This remote quarry is located on the upland between Cwm Penmachno and Cwm Teigl, at more than 450m OD. In about 1812 serious commercial exploitation started on an ancient vernacular site (1760s?) some distance to the south of the later main complex. From the 1860s operations were concentrated at what became the main site, and on a substantial scale. It began first by pit working, later underground to the south of the main mill, and then by extensive underground workings on eight levels to the east. Notably, one central steam engine was put to serve all the power needs of the quarry. Designed as a multi-purpose machine, operating the mills, hauling from a shaft and winding several inclines, including the quarry's main exit incline which, unusually, ran uphill from the mill area to reach the contour section of the exit railway. The engine house, built by a local contractor, is still evidently the focus of the site, with its tall square-plan stack. No other Welsh slate quarry was arranged around a central power source in this way, and it was only at Delabole in Cornwall that anything comparable is evident. However, it did not last, going out of use before 1888. Near the mill was built a small village of houses, barracks, a shop and a schoolroom/chapel for quarrymen and their families.
This was a large site with an extensive locomotive powered rail network which involved three counter-balance inclines and one uphaulage incline, a 2 ft (0.6m) gauge affair over 5.6 km, and with its own tramway connection to Blaenau Ffestiniog. However, despite all this output rarely, if ever, exceeded 6000 tons per annum. In 1887 output was 3187 tons from 130 men. By 1898 109 men were employed reduced to 73 in 1937-8. The quarry closed and reopened several times, production finally ceasing in 1953.
Remains are extensive. These include workings, above and below ground (on several levels), inclines, the village buildings, manager's house, engine house with stack and fuel store, and the main mill.

Sources:
D.Gwyn, Welsh Slate: the Archaeology and History of an Industry (RCAHMW 2015), esp.pp.88 (Fig.79), 89, 189 & 192 (Fig.180).
Ordnance Survey County Series 25-inch map: sheet Caernarvonshire XXIX.11, editions of 1889 & 1900.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), pp.138-9.

David Leighton & Toby Driver, RCAHMW, 2 March 2015