Tamala Limestone

The Tamala Limestone stretches in a narrow band more than 1000 km along the W coast of Australia from Shark Bay in the north, past Perth, nearly to Albany. The Tamala is an 'aeolian calcarenite' where the term 'aeolian' refers to the wind-blown origin of the sediment. The term 'calcarenite' indicates that the majority of the sand grains composing the original sediment were calcium carbonate (either calcite or aragonite). These carbonate grains are predominantly shell fragments of molluscs, bryozoans and foraminiferas as well as calcareous algae. Quartz sand grains are present in variable amounts.

The Tamala Limestone originated as sands brought ashore by waves and then blown by the prevailing westerly winds into high dunes stretching along the coast. Evidence for the wind-blown origin can be found in fairly steep crossbeds observed in road cuts and along the sides of hills throughout the Pinnacles area.

The Tamala Limestone is thought to owe some its great extent to the recent Pleistocene glaciation. During the last glacial maximum between 80,000 and 20,000 years ago, enough water was tied up in continental ice sheets that sea level dropped by more than 100 meters. This drop exposed many thousands of square kilometers of the West Australian continental shelf which served as a source for the broken up shell fragments that were blown ashore to form the dunes.

Note the weathering difference between the top and the bottom of this pinnacle which is most pronounced because of the obvious crossbedding in the upper half.