The Sydney Basin sits on the east coast of Australia. It's made up of a basin filled with near horizontal sandstones and shales of Permian to Triassic age that overlie older basement rocks of the Lachlan Fold Belt. The sedimentary rocks have been subject to uplift with gentle folding and minor faulting during the formation of the Great Dividing Range.
The topography of the area was influenced by the uplifting at the end of the Triassic period and subsequent erosion. The widespread volcanic activity in New South Wales and eastern Australia in Jurassic and Tertiary times probably influenced the overall topography and geology of the Sydney Basin.
Today's topography of the Hornsby Plateau was produced by erosion over a long period of time following uplift of the area about 80 million years ago, associated with rifting and opening of the Tasman Sea. The plateau has been eroded by fresh water streams, which would have cut a maze of deep V-shaped valleys with intervening rocky ridges into the Hawkesbury Sandstone. Some leftover shales of the Ashfield (Shale) Group occur on the ridges in the area, but most have been eroded.
The Berowra Valley has been exposed to a number of big changes in sea level beginning 200,000 years ago. The last stage in the development of the present topography of Berowra Valley was the rising sea level, which occurred as the Pleistocene glaciation waned about 12,000 years ago. A series of changes began in the relative levels of the land and the sea which reduced the sea level and moved the coastline many kilometres offshore, causing rivers to cut down into their beds, lowering them towards the sea level, thus deepening the coastal gorges. The returning sea rose to a higher level than previously, drowning the river mouths. About 6000 years ago these changes stabilized to produce the present topography. The Hawkesbury River and Berowra Creek are, like Sydney Harbour, typical examples of these drowned river valleys.
Erosion by coastal streams have created a landscape of deep cliffed gorges and remains of plateaus across. The Sydney Basin Bioregion includes coastal landscapes of cliffs, beaches and estuaries.
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