Sydney Basin - Geology

Geology

Sydney is mostly Triassic rock, with some recent igneous dykes and the volcanic neck. The Hawkesbury sandstone is some 200 metres (600 feet) thick, with shale lenses and fossil riverbeds dotted throughout it. The sandstone was laid down in the Middle Triassic period between 180 and 220 million years ago. It consists of massive and cross-bedded sheet bedforms with minor (less than five per cent) siltstone and mudstone beds, which contain fish fossils in some locations.

The quartz-rich, nutrient-poor, sandy sediment was derived from the older continental area southwest of the Sydney Basin. A thick blanket, up to 274 m, of this quartz-rich sediment was deposited over the area. The present day surface of the area consists of alluvial soils and sands deposited on the weathered sandstone, with clayey soils developed on the weathered shale.

On separate ridges, a capping of Wianamatta shale makes richer soil, and below the sandstone, assorted shales, mudstones and other sedimentary layers go all the way down to the Permian coal measures, deep below Sydney.

At some time in the past, the whole Sydney area was worn down to a flat plain, close to sea level. The land then rose up, the rivers and streams cut down into the rock. This flow produced a fern-leaf pattern of drainage that cut deeply between the tough sandstone which survived in ridges, high above the water, and in cliffs, where pieces of rock fell away, whenever a shale lens or a softer sandstone bed was eaten out, and a joint in the sandstone was eroded.

In recent times, the sea levels rose again, filling the river valleys. The Blue Mountains are the same sandstone, raised up in the past, except for a few ancient volcanic remnants like Mount Wilson, and Mount Tomah, where the Mount Tomah Gardens are located.

There are some remnants of more recent volcanoes around, but almost everything seen in Sydney is a good old-fashioned sedimentary rock, lying in almost horizontal layers.