Clogwyn y Fuwch Slate Quarry is located on the north side of Mynydd Deulyn, above the Afon Crafnant. Workings are mostly underground and may have begun as early as the eighteenth century. It was operated by William Turner when he came to Wales from the north of England in 1812, before moving to Blaenau Ffestiniog; this could therefore be the prototype for underground workings in Wales. The series of large openings up the hillside and the use of cut-and-cover entrance are redolent of Lake District practice.
The site is unusual with workings on six levels going into a near-vertical scarp of some 400 feet on very steeply sloping ground. Sledges were originally used but later an incline brought material down to road level. Although the product was perishable work continued into the twentieth century with output at about 200-300 tons per annum.
In 1991, remains survived on all six levels. Remains of the sledgeway and incline also surviveed and, at the foot of the workings, the walls of a tiny mill. The quarry is visible on the Ordnance Survey County Series (Caernarvonshire XVIII), dated 1890.

Sources:
David Gwyn & Merfyn Williams (1996) `A Guide to the Industrial Archaeology of North West Wales?. Association for Industrial Archaeology.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), p.69.

David Leighton & Claire Parry, RCAHMW, 22 January 2015.