Long Meg & Her Daughters 20
Image by greywether

I would say I am 99% sure this is modern. Reminds me of the quarry markings on the Chatton Park fort rockart panel http://www.themodernantiquarian.com/post/12207 only not as obvious of course!!

It's too straight and - erm - boring to be prehistoric IMHO

Isn't the Long Meg circle partially restored? Might this strange groove relate to the movement of the stones during restoration? This is purely speculation.

Gotta run. I have a destructive 2 year old to look after.

K x