Australia: The Land Where Time Began
Silcretes, strongly indurated siliceous material, that form as a result of low-temperature silicification of weathered bedrock, regolith and/or sediments that are unconsolidated, at or near the surface. Silcretes are often outcrop prominently as low cliff lines, as they are very resistant to weathering. Silcretes are comparatively simple materials, both chemically and mineralogically, mostly containing more than 95 % silica in some samples, and are predominantly composed of quartz, 85 % silica being the arbitrary lower limit for rocks classified as Silcretes (Summerfield, 1983).
Other rock types have sometimes been interpreted as silcrete in the archaeological literature, such as flint or chert, that are of marine or replacement origin, metamorphic quartzite and silicified sandstone that formed deeper than the near-surface layers (e.g. Sullivan & Simmons, 1979). Careful examination of the field evidence associated with a particular sample is required to distinguish silcrete from other siliceous rocks, silcrete that is in situ always occurs as layers or blocks near the surface. In Australia and America some artefacts made from siliceous material have been re-examined and some of those that were originally classified as 'silicified sediment', 'orthoquartzite' or 'quartzite' were identified as silcrete (Finlayson & Webb).
Silcrete is composed of detrital material, inherited from the host material, that is dominated by quartz and minor amounts of other heavy minerals, and secondary silica that was deposited during the formation of the silcrete, that is dominated by quartz usually with minor chalcedony and/or opal (Nash and Ullyott).
The grain size of the host material, from mud to gravel, is largely reflected in the micromorphology of the silcrete that is very variable. 4 silcrete types have been recognised (Summerfield, 1983) based on microfabric.
Antarctica is the only continent where silcrete has not been found, and Australia is the continent on which it has the widest spread, though it is common in Africa and Europe, especially England and France (Nash & Ullyott).
Australian silcrete distribution
Distributed widely across the continent of Australia (Stephens, 1971) silcretes have been found to occur in 2 distinct geographical associations, inland and eastern (Webb & Golding, 1998). Occurring extensively across arid regions of central Australia (Wopfner, 1978; Webb & Golding, 1998), inland silcretes have been subdivided into 2 types, pedogenic and groundwater (Nash & Ullyott).
Pedogenic silcretes are usually found as low cliffs lines, outcropping around the edges of mesas and cuestas in surface or near-surface layers 0.5-2.0 m thick, characterised by irregular bases that merge with the underlying sediments, comprising nodular, columnar and/or massive layers resulting from rainwater percolating through the soil profile.
Groundwater silcretes, less common than the pedogenic type, are massive, with sharp upper and lower boundaries, lacking such pedogenic features as columns and nodules, generally being found topographically lower in the landscape, the authors suggesting they probably formed by silicification at the water table.
In the more humid parts of Australia east of the Great Dividing Range eastern silcretes occur from Tasmania to north Queensland (Young, 1985; Webb et al., 1994). As with the inland groundwater silcretes, they lack the features of pedogenic silcretes, generally displaying a close spatial relationship to basalts, hence the term 'sub-basaltic' silcretes, and have been suggested to be genetically related to basalts (Webb & Golding, 1998). The availability of silcrete over wide areas of Australia is reflected in the wide distribution of its occurrence in the archaeological lithic assemblages over much of Australia (White & O'Connell, 1982; Mulvaney & Kamminga, 1999). Large areas of the western New South Wales plains are covered with 'gibber', silcrete gravels, with the result that is probably the most common material used for flaked artefacts (Allen, 1998; Hiscock & Allen, 2000; Doelman et al., 2001; Shiner et al., 2005; Holdaway et al., 2006).
Eastern association (sub basaltic) silcretes
Mechanical properties and microstructure
The authors found when they compared the mechanical properties of the samples in their study with the microstructure of the samples they found a close relationship between the amount of matrix and the compressive strength (Fig. 4 (A), Source 1), especially for silcrete with a microcrystalline matrix where the correlation was strong, r2 = 0.96. Where there are few impurities in microcrystalline silcrete its compressive strength is high as a result of its vert strong quartzose mineralogy and a very small grain size that is uniform, fine-grained material are stronger than coarse-grained lithologies with the same composition, as they fracture predominantly along grain boundaries, fracturing in the coarse-grained material is predominantly transgranular (Davidage, 1979; Mardon et al., 1990). The microcrystalline quartz matrix effectively blunts any cracks developing in larger quartz clasts. relatively large particles of weaker non-silica minerals, mostly clay and iron oxide, lower the compressive strength (fig. 4 (A), Source 1).
For medium-grained and finer-grained silcretes moderate negative correlations were found between average grain size and compressive strength, except for samples with high impurities content (Fig. 4 (B), Source 1), as expected from the general relationship between grain size and strength. The authors suggest the correlation is probably weakened by the large variations that occur between the percentage of matrix and cement (Fig. 4 (A), Source 1).
