Glanrafon Slate Quarry is located to the north of Rhyd-Ddu, at the south end of Llyn Cwellyn. An open working, it is the largest in the Cwm Gwyrfai district but was only a small-scale opeartion up to the late 1870s. It then developed with the coming of the railway, and produced 1725 tons with 92 men in 1882, possibly double that in the years after.
The quarry consisted of one large and several subsidiary workings. As the main working deepened material was raised by powered incline, or possibly a water balance. Pumping was done by flat rods from a remote water wheel, before a drainage tunnel was cut. Material was taken by incline to a large mills area (double mill with central wheel); later a tramway tunnel was cut and several locomotives were used on site. A short, followed by a longer, incline descended to make a triangular junction with the North Wales Narrow Gauge Railway. The quarry closed in about 1915.
The main working is now filled with water. There are spoil tips and derelict buildings, including Glanrafon Slate Quarrymen's Barracks (NPRN 415217) on the site. The quarry is depicted in-use on Ordnance Survey County series mapping and it has been photographed by RCAHMW during aerial reconnaissance.

Sources:
OS County Series 25-inch map: sheet Caernarvonshire XXL.8, editions of 1899, 1900 & 1915.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), p.89-90.

David Leighton, RCAHMW, 4 February 2015