The Cumberland Plain is the surface expression of the Cumberland Basin of New South Wales, Australia. Cumberland Basin is the preferred physiographic and geological term for the low-lying plain of the Permian-Triassic Sydney Basin found between Sydney and the Blue Mountains. It is a structural sub-basin of the Sydney Basin. Increasingly in recent years, the Press and politicians have been tending to confuse the Cumberland Plain/Basin with the entire Sydney Basin (and referring to the Cumberland Basin as the Sydney Basin). Why the region is politically prominent, and has also become increasingly referred to as "Western Sydney" is because it is where Sydney's population is most aggressively expanding into. Within, it the NSW Government has designated two particular foci of planned population growth, called the Northwest and Southwest Growth Areas. Between these a new international airport is to be constructed, the Badgerys Creek Airport, also known as Sydney's Second Airport. The Cumberland Plain consists of not exactly flat "plains" and overall it is a low-lying area (largely over shale and labile sandstone) which derives its recognition largely by comparison with the surrounding uplands of harder quartzose Hawkesbury Sandstone. Relative to the surrounding higher sandstone lands (Hornsby and Blue Mountains Plateaux) it was an early matter of debate in Sydney physiographic circles as to whether the Cumberland Plain had gone down (sunken), or the surrounding plateaux had been raised up. Despite much study, especially along the western side at the Lapstone Structural Zone (a.k.a. Lapstone Monocline) this complex matter is still not fully understood. The Cumberland Plains have an area of roughly 2750 km2. The Wianamatta "Plain", with many undulating hills of Wianamatta Group shales and sandstones, bounded by the Woronora and Illawarra Plateaus to the south, the Blue Mountains Plateau to the west and the Hornsby Plateau to the north/northeast. At the front of the Blue Mountains Plateau runs the Lapstone Structural Complex, which forms the western edge of the Cumberland Basin and Cumberland Plain. This is a north-south trending collection of reverse faults and monoclinal folds which extends for over 100 km. At the opposite side of the Cumberland Plain the Hornsby Plateau is fronted by the Hornsby Warp. That warp is topographically subtle in comparison to the Lapstone Structural Complex, and it is a feature which is poorly defined and inadequately defined in literature.