The Kenniff Cave silcrete has a relatively weaker compressive strength, being among the samples that have cement instead of matrix. It has been found from experimental studies on granular lithologies that contain little matrix or cement indicate that most tensile cracks are initiated near the boundaries of larger grains (Gallagher et al., 1974; Zhang et al., 1990; Malan & Napier, 1995). As a result a large contact area between adjacent particles lowers the compressive strength markedly. In the Kenniff Cave silcretes the dominance of syntaxial overgrowth cement results in most clasts being effectively in contact, hence the relatively low compressive strength. Any increase in strength that results from microcracks being filled and point defects in quartz clasts by syntaxial overgrowth cement is overridden by this effect (Joesten, 1991; Goldstein & Rossi, 2002).
No relationship has been found between fracture toughness and matrix percentage or grain size in the silcretes sampled. It had been found previously that an increased grain size in rocks of similar mineralogical composition resulted in a decreased fracture toughness where grain sizes are smaller than 0.2 mm (Huang & Wang, 1985; Whittaker et al., 1992; Domanski & Webb, 2000).
The authors suggest that as this correlation is not present in the samples they tested it is probable that it was overridden by the variability of microstructure in sorting and microcrystalline quartz matrix percentage, chalcedonic cement and/or voids. They suggest that crack branching may be caused by microstructural heterogeneities, the energy for fracture propagation being dissipated (Atkinson & Meredith, 1987).
According to the authors, the importance of silcrete compressive strength in the formation of flakes is highlighted by the present study. Silcretes with a high index of stiffness and high compressive strength, >500 MPa, that is comparable to those of obsidian and flint (Domanski et al., 1994, Table 4), are suitable for systematic blade production. Microstructure has a large influence in determining the compressive strength of the different types of silcrete, hence compressive strength is positively correlated with the percentage of microcrystalline matrix and correlates negatively with the average grain size. As a result a good indication of the flaking properties of a sample can be obtained by the use of a hand lens.
Patterns of raw material selection, as well as stone tool manufacture, maintenance and discard, and effectively the stone tool assemblage composition, are strongly influenced by the mechanical properties of the different types of silcrete. Microcrystalline silcrete from Olive Downs has mechanical properties that are comparable to those of Australian flint, which reflects the petrological similarities of the 2 lithologies, especially the groundmass of quartz crystals that are very fine. A high compression-bending stiffness is given to microcrystalline silcrete by the high values of compression and tensile strength, keeping the fracture direction stable and preventing step fracture terminations, hence enabling the systematic manufacture of blades that are long and regular, micro-blades and the detachment of small flakes when stone tools are retouched. Substantial edge holding propertied also result from the high strength for working soft material, such as meat and skin, and hard organic material such as wood and bone. For the more curated tools, such as tula adzes, requiring a greater degree of sharpening and retouch, selection of high quality microcrystalline silcrete is evident.
The mechanical properties of the fine-grained silcretes from Kenniff Cave are comparable to those of microcrystalline silcrete and Australian flint, making them suitable for the manufacture of blades and blade-based implements requiring fine retouching. An increased susceptibility of fine-grained silcrete to edge fracturing compared to flint, as a result of its lower tensile strength, make it less suitable for the manufacture of tools for working with hard organic materials such as wood, e.g., tula and burren adzes.
Silcrete from quarries such as at Lake Mungo, Green Gully and Gorge Quarry is medium-grained with poor flaking properties, as a result of low compressive and tensile strength, hence the medium-grained silcrete was often used in the manufacture of flakes, as it was much less suitable for the manufacture of blades and retouched tools.
According to the authors, the mechanical properties of silcrete strongly influenced the choice of a silcrete lithology for the manufacture of a particular type of artefact, though other factors, such as source proximity and availability of material was also involved in the choice.
The proliferation of tula adzes, backed artefacts and points, that were associated with retouching by pressure and delicate percussion required the use of high quality silcrete, microcrystalline or fine-grained, for these tools that were more curated (Flenniken & White, 1985; Mulvaney & Kamminga, 1999). Greater selectivity and an increasing emphasis on extending use-life of stone tools characterised the procurement of raw material. The change in technology that occurred at some sites in the mid-Holocene was accompanied by a change in the predominant artefact lithology, though at sites where high-quality silcrete was already being used for flakes and scrapers, a change in lithology was not required for the production of the more curated tools.
The authors suggest that in the selection of raw material, the manufacture of artefacts, maintenance and discard, material determinism clearly played a major role, and they suggest material determinism can explain much of the variability in silcrete artefact morphology, as well as in the composition of assemblages, other components of variability in artefact morphology and the composition of assemblages are subordinate to the raw material's physical and mechanical properties.
It other parts of the world the primacy of the raw material's nature in the manufacture of tools has also been noted (e.g. Ambrose & Lorenz, 1990; Ambrose, 2002; Byrne, 2004), and an editorial introduction (Speth, 1977) 'the worker of siliceous rock ... is bound within a mechanical straitjacket because the resistive nature of the raw material itself imposes severe limitation on the final form a piece may take and and on the techniques that may be used to achieve it'.
For more detailed information and illustrations see Source 1
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