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Geology

Sydney is made up of mostly Triassic rock with some recent igneous dykes and volcanic necks (typically found in the Prospect dolerite intrusion, west of Sydney). The Sydney Basin was formed in the early Triassic period. The sand that was to become the sandstone of today was laid down between 360 and 200 million years ago. The sandstone has shale lenses and fossil riverbeds.

The Sydney Basin bioregion includes coastal features of cliffs, beaches, and estuaries. Deep river valleys known as rias were carved during the Triassic period in the Hawkesbury sandstone of the coastal region. The rising sea level between 18,000 and 6,000 years ago flooded the rias to form estuaries and deep harbours. Port Jackson, better known as Sydney Harbour, is one such ria. Sydney features two major soil types: sandy soils (which originate from the Hawkesbury sandstone) and clay (which are from shales and volcanic rocks), though some soils may be a mixture of the two.

Directly overlying the older Hawkesbury sandstone is the Wianamatta shale, a geological feature found in western Sydney that was deposited in connection with a large river delta during the Middle Triassic. The Wianamatta shale generally comprises fine grained sedimentary rocks such as shales, mudstones, ironstones, siltstones and laminites, with less common sandstone units. The Wianamatta Group is made up of Bringelly Shale, Minchinbury Sandstone and Ashfield Shale.