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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