High Quality Content by WIKIPEDIA articles! The slate industry in Wales began during the Roman period when slate was used to roof the fort at Segontium, now Caernarfon. The slate industry grew slowly until the early 18th century, then expanded rapidly until the late 19th century, at which time the most important slate producing areas were in northwest Wales, including the Penrhyn Quarry near Bethesda, the Dinorwic Quarry near Llanberis, the Nantlle Valley quarries, and Blaenau Ffestiniog, where the slate was mined rather than quarried. Penrhyn and Dinorwig were the two largest slate quarries in the world, and the Oakeley mine at Blaenau Ffestiniog was the largest slate mine in the world. Slate is mainly used for roofing, but is also produced as thicker slab for a variety of uses including flooring, worktops and headstones.