The geography of Sydney is characterised by its coastal location on a basin bordered by the Pacific Ocean to the east, the Blue Mountains to the west, the Hawkesbury River to the north and the Woronora Plateau to the south. Sydney lies on a submergent coastline on the east coast of New South Wales, where the ocean level has risen to flood deep river valleys (rias) carved in the Sydney sandstone. Port Jackson, better known as Sydney Harbour, is one such ria.
The Sydney area lies on Triassic shales and sandstones. The region mostly consists of low rolling hills and wide valleys in a rain shadow area that is shielded by the Great Dividing Range.Rain Shadows by Don White. Australian Weather News. Willy Weather. Retrieved 24 May 2021. Sydney sprawls over two major regions: the Cumberland Plain, a relatively flat region lying to the west of Sydney Harbour, and the Hornsby Plateau, a plateau north of the Harbour rising to 200 metres and dissected by steep valleys.Sydney Basin-Subregions Sydney's native plant species are predominantly eucalyptus trees, and its soils are usually red and yellow in texture. The endemic flora is home to a variety of bird, insect, reptile and mammal species, which are conspicuous in urban areas.
There are more than 70 harbour and ocean beaches in the urban area. Most of Sydney's water storages are on tributaries of the Nepean River. Parramatta River drains a large area of Sydney's western suburbs. With 5,005,400 inhabitants (as of 2016) and an urban population density of 2037 people per square kilometre, Sydney's urban area covers , comprising 35% of Sydney and is constantly growing.
Geology
The rising sea level between 18,000 and 6,000 years ago flooded the rias to form estuaries and deep harbours. Erosion by coastal streams has created a landscape of deep canyons and remnant plateaus. Sea level reached a highstand of approximately above the present level around 7,400 years ago, inundating former river valleys and contributing to the formation of the modern Sydney Harbour estuary; sea level subsequently declined gradually towards its present position.
The city has fault lines which run considerably deep beneath the Sydney basin, dating back to when New Zealand started breaking away from Australia more than 85 million years ago. To the east the basin continues to the edge of the continental shelf. The edge of the Australian continental shelf lies about east of Sydney Heads, beyond which begins the Tasman Abyssal Plain. The centre of the basin is located around west of the Sydney central business district at Fairfield.
At a time in the past, monocline formed to the west of Sydney. The monocline is a sloping bend that raises the sandstone well above where it is expected to be seen, and this is why the whole of the visible top of the Blue Mountains is made of sandstone. Sandstone slopes in the Sydney area are on three sides: to the west the Blue Mountains, and to the north and south, the Hornsby and Woronora plateaux'. During the age of dinosaurs, Australia separated from Gondwana and began a gradual northward drift driven by the movement of tectonic plates. Around 120 million years ago, New Zealand rifted away from Australia, opening the Tasman Sea and leaving the eastern margin of the continent with a steep, narrow continental shelf. Exposed to wave action and marine weathering, cliffs of Hawkesbury Sandstone progressively retreated, producing a highly indented coastline. This erosion left behind a broad, relatively flat rock platform that was subsequently mantled by sand delivered to the coast by ancient river systems.
The Eastern Suburbs landscape was already undergoing significant geological modification, characterised by widespread thrusting, bending, and folding. These processes progressively produced a broad, bowl-shaped structural depression now known as the Sydney Basin. Over time, the basin accumulated thick sequences of sediment transported by ancient river systems. Some of this material was organic in origin and was later transformed into coal, explaining the presence of coal seams at the surface to the north, south, and west of Sydney, while equivalent layers occur at depths of up to 900 metres beneath the Sydney region itself. The dominant deposit, however, was a sandstone unit approximately 300 metres thick, which lithified to form the Hawkesbury Sandstone that dominates much of the modern landscape.
Sydney features two major soils; sandy soils (such as red, brown and yellow podsols, grey and brown tenosols, lithosols, kandosols and kurosols)Physical geography Bioregional assessments. 21 January 2019. Retrieved 24 May 2022. which originate from the Hawkesbury sandstone and have low fertility, and clay soils (which are from shales and volcanic rocks), though some soils may be a mixture of the aforementioned soils (such as sandy clay loams and silty clay loam).Gray, J.M. & Murphy, B.W. (2002), Predicting Soil Distribution, Joint Dept. of Land & Water Conservation (DLWC) & Aust. Society for Soil Science Technical Poster, DLWC, Sydney.Bannerman and Hazelton (1990), Reference to the 100,000 Soil Landscapes Sheet and Notes of the Penrith area, Soil Conservation Services of NSW. Sydney has a number of islands in its harbour and surrounding rivers. Such islands include, Shark Island, Cockatoo Island, Clark Island, Snapper Island, Spectacle Island and Goat Island in Port Jackson. Bare Island in Botany Bay. Lion Island, Long Island and Milson Island in Hawkesbury River. Scotland Island in the Northern Beaches. And Rodd Island in Parramatta River.
