The Cumberland Plain, also known as Cumberland Basin, is a relatively flat region lying to the west of Sydney CBD in New South Wales, Australia. An IBRA biogeographic region, 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, and it is a structural sub-basin of the Sydney Basin.
The Cumberland Plain has an area of roughly , which lies on Triassic shales and sandstones. Shaping the geography of Sydney, it extends from north of Windsor in the north, to Picton in the south; and from the Nepean-Hawkesbury River in the west almost to Sydney City, and includes parts of the Inner West and Northern Suburbs in the east. Much of the Sydney metropolitan area is located on the Plain. The Hornsby Plateau is located to the north and is dissected by steep valleys.
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 Arthur Phillip in honour of Ernest Augustus, Duke of Cumberland. Being the most populous region in Australia, the Cumberland Plain is one of the fastest-growing areas of the country in terms of population and it is home to a variety of Australian animal species, which are observable in the urban environments.
Geography
The Cumberland Plain stretches from Ryde in the east to the Nepean River in the west and from Cattai in the north to Thirlmere in the far south. The Cumberland Plain sprawls over south-western Sydney, and into the Greater Western Sydney area. It is within the local government areas of Blacktown, Burwood, Camden, the western portion of Campbelltown, Canada Bay, Canterbury-Bankstown, Cumberland, Fairfield, Georges River, Hawkesbury, the Western outskirts of the Inner West Council, Liverpool, Parramatta, Penrith, Ryde, Strathfield, The Hills Shire, Wingecarribee, and Wollondilly.
The Cumberland Plain is separated by the more hilly areas that fall into other Sydney regions such as Sydney CBD (in the City of Sydney), North Shore, Northern Beaches, Eastern Suburbs, Southern Sydney (including Sutherland Shire), Hills District, Hornsby Shire and the Forest District, which areas that border the Cumberland Plain.
The region mostly consists of low rolling hills and wide valleys in a rain shadow area near the Blue Mountains. The annual rainfall of the plain is typically around 700–900 mm, and is generally lower than the elevated terrain that partially surrounds it. Common vegetation in the Cumberland Plain is eucalyptus trees. The sclerophyll woodlands are situated on a nutrient-poor alluvium deposited by the Nepean River from sandstone and shale bedrock in the Blue Mountains. Despite this, they support a tremendous regional biodiversity. 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.NPWS (2002a) Native Vegetation of the Cumberland Plain – Final Edition, NPWS, Sydney.
Geology
The Sydney region lies on a submergent coastline where the ocean level has risen to flood deep rias. The Cumberland basin is a structural and topographical depression within the sedimentary Sydney Basin in Australia's southeast. Formed about 80 million years ago, the Cumberland Plain consists of not exactly flat plains; 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 Sydney sandstone. Relative to the surrounding higher sandstone lands, Hornsby Plateau and Blue Mountains Plateaux (which was uplifted 90 million years ago), it was an early matter of debate in Sydney physiographic circles as to whether the Cumberland Plain had gone down, or the surrounding plateaux had been raised up. The Cumberland Basin's perimeters had therefore developed in various ways and at different time period, where volcanic and tectonic movements had accompanied the uplift of Australia's Eastern Highlands within the time zone of 80–10 million years ago.
To the south, the Cumberland Basin's topography is closely linked to the development of the Camden Syncline, which was active in the late Permian to the Middle Triassic period. The Botany Basin, which would have started to settle since the Oligocene, sits within this Lachlan Transverse Zone, which hints that this area of the Basin would have already existed at this stage. During the Cenozoic period, the Penrith and Botany basins may have acted as the Cumberland Basin's drainage, thereby explicating the incomprehensible activity of rivers within the area. Despite much study, especially along the western side at the Lapstone Monocline, this complex matter is still not fully understood. There are volcanic rocks from low hills in the shale landscapes. Swamps and lagoons are existent on the floodplain of the Nepean River.
