4. Geological Descriptions

The rhyolite: a typically black to dark grey-coloured, fine-grained, acid igneous rock that outcrops above the quarried tuffite exposures in Zone 1; it weathers to a cream to light brown colour depending on the amount of secondary iron alteration available (Fig. 17: block of flow-banded rhyolite on the beach from which flakes had been struck). The thickness of the patina mainly reflects the mineralogy and lithological characteristics of the different volcanic flow units and their weathering histories. The surfaces of worked fragments of rhyolite (and tuffite) from the beach deposit showed good evidence of textures and microstructures clearly revealed by the chemical action of the sea water (Fig. 18: struck flake from beach deposit). These microstructures correspond generally with the micro and macrostructures exhibited by rocks in the exposed quarry face in Zone 1. Some weathered rock surfaces contain eutaxitic textures, possibly linked to horizons with well-developed platey cleavage. The rhyolite rocks are well jointed and banded and shows evidence of pyroclastic flow (Fig. 18). Although probably from the same parent magma source, the tuffites resemble welded ash-fall deposits.

Figure 17: Block of flow-banded rhyolite on the beach from which flakes had been struck
Figure 18: Struck flake from beach deposit

The tuffites: the junction between the tuffites and overlying rhyolite is not clearly exposed on the site. However, small outcrops adjacent to the site (in very difficult terrain) suggest a downward transition from rhyolite to tuffite. Tuffite is exposed in the quarry face, excavated during prehistoric times, in Zone 1. Much evidence was seen to suggest that rhyolite and tuffite boulders and cobbles in the beach deposits had been utilised for tool making. There is evidence further along the hillside and beach to suggest that the finer grained tuffites, exposed at the Zone 3 shore-line working sites, rest on intermediate and coarse chloritic ash tuffs and metasediments.

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