The capstone with one of the many strange little holes that appear all over the Quoit.
Image credit: A R Cane
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I think these may have been drilled to take gunpowder, which was then set off like a firecracker. Before TV was invented, like.
Ahaa! That fits in with the story of the farmer then who tried to destroy it and his attempt to build a cow shed on the same site, hence the five pillars next door. And we think we live in exciting times.......
No, they're decoration, by the original builders. The hole will not be truly round, as it would be if it had been drilled recently.