Saturday 6 August 2022

Silbury Hill Bluestone Descriptions by Rob Ixer

Three preselite-spotted dolerite samples from the summit of Silbury Hill and a single flake from the Alexander Keiller Museum. They were from sub-soil and topsoil excavated material; all four appeared to have been flaked.


photo - Rosie Ixer

The three struck flakes of preselite-spotted dolerite from the Preseli Hills vary slightly in the amount and size and density of their characteristic pale spotting. In two very similar samples (1.6 and 2.2g) pale pinkish grey spots 0.5 to 0.7mm in diameter are present in hand specimen and in the third, the largest sample, similar size spots are visible in polished thin section but not in hand specimen.

Petrographically the third preselite is a highly altered ophitic dolerite now comprising clinopyroxene-altered plagioclase-altered iron titanium oxide minerals with abundant secondary chlorite and epidote. It has millimetre diameter, metamorphic alteration spots that carry characteristic chrome-rich spinels.

The transmitted and reflected light petrography of this sample is almost indistinguishable from that belonging to preselites from Carn Menyn and members of the SH33group.

The size, shape and macroscopical lithological characteristics of the 1968–70 spotted dolerite flake from the Alexander Keiller Museum suggest that it belongs with the three more recently excavated, spotted dolerite flakes found on Silbury Hill. The metamorphic spots are slightly larger but this is not of significance.

Although none of the flakes could be fitted together all four could have been struck from a larger artefact. They do appear to belong together.


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