page_title: Cumberland Plain
section_title: Geology
hierarchical_section_title: Cumberland Plain / Geology
caption_attribution_description: English: This shows the Headwaters of the Parramatta River at the confluence of Darling Mills Creek and Toongabbie Creek.
context_page_description: The Cumberland Plain, an IBRA biogeographic region, is a relatively flat region lying to the west of Sydney CBD in 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, and it is a structural sub-basin of the Sydney Basin.
  The Cumberland Plain has an area of roughly 2,750 square kilometres. Shaping the geography of Sydney, it 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 Hornsby Plateau is located to the north and is dissected by steep valleys.
  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. 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.
context_section_description: 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 and Blue Mountains Plateaux, 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. 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.
  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 100 kilometres (62 mi). 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.
  Prospect Hill 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, lies on a prominent ridge that is between 130 to 140 metres (430 to 460 ft) 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.
caption: The Parramatta River is the most prominent river that flows within the Cumberland Plain.
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