Fron Boeth Slate Quarry was opened in 1886 as an extension to Pant Mawr Quarry (NPRN 408873) with which it amalgamated. The 1st edition 25-inch O. S. map, 1889, shows just one adit under construction, accessed by a zig-zag path from Pant Mawr's exit tramway above (NPRN 408876). The mill was constructed on waste from this adit, and is shown on the 2nd edition of 1900, 50 metres long and 14 metres wide. It was driven by a steam engine, and a Lancashire boiler survives in the boiler house at the north-west end of the building. The layout of three saws and three dressing machines occupied only half of the building, so it was probably never fully developed. The 3rd edition of the O. S. map shows the whole site disused.
W J Crompton, RCAHMW, 1 April 2009.