Alexandra Quarry is located on the east side of Moel Tryfan, to the south-east of Rhosgadfan. It was opened in about 1862 and at its peak produced 6000 tons per year employing more than 200 men. The quarry was a substantial open pit working with steam-powered mills etc. and an extensive locomotive worked internal tramroad system. It was apparently worked with Moel Tryfan to the south-west (NPRN 40560) from 1918. It worked only sporadically after the First World War. Although it closed in the late 1930s, when the Moel Tryfan workings broke through it was used as a source by that quarry until the mid-1960s, all reduction taking place in their mill. In its final days all transport was done by lorry. Its development is variously depicted on Ordnance Survey County series maps (Caernarvon. XXI.2 1889, 1900, 1915). A detached portion of the quarry lies to the west (NPRN 400656).
Due to bulk working the site is much disturbed. Apart from the pits there are substantial remains (in 1991) of the mills and other structures including possible Blondin bases. A most notable feature is the tramroad formation.

sources:
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), p.85-6.
Lindsay 1974 (Hist. N Wales Slate Id.), p.310.

David Leighton & J.Wiles, RCAHMW, 12 January 2015