I wonder... if you say they had markings on them then maybe they were buried becuase they were sacred to hide them from incomers/invading tribes ( in the way that certain hoards have been buried and in modern times nazi treasures hidden in the vatican during the war) the close proximity of the yew tree(either planted by them or already present) may serve as a marker if these people were ever to return, maybe they were defeated and unable to reclaim their sacred stone.
just a thought!
caroline