Bryn Celli Ddu 12
Image by thesweetcheat

have you looked into where the nearest source of fossillised wood has been found to the site?
Is it unique to the area or is there a wide girth of finds?
PeteG

There are no fossil tree sites known in the area. I asked an old lecturer of mine who thought it could be a schist or other metamorphic rock. The problem remains though, that if it's rock worked to a cylindrical shape, why did they do such a crap job? You couldn't accidentally create the irregularities of wood grain.