Australia: The Land Where Time Began

A biography of the Australian continent 

Green Gully Quarry -  near Keilor

At the Green Gully Site near Keilor, central Victoria, there are outcrops up to 2 m thick of discontinuous silcrete lenses in the banks of the Maribyrnong River that formed in sediments directly beneath the basalts in the Tertiary (Joyce et al., 1983), medium-grained and coarse-grained silcrete being common, with fine-grained and microcrystalline silcrete being less common. Microcrystalline silcrete has been called amorphous, opaline or cherty (Tunn, 1998; Witter & Simmons, 1978).

In the Maribyrnong River Valley there are 10 known silcrete quarries (Tunn, 1998), of which the Green Gully site was the only site that was sampled, the silcrete from this site, that is brown to greyish yellow in colour, has irregular granular fracture surfaces. This silcrete is medium-grained with a microfabric that is grain-supported, and consists predominantly of angular to sub-angular quartz clasts of medium-sand-size, that are moderately sorted, in a microcrystalline quartz matrix (Table 1 & Fig. 3 (A), Source 3), the quartz clasts ranging from very fine-grained, 0.012 mm in diameter, to the occasional very coarse-grained that are up to 3 mm in diameter.

The silcrete of Green Gully has been found by mechanical tests to have poor flaking and edge-holding properties as well as low resharpening qualities. The high fracture toughness indicates it is difficult to knap, and it is indicated by the low compressive strength (350 MPa), low index of stiffness (median 4.6) and low tensile strength (28 MPa) to lack compression-bending stiffness that is necessary to maintain the fracture propagation directional stability, the result being a high frequency of step fracture terminations.

In the Maribyrnong River valley there are 2 lithic assemblages that have been identified (Mulvaney, 1970; Witter & Simons, 1978; Munro, 1998). There are relatively small numbers of artefacts in the lower assemblage, mostly scrapers and notches, micro-blade production, with backed artefacts, large scrapers and notches being emphasised in the upper small tool industry. The frequent selection of microcrystalline and fine-grained silcrete for tool-making, such as backed artefacts that have been delicately retouched, though silcrete varieties are relatively uncommon at the quarries that are close at hand, is a striking feature, according to the authors, of the younger assemblage. The coarser-grained silcrete was used for about 10 % of the retouched implements, as it was less suitable for artefacts and tools that required repeated resharpening as a result of its poor flaking properties.

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