Slate was mined at Croesor Quarry from the 1850s to1878, and from 1895 to 1930. The site lies high up on the steep southern slope of Cwm Croesor, reached from Croesor village by a track rising steadily along the valley side. All quarrying was done in underground chambers, and a policy of backfilling meant that a relatively small amount of waste was tipped on the valley side. Access was by a single adit leading to underground inclines connecting seven working levels, and a single waste tip formed a platform at adit level on which the surface buildings stood. The first mill dates from c1860 and was powered by a 28ft (8.5m) diameter waterwheel. In 1866 an extension on the north side contained 12 or 14 saws, driven by a 39ft (11.9m) diameter wheel beneath floor level. The specatular exit incline was built in 1864 to connect to the Croesor Tramway (NPRN 34933). A second adit was begun towards the lower workings but not completed; the track of an incline below it may have been intended to carry a water balance trolley to work a haulage system in this adit.
The 1895 reopening brought increased mechanisation, including replacement of the waterwheels by a Pelton turbine, and electrification from 1904 with a power station in the valley below (NPRN 33745). Electric locomotives with overhead wires were introduced in 1905. But production declined in the early 20th century, and final closure came in 1930. The underground chambers were used for the storage of explosives until the 1970s, but most surface buildings were cleared in the 1980s and the mine is flooded to adit level. The ruins of the mill and the incline drumhouse. without its drum, survive.
W J Crompton, RCAHMW, 30 July 2009.