The oval-shaped ridge was made many millions of years ago when volcanic material from the Earth's core actuated upwards and then sideways.was laid as sediment more than 200 million years ago in the Triassic period.
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.
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.
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.
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.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'.Being very porous, the Sydney sandstone has shale lenses and fossil riverbeds dotted throughout and it is some 200 metres (656 feet) thick.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.Where the Nepean turns east it becomes the Hawkesbury River, which winds through the Hornsby Plateau before emptying into Broken Bay.