[3] The Cumberland Plain has an area of roughly 2,750 square kilometres (1,060 sq mi), which lies on Triassic shales and sandstones.

[5] 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.

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.

[6] The region mostly consists of low rolling hills and wide valleys in a rain shadow area near the Blue Mountains.

The sclerophyll woodlands are situated on a nutrient-poor alluvium deposited by the Nepean River from sandstone and shale bedrock in the Blue Mountains.

[2] 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.

This is a north-south trending collection of reverse faults and monoclinal folds which extends generally north south for over 100 kilometres (62 mi).

The oval-shaped ridge was made many millions of years ago when volcanic material from the Earth's core actuated upwards and then sideways.

Western Sydney Parklands and the surrounding suburbs (such as Cecil Hills and Horsley Park), for instance, lie on a prominent ridge that is between 130 and 140 metres (430 and 460 ft) high.

[16] The moderately fertile soils in the plain are usually red and yellow in texture due to their clay-rich nature,[17][18] where they turn coarser and are sometimes affected by salt in confluent valley floors.

Poor rocky soils, frequently on older gravels, and high quality loams are present on contemporary floodplain alluvium.

[22] The south and southwest of Sydney is drained by the Georges River, flowing north from its source near Appin, towards Liverpool and then turning east towards Botany Bay.

The other major tributary of Botany Bay is the Cooks River, running through the inner-south western suburbs of Canterbury and Tempe.

[23] Dry and wet sclerophyll forests generally lie on the Hornsby plateau, an elevated region north of the Plain.

This confirmed earlier accounts by Governor Arthur 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..."[25][26] The Cumberland Plain is home to a variety of bird, insect, reptile and mammal species, including bats.

Cumberland Plain Woodland, of which around six per cent remains in isolated stands, was the first Australian ecological community to be assigned this status.

[37] The western portion of the Cumberland Plain mainly consists of sparsely populated, vast, rural grasslands with undulating hills and scenic vistas.

The Sydney western suburbs of Mount Vernon, Kemps Creek, Orchard Hills, Luddenham, Mulgoa, Bringelly, Silverdale and Horsley Park, among others, lie in this agricultural countryside, adjacent to the footsteps of the Blue Mountains westwards of these country plains.

The area around the site of Regentville has remained largely rural, if hemmed in somewhat by the modern residential suburbs of Jamisontown and Glenmore Park.

[39] In the 1800s, John Blaxland built an original wooden weir at "Grove Farm" (now known as Wallacia) for a sandstone flour mill and additional brewery.