The Blue Mountains are a mountainous region and a mountain range located in New South Wales, Australia. The region is considered to be part of the western outskirts of the Greater Sydney area. The region borders on Sydney's main metropolitan area, its foothills starting about 50 kilometres (31 mi) west of centre of the state capital, close to Penrith. The public's understanding of the extent of the Blue Mountains is varied, as it forms only part of an extensive mountainous area associated with the Great Dividing Range. As defined in 1970, the Blue Mountains region is bounded by the Nepean and Hawkesbury rivers in the east, the Coxs River and Lake Burragorang to the west and south, and the Wolgan and Colo rivers to the north. Geologically, it is situated in the central parts of the Sydney Basin.