The center for the Welsh slate-mining industry is Blaenau Ffestiniog, surrounded by massive, smooth and gleaming waste-heaps left by the slate quarries. Slate, the mineral which dominates this environment, is everywhere in evidence and in every conceivable shade, from the brightest to the darkest silver-gray, providing a cladding for the little
terraced houses, built out of rough stone and granite, which line the main street, and even visible in the enclosures reserved for sheep grazing.
This long seam of Cambrian slate extends from the Carnedds on the northwest flank of the Snowdon massif as far as Nantlle and was formed during the Palaeozoic period about 600 million years ago. Whereas at Penrhyn and Dinorwc it was possible to mine the slate using open-cast methods, the more unfavorable diagonal seam at Blaenau required tunnels and shafts. Large-scale slate quarrying was begun at the beginning of the 19th century by the Liverpool mining speculator Samuel Holland at the Oakeley Slate Quarries, his example being immediately followed by other, mainly English entrepreneurs. The layers of slate, after preliminary working, were transported for export through the Vale of Ffestiniog on a narrow-gauge railroad to the nearby harbor of Porthmadog. From there the much prized weather-resistant building stone was shipped all over the world. At the height of the slate boom, around the turn of the century, the population of Blaenau almost reached 12,000 and its 18 mines, which employed some 4,000 men - almost a quarter of the slate-workers in North Wales - produced some 140,000 tons each year. Today there are just two open-cast mines left which only employ 150 men.