The South Downs National Park's chalk downland is a feature that sets it apart from other national parks in Britain. However, almost a quarter (23%) of the national park consists of a quite different and strongly contrasting physiographic region, the western Weald, whose densely wooded hills and vales are based on an older Wealden geology of resistant sandstones and softer clays. The highest point in the national park, Blackdown, at above sea level, is in fact situated in the Weald, on the Greensand Ridge, whereas the highest point on the chalk escarpment of the South Downs, Butser Hill, has an elevation of above sea level.