The slate quarry at Tyllau Duon, Llanelian is a spectacular series of cliff-face workings on the north-east coast of Anglesey, described as 'Old Quarr'y' when the Ordnance Survey first-edition 25-inch map was published in 1889.
Remains include small working faces, some buildings and traces of an access track. 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.