shoreline. In the southeast of the county, beyond Beachy Head, lie the reclaimed marshes of Pevensey Levels, historically an important point of entry into Britain for early invaders. A further line of cliffs lies along the coast eastward past Hastings.