The Blue Mountains (Gundungurra/Dharug: Colomatta or Gulumada) 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.
The Blue Mountains Range comprises a range of mountains, plateau escarpments extending off the Great Dividing Range about 4.8 kilometres (3.0 mi) northwest of Wolgan Gap in a generally southeasterly direction for about 96 kilometres (60 mi), terminating at Emu Plains. For about two-thirds of its length it is traversed by the Great Western Highway, the Main Western railway line and the proposed Blue Mountains tunnel. Several established towns are situated on its heights, including Katoomba, Blackheath, Mount Victoria, and Springwood. The range forms the watershed between Coxs River to the south and the Grose and Wolgan rivers to the north. The range contains the Explorer Range and the Bell Range.
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