I'd noticed this on the UCET forum a while ago, but had not got round to having a look until today.http://www.ucet.org.uk/~ucet9504/index.php/forum/21-knowledge-bank/870-menai-straits-adit
The footpath in question is next to the (self) grandly named Rhianfa Château. The spring tide was fully in, so a bit of crag hopping sport was called for. I was hoping to traverse round and exit via the Gazelle pub. I came across Ian's adit quite soon. It was fairly low but had and almost geometrical arched roof.A bit further round there was another adit, maybe 20 feet long. Further on again there was a trench quarried out parallel to the Straits. This section had obvious and profuse quartz veining. I can't say I noticed any mineralisation though. Further on things got tricky - a private establishment with vertical walls dropping into the briney putting a tin hat on my traversing. So it was a reverse, and slightly higher traverse back to Plas Rhianfa due to the incoming tide. There was also a bricked up area at some point, but I don't know if that was adit related or mystery pipe related, presumably bringing dubious matter from someone's property. Ian gave it a 1/100, but then again he may have visited during low tide!