Cwmorthin Slate Quarry was worked from c1810 to 1939, leaving a complex of surface remains overlying a still more complex series of underground workings, mainly in the Back and Old Veins of the Ffestiniog Series. The earliest working, from c1810, was abandoned by 1830, but was recommenced in 1840. The main development, by the Cwm-Orthin Slate Co from 1861, saw the mechanisation of the quarry with three slate mills, Lake Mill (NPRN 408410), Cross Mill (NPRN 408419) and Lower Mills (NPRN 408420). The tramway connection to the Ffestiniog Railway (NPRN 400887and 408421) was also developed in this period. Taken over by the New Cwmorthin Company 1876 to 1888, work continued with a major underground collapse in 1884 closing half the workings. The New Welsh Slate Company, 1889-1900, developed five underground floors below lake level, but continuing boundary disputes with the Oakeley Quarries Company brought major work to a close. The quarry was taken over by Oakeley in 1900 and abandoned until the 1920s. In 1925 the Lower Mills and some of the tramways were restored. In the early 1930s the flooded levels were drained and worked from the Oakeley side of the hill, but work ceased in 1939. Post-war attempts to strip off the surface rock failed, and the quarry was sold in 1970. Since then sporadic working of the waste tips has destroyed many surface features.
W J Crompton, RCAHMW, 5 November 2008.