Chwarel Cefn Du, slate quarry originated in 1802 when some little early diggings such as Cerrig y Pigia, Chwarel Huw Dafydd, Chwarel Morgan, Chwarel Owen, Chwarel y Maen etc, were amalgamated into the Cilgwyn & Cefn-du Co. The site consisted of several pits and a locomotive tramway system. By 1879 there were said to be four steam engines, two water wheels and a water turbine on the site. Chwarel Cefn Du closed in 1928.
The remains consist of pits and tunnels which are much collapsed; there are several ruined buildings in the mills area including the remnants of a large mill, partly built of an unusual green country rock. The tramway formation to the Ffridd incline is still visible, near which are two barracks.
