Hello Hob... yes that white stone is conspicuously different than the rest of the fort's perimeter rubble embankment. We should be taking a look again on Oct 2nd, but given there are no geologists in the party!

The grid ref you gave for the well / stone places it on the A55 on Anglesey! ;o) The ref should be SH751738 http://www.megalithic.co.uk/article.php?sid=8255 ; this area is going to be well visited by me over the coming year, so I will resolve to investigate.

Cheers

Tim