Hmmm. not to disagree with your actual premise, LS, but not sure about those examples. They look like they could be 'cairn circles' to me - the retaining 'wall' to keep in cairn material.

Like this http://www.themodernantiquarian.com/post/19469 or a 'single' version of this http://www.themodernantiquarian.com/post/9633

love

Moth