Australia: The Land Where Time Began

A biography of the Australian continent 

Pinnacle Desert, Western Australia

The Pinncale desert

In this desert, about 180 km north of Perth, and several km inland, there are thousands of pillars, some reaching 5 m high. They are formed of an aeolian calcarenite, the residual Tamala Limestone, being formed by calcareous sand transported by the wind. This unit extends from Shark Bay south to the southernmost point of Western Australia. It also extends in patches towards Albany along the southern coast. The constituent calcareous grains were then cemented together by secondary calcite as seepage from the surface, when it was still buried in the sand, that dissolved the grains in the upper levels, depositing the calcite deeper in the sand column. The sand grains are formed of fragments from the shells of a variety of marine invertebrates, mostly molluscs, bryozoa, foraminifera and calcareous algae. The shells were pulverised by the action of the waves and mixed with quartz sand.

About 500,000 years ago shallow marine seas covered the area. The Tamala Limestone that formed by the hardening of the lime deposited in this sea. About 25,000 years ago the sea receded and dunes accumulated which were composed of sand, shells and calcium carbonate-rich debris. Once vegetation began to consolidate the dunes humic acid accumulated from the biological material in the soil. The resulting acidic seepage water leached calcium carbonate from the upper layers of the dunes which formed a limestone cap rock some distance below the dune surface. Once the calcium carbonate has been leached from the upper levels of the dunes the seepage water began dissolving the cap rock. At first channels were dissolved through the limestone of the caprock at points of weakness like cracks, joints or holes left by decayed roots, and the limestone was redeposited further down in the dunes in stalagmite-like formations. Eventually the caprock was dissolved away resulting in the limestone columns surrounded by leached quartzite sand. 

At some time in the recent past windblown sand covered the consolidated dunes. The vegetation was smothered, then the sand moved on leaving the previously consolidated dune unprotected from the forces of erosion. The dunes were then moved away to expose the limestone columns. 

Sources & Further reading

Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003


Geology of the Northern Perth Basin, Western Australia - a field guide  

Author: M. H. Monroe
Last Updated  17/04/2011


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