The mine of Cwm Orthin is situated in the valley of that name just to the west of the town called Blaenau Ffestiniog above the village of Tan y Grisiau. Cwm Orthin has substantial underground workings and it had a notorious reputation for bad working conditions so much so it was nick named 'The Slaughter House'. The mine was first opened in 1810 and was originally worked as a quarry. In 1860 it was connected to the Ffestiniog railway and then extensive underground development commenced. Chambering originally was all above the water table and material was all brought out at lake level. As progression moved downwards pumping and haulage was needed this was originally steam. There was however a serious collapse in 1884 this caused a dramatic decrease in production. In 1900 it was incorporated in Oakeley Quarry all surfaces workings and the use of the Cwm Orthin tramway were abandoned. When Cwm Orthin was taken over by Oakeley the mines were joined by the means of a new tunnel called 'level french' this joined Cwm Orthin at level E. This allowed all the drainage and power to be done through Oakeley Quarry.A big re- development project occurred around 1925 and several other buildings were built but all to no avail. Underground work however continued until the mine was closed along with Oakeley in 1970. It was worked however in the 1980's on a very small scale in one chamber; transport underground was by the use of a Land Rover. It finally closed in 1997 after some untopping work had taken place. Cwm Orthin quarry had in its day up to three mills, Lake Mill, Cross Mill and London/Lower mill.
