Earlier ...
FourWinds wrote:
There's a big old natural lump of rock that you pass on the way to the Holyhead village. I wonder if it's that. It's been so long since I passed it that I can't quite remember what it looks like. I do remember that it's near to a t-junction.
After seeing it on the map it is the one I was thinking of. I have always considered it to be natural, but only because it ain't on the OS. I've never stopped to look at it. 