In 1861 the Glan-y-Pwll Slate & Slab Company Ltd. was incorporated to work the quarry. In 1867 the incline was constructed to connect to the Ffestiniog Railway. The quarry closed in 1870, with the company going into liquidation two years later. After this failure the owning company was acquired by the owners of the neighbouring Oakeley quarry which was mining the north slopes of Allt-fawr. After the amalgamation, the Nyth-y-Gigfran and Oakeley workings were joined together underground and the Nyth-y-Gigfran chambers were worked through to Oakeley. Thus Nyth-y-Gigfran became part of the largest underground slate mine in the world, and was worked until Oakeley closed in 1969.