I've never seen the original, only pics , but it has never seemd to me to have the same feel as the other rock art in the area, I'd be comfortable with the idea its Roman, or at least later than the other stuff in the area. There are modern carvings up there too, don't see why there can't always have been people wanting to create their own, even though the original meaning or purpose is lost. Its human nature to make graffitti!