Loathe though I am to disagree with Big Stan, I don't think it's a cross base. The socket would be more rectangular if that were so.

I'd rather think of it as a prehistoric basin, related to the rock shelter, altered in later times as the site was co-opted by early Cuthbertians, possibly used as a font. And seeing how there's bog all evidence one way or another, I can't see how this theory could be proved false*.

* Or true of course ;)