I think location comes into it a bit as well e.g. rock art more likely to be in areas with nice views/high up/ near river or trackway routes?
Damnation and curses and wailing!
That is indeed the case.
Back to the pondering
I'd go along with that NS. Except the Copt Howe one seems to be quite high up .... hhhmmmm
If it's big and it's in Ireland then it's a bullaun. That's all I can say for sure ... bugger!