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.
The plain extends from 10 kilometres north of Windsor in the north, to Picton in the south; and from the Nepean-Hawkesbury River in the west almost to Sydney City's Inner West in the east. Much of the Sydney metropolitan area is located on the Plain.
The plain takes its name from Cumberland County, in which it is situated, one of the cadastral land divisions of New South Wales. The name Cumberland was conferred on the County by Governor Phillip in honour of Ernest Augustus, Duke of Cumberland.
The Cumberland Plain is located within the local government areas of Auburn Council, Baulkham Hills Shire, City of Blacktown, Camden Council, City of Canada Bay, City of Fairfield, City of Hawkesbury, Hornsby Shire, City of Holroyd, City of Liverpool, City of Parramatta, City of Penrith, City of Ryde and Wollondilly Shire; and parts of City of Campbelltown and several inner-western councils. The Hawkesbury, Nepean, Parramatta and Cooks rivers run through parts of the plain.
The area lies on Triassic shales and sandstones. The region mostly consists of low rolling hills and wide valleys in a rain shadow area near the Blue Mountains. There are volcanic rocks from low hills in the shale landscapes. Swamps and lagoons are existent on the floodplain of the Nepean River. Soils are usually red and yellow in texture.
In 1820s, Peter Cunningham described the country west of Parramatta and Liverpool as "a fine timbered country, perfectly clear of bush, through which you might, generally speaking, drive a gig in all directions, without any impediment in the shape of rocks, scrubs, or close forest". This confirmed earlier accounts by Governor Phillip, who suggested that the trees were "growing at a distance of some twenty to forty feet from each other, and in general entirely free from brushwood..."
At the time of European settlement, the Cumberland Plain contained 1,070 km² of woodlands and forests. The westward expansion of Sydney over the plain has placed enormous pressure on the woodlands and other local ecological communities, only 13% of which remain uncleared. Cleared and used first for agriculture and then for urban development, most of the ecological communities that originally flourished on the plain are now considered endangered. They include:
- Cumberland Plain woodland
- Shale/sandstone transition forest
- Sydney coastal river-flat forest
- Elderslie banksia scrub
- Blue gum high forest
- Sydney turpentine ironbark forest
- Western Sydney dry rainforest
- Castlereagh swamp woodland
- Agnes Banks woodland
- Cooks River/Castlereagh ironbark forest
- Moist shale woodland
- Shale gravel transition forest
Under Federal environmental legislation, six of the above ecological communities are protected as four "matters of national environmental significance". Some are grouped together into broader communities that share similarities in landscape position, structure and/or species.
The four nationally defined and protected threatened ecological communities are: Blue Gum High Forest of the Sydney Basin Bioregion; Cumberland Plain Shale Woodlands and Shale-Gravel Transition Forest; Shale/Sandstone Transition Forest; Turpentine-Ironbark Forest in the Sydney Basin Bioregion; and Western Sydney Dry Rainforest and Moist Woodland on Shale.
Cumberland Plain communities are protected in a number of council reserves, plus the Lower Prospect Canal Reserve, Scheyville National Park, Windsor Downs Nature Reserve, Leacock Regional Park and Mulgoa Nature Reserve and Mount Annan Botanic Garden. Cumberland Plain Woodland, of which around six per cent remains in isolated stands, was the first Australian ecological community to be assigned this status.
- Carter, Lewis, 2011. Tectonic Control of Cenozoic Deposition in the Cumberland Basin, Penrith/Hawkesbury Region, New South Wales. Bachelor of Science (Honours), School of Earth & Environmental Sciences, University of Wollongong. 
- "Cumberland". Geographical Names Register (GNR) of NSW. Geographical Names Board of New South Wales. Retrieved 4 August 2013.
- Kohen, J., The Impact of Fire: An Historical Perspective, in Australian Plants Online, Society for Growing Australian Plants, September 1996
- Botanic Gardens Trust - Mount Anna Botanic Garden webpage
- Australian Government: Department of the Environment and Heritage - Cumberland Plain Woodland: Woodlands Vanishing from Sydney's Outskirts