Hi Dave, it sounds like Callum’s traverse in the slate quarry above Rachub, near Bethesda. ‘Flattening the Curve’ (done during the plague). Page 266 in the new guide - ‘Bethesda Oddities’.
Defo a local’s venue. Think Zippy has done a few new routes around there too.
Went up to have a play on Super collider last year. Sharp pockets, long walk, seepy. Wasn’t smitten enough to return.