Cwmorthin Quarry is a slate quarry west of the village of Tanygrisiau, north Wales. Quarrying on the site started in 1810. In 1860 it was connected to the Ffestiniog Railway. In 1900 it was acquired by the nearby Oakeley Quarry and the two were connected underground. In 1970 it closed along with Oakeley. There was small-scale working in the 1980s and 1990s, the mine finally closed in 1997.
Source: Wikipedia