The Blue Mountains, is a mountainous region in New South Wales, Australia, which borders Sydney's metropolitan area, beginning approximately 50 kilometres west of the city's centre. The area is generally considered to begin on the west side of the Nepean River and to extend westward as far as Coxs River. Consisting of a sandstone plateau, the area is dissected by gorges of up to 760 metres in depth, and has high points up to 1,190 metres above sea level. A large part of the Blue Mountains is incorporated in the Greater Blue Mountains Area World Heritage Site and its constituent seven national parks and a conservation reserve.