Gallt y Llan slate quarry is a hillside terrace working sited on the very steep north-east facing slope of Clogwyn Mawr, rising from around 110m O.D. to nearly 300m O.D. It opened in about 1811 and operated intermittently over a long period. The ease of working and access to the valley road favoured economic working. But with a high sulphur content the product was poor. In 1882 the quarry produced 90 tons employing three men. Slates from the quarry found favour with the Arts and Crafts movement in the late nineteenth and early twentieth century. In Conwy and Deganwy a number of houses designed by the architect Herbert Luck North are roofed with their distinctive blue and green slates.
Remains include stone-built incline formation, small ruined buildings and tips extending to the south-east of the workings.

Sources:
D.Gwyn, Welsh Slate: the Archaeology and History of an Industry (RCAHMW 2015), p.29 & Fig.19.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), 38.
Recorded during RCAHMW aerial reconnaissance on 7th June 2013.

David Leighton & T. Driver, RCAHMW, 2 February 2015