In reply to "I wonder if there are similar stones on The Cheesewring":-
http://www.themodernantiquarian.com/post/56917
http://www.themodernantiquarian.com/post/56918
http://www.themodernantiquarian.com/post/56919
There are similar stones on the tops of many Tors on Dartmoor, Kes Tor having some fine examples. I was always lead to believe they were formed naturally by wind , water moving small stones over many years.
Peace , Lubin