Slate is found in veins between other types of rock and the angle of dipping of the vein decides the method of extraction.
There are three basic types of slate quarry – underground, open and pit. In underground workings, tunnels are drilled to intercept the vein and then enlarged to remove the slate – pillars being left to hold up the roof. Open quarries follow the vein on the slope of the mountain and often use a system of galleries up to 70 feet high, the greatest exponent of this type of quarrying being Penrhyn at Bethesda. Pit workings are necessary where the vein dips almost vertically and are most common around Nantlle. It was not uncommon to find examples of more than one method of extraction in the same quarry.
Words and photo by kind permission of Dave Sallery. For more information from his tremendous website, please click here.