Australia: The Land Where Time Began

A biography of the Australian continent 

Outcrops of pedogenic silcrete occur in a layer in low scarps in the Sturt National Park, western New South Wales, 25 km east of Tibooburra,  that is laterally continuous (Watts, 1978), as well as extensive 'gibber' pavements of closely packed cobbles that are sub-spherical, the silcrete occurring as microcrystalline, medium-sized and fine-grained varieties (Table 1, Doelman et al., 2001).

Though there are many silcrete outcrops in the area, only those with a massive appearance were quarried. Quarries are mostly present at elevated positions on the crests of the escarpments and in the stream gullies. From the Tibooburra area the 2 quarries that were sampled by the authors were the Olive Downs Quarry, of microcrystalline silcrete, and Gorge Quarry, fine-grained and medium-grained silcrete.

Conchoidal fracturing is present in the Olive Downs silcrete, of light brownish-grey to dull red colour, and it is microcrystalline, predominantly consisting of a microcrystalline quartz matrix with angular clasts of mostly silt-sized quartz that are scattered throughout the matrix. Also in the silcrete are occasional layers and patches of coarser quartz clasts, up to 2 mm in diameter (Fig. 2 (B) & Table 1, Source 1) and clay that is partially silicified, iron oxide minerals that are very fine-grained, as well as small cavities that are up to 1 mm in diameter.

The microcrystalline silcrete from Olive Downs has been found to have mechanical properties that are similar to those of Australian flint (Doelman et al., 2001, table 1), in fracture toughness, compressive strength and index of stiffness, as well as in their petrology, in the form of very fine quartz crystals forming an interlocking matrix. According to the authors, the flaking quality of some samples is markedly decreased by the presence of soft clay, iron oxide and organic inclusions that lower to a significant degree the fracture toughness and compressive strength.

For more detailed information and illustrations see Source 1

Sources & Further reading

1. Webb, J.A. & Domanski, M, The Relationship Between Lithology, Flaking Properties & Artefact Manufacture for Australian Silcretes, Archaeometry, Oxford University, Archaeometry, 50, 4 (2008) 555-575

Stone tools