These sandy beaches have been winnowed out of the surrounding country rock - which is a melange of sandstone and conglomerates (with many mega-boulders of dolerite). The rock accumulated during times of tectonic instability 60-40 million years ago (early Tertiary Period) when the region was repeatedly shattered by earthquakes associated with the separation of Tasmania from the Antarctic tectonic plate.

The Alum Cliffs are the defining feature of Hinsby Beach. ‘Alum’ refers to the powdery white clay that accumulates on weathered surfaces there. The bulk of the cliffs are sandstones and mudstones of Permian age (around 250 Mya), (similar to the rocks exposed at Grange Quarry Quarry). Jurassic dolerite forms the western edge of the cliffs : see note below by local geologist Simon Stephens (1988 : ‘Taroona, 1808-1986 : farm lands to a garden suburb / written by past and present residents of Taroona’.

p.12 - p.20 Geology by Simon Stephens.pdf

Other geologists have also commented on the Alum Cliffs…

1. Patricia Roberts-Thompson (http://taroona.tas.au/assets/document/1354363720-a_walk_along_alum_cliffs.pdf) notes that the first recorded reference to the Alum Cliffs was in 1847. These rocks, Permian mudstone (250 million years old) contain iron pyrites and, as the rock weathers, the pyrite oxidises and produces sulphuric acid which reacts with the limey clay to produce alum. There is no evidence that the alum has been extracted from our cliffs for commercial purposes.
2. Simon Stephen (pers comm) commented that the mudstone on the Alum Cliffs contains much sulphur so that when struck a strong smell is emitted. “Much of the sulphur manifests itself as a white encrustation on the sheltered areas of the cliffs. It has a distinct bitter taste…”
3. David Leaman’s 1999 book ‘Walk into History’ states:

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