The oldest rocks in the Taroona area are exposed at the Grange Quarry, Cartwright Reserve.
The succession of fine sandstones and mudstones here were deposited in icy polar-waters around 280-260 million years ago (early Permian Period) when Tasmania was still part of Gondwana (ie attached to Antarctica and close to the South Pole).
The rocks are quite fossiliferous – with mainly now-extinct Brachiopods (bivalve shells) and Bryozoa (colonies of marine invertebrates (that look like, but are not corals). The sedimentary sequence has been hardened (‘cooked’) by metamorphism when igneous dolerite intruded nearby, 200-160 million of years ago (Jurassic Period). Adjacent to the quarry a major fault (Taroona Fault) follows the Main Road south to Jenkins St, then downhill to the southern end of Hinsby Beach and then defines the edge of the Alum Cliffs. The fault was created during mega-continental fracturing during early Tertiary time.
More details of the geology are given by Simon Stephens at the end of Chapter One of the Taroona Book ⇒.
p.12 - p.20 Geology by Simon Stephens.pdf
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