Above Tan-y-grisiau is Cwmorthin valley where there are extensive slate quarry workings. The entrance to the valley is essentially a step feature - not quite a 'hanging valley' over a hard lip of rhyolite. The larger glacier that occupied the Ffestiniog Valley was joined by the Cwmorthin glacier and the rhyolite is smoothed through heavy glaciation. PHOTO: The view looks up the Cwmorthin valley, the entrance road being on the left of the image.