Slate quarry. View of the Penrhyn slate quarry, in Bethesda, Wales. The terraced effect is to provide access to more of the pit face than would be available if it were one large wall. The terraces are designed to be wide enough for vehicles to negotiate. Two excavators can be seen working the exposed pit face (lower right). The lower of the two is loading a dumper truck with freshly excavated slate. This is one of the largest slate quarries in the world, measuring more than 200 hectares in area, and being over 350 metres deep.