There are a number of similar motifs on the north york moors, I'm afraid I can point to any on the WWW but Chappell and Brown document them in Prehistoric Rock Art of the NYM.
Yes, I'm fairly sure they are artificial and considering the work involved I'd guess they aren't simply defacement (they are also separate from
the rings and are parallel to the gutter like lines nearby).
We couldn't tell much while there because of their position and moss covering but I'm sending some pics to Blaze for her opinion.