When I was a student I made a film about the decline of the Welsh slate industry. One of the quarries we filmed in was the Dorethea Quarry at Nantlle Valley. Nowadays it is an extraordinary walk where you can see many of the structures that supported wires across the quarry to bring up the slate. There is also the house for the beam engine that pumped water from the quarry. Well worth a visit. By squeezing through a fence you can visit the manager’s house with stables adorned by extraordinary ceramic tiles. 

Information about Dorethea is found on https://www.llechi.cymru/slateareas/dyffrynnantlle