Hawkesbury sandstone
Being very porous, the Sydney sandstone (also known as the Hawkesbury sandstone) has shale lenses and fossil riverbeds dotted throughout and it is some thick. The sandstone was probably deposited in a freshwater delta and is the caprock which controls the erosion and scarp retreat of the Illawarra escarpment. Six kilometres of sandstone and shale lie under Sydney and there are volcanic rocks from low hills in the shale landscapes.. The sand that was to become the Hawkesbury sandstone of today was washed from Broken Hill and laid down about 200 million years ago. The ripple marks from the ancient river that brought the grains of sand are distinctive and easily seen, telling geologists that the sand comes from rocks formed between 500 and 700 million years ago far to the south. This means that the highest part of the visible lines almost always faces approximately south. The Basin's sedimentary rocks have been subject to uplift with gentle folding and minor faulting during the formation of the Great Dividing Range.
Sydney sandstone has commonly been interpreted as having formed within a large braided river system, similar in scale and depositional style to the modern Brahmaputra River in Bangladesh. The sands were deposited by a vast braided river system whose headwaters lay in Antarctica, at a time when Australia formed part of the supercontinent Gondwana and the Sydney region was situated at approximately 60° south latitude. During this period, no polar ice caps were present, and the climate was warm to hot, reflecting atmospheric carbon dioxide concentrations that were at least three times higher than those of today.
Bringelly Shale and Minchinbury Sandstone are often seen in the greater western parts of Sydney. Ashfield Shale is observed in the inner western suburbs. These components are part of the Wianamatta Shale group. Mittagong Formation sighted in a few areas in northern Sydney.Chris Herbert. Geology of the Sydney 1:100,000 Sheet 9130 The Prospect dolerite intrusion in western Sydney is the largest assemblage of igneous rock in Sydney. The oval-shaped ridge was made many millions of years ago when volcanic material from the Earth's upper mantle moved upwards and then sideways.Compton, K., Mindat: Prospect, New South Wales 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.Conybeare Morrison, Prospect Hill Conservation Management Plan, Holroyd City Council, 2005 The Gap, an ocean cliff on the South Head peninsula in Watsons Bay, was laid as sediment more than 200 million years ago in the Triassic period. During the Jurassic period, a cataclysmic event resulted in an enormous crack forming within the strata. The Gap itself forms a sequence that continues offshore to the edge of the Sahul Shelf.
Rivers and creeks
Overall, Sydney has nine rivers and just over a hundred creeks. The Nepean River rises to the south in the Woronora Plateau, and wraps around the western edge of the city. Swamps and lagoons are existent on the floodplain of the Nepean River, one being Bents Basin, which is also a recreational area. Where the Nepean turns east it becomes the Hawkesbury River, which winds through the Hornsby Plateau before emptying into Broken Bay. Broken Bay and the lower Hawkesbury form the commonly accepted boundary between Sydney and the Central Coast to the north. The remaining section of Warragamba River flows north-east from the Warragamba Dam spillway to its confluence with the Nepean River.
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. The Georges River estuary separates the main part of Sydney's urban area from the Sutherland Shire. The Woronora River, on the southern edge of the Sydney Plain, flows in a steep-sided valley from the Woronora Dam to the eastern estuary of the Georges River. The Hacking River is further south and runs through The Royal National Park into Port Hacking which forms the southern boundary of the Sutherland Shire.
Parramatta River's headwaters are several local creeks including Toongabbie Creek and Hunts Creek, part of the upper Parramatta river catchment area. Duck River is the river's southern tributary. Hunt's creek flows from Lake Parramatta, a few kilometres North of Parramatta. At east Parramatta the river becomes a tidal estuary that flows into Port Jackson, commonly known as Sydney harbour. Other major tributaries flow into Port Jackson from the North Shore and are the Lane Cove River and Middle Harbour Creek.
Minor waterways draining Sydney's western suburbs include South Creek and Eastern Creek, flowing into the Hawkesbury, and Prospect Creek draining into the Georges River. Cowan Creek and Berowra Creek run north from the Upper North Shore to the Hawkesbury river. The creeks draining into Georges River were described as "a chain of ponds" by the early European settlers, because these creeks were more like a series of ponds rather than an unbroken stream, before the disturbance of their banks by the settlers.
Ecology
The predominant plant communities in the Sydney region are grassy woodlands, which are open woodlands scattered with sclerophyllous shrubs and sparse grass in the understory. Wet and dry sclerophyll forest exist in some pockets, with heathlands and temperate/subtropical rainforests occurring in a few areas as well, thus making the Sydney region have distinct biomes. Dry sclerophyll forests, reminiscent of Mediterranean forests, are the most widespread forest type in the region, which occur sporadically in the Cumberland Plain and would feature dry, eucalyptus woodland. On the other hand, wet sclerophyll forests, which fall under temperate forests, have narrow, relatively tall, dense trees with a lush, moist understorey of fleecy shrubs and tree ferns. They are found in the cooler, wetter areas such as Northern Suburbs, Forest District, North Shore and in the Blue Mountains.