A now sparsely populated glacial valley running between Rhyd Ddu and the coast.

Roofing slates excavated at the nearby Roman fort of Segontium, originated from the valley and by the eighteenth century, Cilgwyn quarry was well-known for its brightly red slates.Unlike the larger enterprises run in the area, the slate quarries situated in Nantlle were run independently and on a much smaller scale, but the valley was still one of the world-leading producers of slate in the nineteenth century. The open cast mining and innovative transportation systems they used were of great interest to mining engineers from France and Germany who repeatedly visited the quarries from the 1850s up to the 1880s and took detailed notes to encourage the installation of similarly efficient systems in their home nations, an early form of industrial espionage!