Glynrhonwy Uchaf Slate Quarry, located on the north side of Llanberis, was a mid-nineteenth century development. It was essentially two pits operated in tandem, with mills and a complex, partly locomotive-worked internal rail network. Spectacularly located tracks include some cantilevered along rock faces. In 1882 it produced 2181 tons with 90 men. It was originally connected to the Fridd incline (NPRN 505740), but after amalgamation everything went out through Glynrhonwy Isaf (420629), via a tunnel. The quarry closed in the 1930s.
Impressive pits survive (in 1991) along with rock-cut tramway formations, some tunnelling, inclines and several buildings.

Sources:
Ordnance Survey County series 25-inch map: sheet Caernarvon. XVI.8, editions of 1889, 1900 & 1914.
RCAHMW air photos: AP94-CS 1022 & 1027; AP945131/41-2, 49-50 & 52.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), p.39.

David Leighton, RCAHMW, 4 February 2015