Ta for the info folks. I'll take this as a sorta Gordian knot solution then. If rock art ?=no and diameter>10 and depth>5 then bullaun? = true. Else bullaun? = false. Either way, (stone+hole)*purpose=?????
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?