Llechwedd Slate Quarries is a major slate mine that is now worked as opencast. The quarry was started by John Whitehead Greaves in 1846 and rapidly became one of the largest in the district, one of the most efficient in the industry, and a leader in methodology. The Greaves Trimmer of 1856, for example, became the standard machine in the Industry. Highly mechanised mills were established from 1852, on several levels. Chambering went down almost to sea level with 20 miles of tunnels. Water power was used extensively for driving mills, pumping and haulage, both by water balance and direct cranking - there were several powered inclines, mostly underground and several gravity ones on the surface. Steam was introduced in the 1850s for haulage and later for locomotion. From 1891 electrification was extensively used. Direct incline connection to the Ffestiniog Railway was made in 1856.
In 1882 the quarry produced 24723 tons with 553 men, output increasing in the 1890s. Up to 50 tons per man year were claimed, probably an industry record, and representing a 9-1 waste to make ratio as against a local average of 12-1. Although a working quarry, remains linked to its early use are widespread. From 1972 the lower workings have been used as a tourist facility, allowing visitors to enter the original adit of the quarry and a series of underground chambers (Llechwedd Slate Caverns, nprn 40597).

Sources:
D.Gwyn, Welsh Slate: the Archaeology and History of an Industry (RCAHMW 2015).
David Gwyn & Merfyn Williams (1996) `A Guide to the Industrial Archaeology of North West Wales?. Association for Industrial Archaeology.
Ordnance Survey County Series 25-inch map: sheet Merionethshire IV.5, 9, edition of 1888.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), p.132-3.

David Leighton, RCAHMW, 16 February 2015