The Glynrhonwy Isaf Slate Quarry is located just to the north of Llanberis. An early eighteenth-century working, it was extensively developed in the 1870s. It consisted of one large and several subsidiary pits. A pioneer of internal railways, the quarry operated an extensive locomotive worked rail system on several levels. Self draining, with all downhill working and the facility of dumping waste in nearby Llyn Padarn made this an efficient unit with output of 40 tons per man year achieved. Product was carted to Caernarfon until 1869 when an incline connection was made to a loading point on the main line railway. At one time there appears to have been a link to the Ffridd incline (NPRN 505740) for reasons that remain unclear. This was the largest quarry in the immediate area and produced 1789 tons with 70 men in 1883 though later outputs were greater with 273 men employed in 1898. The payroll had dwindled to just two by the late 1930s when the quarry closed.
Much of the tramway nework can still be traced, including the run via a combined tram and drainage tunnel under the road (now blocked) to a dump in Llyn Padarn, the tipping area now forming a country park. Otherwise, bulk re-working and re-use has disturbed most of the site. The Air Ministry was in occupation from 1940 to 1961 and installed a standard gauge rail network. Most of the buildings are modern, dating from wartime factory use.
Source: A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), p.39.

David Leighton, RCAHMW, 4 February 2015