Ashfield Shale consists mainly of laminite and dark grey siltstone, while Bringelly Shale comprises shale with occasional calcareous claystone, laminite and infrequent coal seams. Minchinbury Sandstone consists predominantly of fine- to medium-grained quartz-lithic sandstone. 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 generally north south for over . 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 Prospect dolerite intrusion in western Sydney is the largest assemblage of igneous rock in the Plain. The oval-shaped ridge was made many millions of years ago when volcanic material from the Earth's core actuated upwards and then sideways. Slow erosion of the overlying layers of sedimentary rock by the flow of rainwater have eventually laid bare the edges of the volcanic and metamorphic rocks of the intrusion. The western suburbs lie on the relatively flat, lowly elevated parts of the Cumberland Plain. Though there are a few hilly or relatively elevated regions on the plain. Western Sydney Parklands and the surrounding suburbs (such as Cecil Hills and Horsley Park), for instance, lie on a prominent ridge that is between high.
Bringelly Shale and Minchinbury Sandstone are often seen in the Plain's west. Ashfield Shale is observed in the inner western suburbs. These components are part of the Wianamatta Shale group. Structurally, the Cumberland Basin underlying the plain is subdivided into several smaller basins, including the Fairfield Basin in the east and the Penrith Basin farther west. The Fairfield Basin is a broad synclinal structure in which substantial thicknesses of the Wianamatta Group have been preserved.
Soils
The soils of the Cumberland Plain, particularly the Cumberland Lowlands, are closely associated with the underlying geology and the region's gently undulating topography. They are predominantly clay-rich and generally have low to moderate natural fertility, with red, brown and yellow soils widespread across the plain. Red to brown clays also occur over volcanic rocks, while older gravel deposits may support relatively poor and stony soils. More recent floodplain alluvium can support higher-quality loams, although soils on some confluent valley floors are coarser and may be affected by salinity.
Much of the Cumberland Lowlands is occupied by the Blacktown soil landscape, a residual landscape formed in situ through the weathering of Wianamatta Group rocks. It occurs extensively across western and south-western Sydney, including around Blacktown, Mount Druitt, Fairfield and Leppington. The Blacktown landscape is characterised by broad, rounded ridges and crests separated by gently inclined slopes, with local relief generally between . Slopes are usually gentle but may reach gradients of approximately 10 per cent, and natural exposures of the underlying shale are uncommon except where the soil cover has been removed. Soil characteristics vary with topographic position. Red and brown podzolic soils occur mainly on crests and upper slopes, grading into deeper yellow podzolic soils on lower slopes and around drainage lines. Under the Australian Soil Classification, many of the strongly acidic, texture-contrast soils of the Cumberland Lowlands correspond broadly to Kurosols, while Sodosols and related soil types also occur.
A typical Blacktown soil profile exhibits a pronounced vertical change in texture. The surface A horizon may comprise up to approximately of friable brownish-black loam or clay loam, commonly containing plant roots and ranging from moderately acidic to neutral. Its colour may vary from brownish black to dark reddish or yellowish brown, and small iron-rich or weathered shale fragments may be present. Beneath it is commonly a hardsetting brown clay loam or silty clay loam forming an A2 horizon. This layer becomes hard when thoroughly dried and can become water-repellent under extremely dry conditions; it is generally moderately to slightly acidic and may contain iron-indurated fragments of weathered shale.
Below the A horizons is a marked transition to a clay-rich B horizon. The upper subsoil consists principally of strongly structured brown light to medium clay, generally becoming heavier with depth. Red, yellow and grey mottling is common and tends to increase farther down the profile, while pH generally ranges from about 4.5 to 6.5. At greater depths, particularly on middle and lower slopes, the brown clay commonly grades into light grey, highly plastic and mottled silty or heavy clay immediately above weathered shale bedrock. This deeper material is generally strongly acidic, with recorded pH values of approximately 4.0–5.5, and may contain weathered ironstone concretions and shale fragments.The depth of these soils varies considerably according to topographic position. Profiles on ridges and crests are commonly less than deep, with the surface horizons overlying as much as of mottled brown clay. On upper and middle slopes they may approach , while lower side-slopes can support profiles exceeding in depth. The latter may contain thick horizons of mottled brown clay over more than of light grey plastic clay.