Pit with tunnels to associated workings.**

A large pit survives, though many tunnels have collapsed. The mill area contains several ruinous buildings, including the main mill. Extensive rubbish tips.

Cefn Ddu, incorporating Chwarel Fawr and several older pits, dates from the eighteenth century or earlier and became part of the Cilgwyn & Cefn Du Company around 1800. It operated with two levels of locomotive tramways, with tunnels connecting the upper districts and inclines linking to the lower level and the integrated mill. Amalgamation in 1878 with Cambrian and Llanberis Slate Company made the Fridd incline extension and its tramway redundant. By the 1880s, the quarry had four steam engines, two water wheels, and a water turbine, later generating its own electricity; output peaked at five thousand six hundred and forty tons with one hundred and ninety-seven men in 1882, before closing around 1928.