Slate Slate Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. It is the finest grained foliated metamorphic rock. Foliation may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression. The foliation in slate is called "slaty cleavage". It is caused by strong compression causing fine grained clay flakes to regrow in planes perpendicular to the compression. When expertly "cut" by striking parallel to the foliation, with a specialized tool in the quarry, many slates
Aberllefenni Slate Quarry slates. Foel Grochan mine consists of eight near-horizontal tunnels at approximately vertical separation. These were bored into the valley side just to the north of the near-vertical narrow vein. Each tunnel connected to a large chamber from which the rock was extracted. These chambers ranged from to in length with of rock left between the bottom of one chamber and the top of the next lower chamber. As more slate was extracted, several of the upper chambers were joined vertically to form an extremely large cavern known as Twll Golau which is open at the top. From the mid-1850s the