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The slate industry in Wales began during the Roman period when slate was used to roof the fort at Segontium, modern Caernarfon. The slate industry grew slowly until the early 18th century, from when it expanded rapidly and reached its peak output in the late 19th century, at which time the most important slate producing areas were in north-west Wales. These included the Penrhyn Quarry near Bethesda, the Dinorwig 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. The Great Depression and the Second World War led to the closure of many smaller quarries, and competition from other roofing materials, particularly tiles, resulted in the closure of most of the larger quarries in the 1960s and 1970s. Slate production continues on a much reduced scale.