The slate quarry at Tyllau Duon, Llanelian is a spectacular series of cliff-face workings on the north-east coast of Anglesey. The quarry was already portrayed as 'Old Quarry' when the Ordnance Survey first-edition 25-inch map was published in 1889. Visible remains include small working faces, some buildings and traces of an access track. But the most notable feature is a flight of steps cut into the cliff to give access from the sea. There is some indication that underground working was attempted.

Sources:
OS County Series mappinf: sheet Anglesey III.8.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), p.25.

RCAHMW, 12 February 2015