Slate is a fine-grained metamorphic rock of great structural integrity and beauty. Its nearly perfect natural cleavage allows it to be split into smooth sheets. Slate’s strength, insolubility and imperviousness makes it one of the most durable and versatile materials in the stone industry. About 300 to 500 million years ago, slates were layers of black mud on the bottom of shallow seas. As epochs passed, the mud hardened by the process of compaction and cementation. At the same time the sea basins sank, allowing vast amount of new sediments to settle upon the hardening mud.
After some 2 to 400,000,000 years the slates-to-be were firm shales that lay beneath younger formations 5 to 6 miles thick.As the Paleozoic Era came to an end, the continents were squeezed by rocks beneath the oceans. They pushed the hardened sea muds upwards, shoving and crumpling them into huge, high folds.
When the shales were squeezed into sharp folds, pressure became so great that they raised nearly 6 miles of rock above them crumpling the top-most shale into mountains. The intense heat and pressure generated caused many particles to recrystallize and flatten, often at right angles to direction of compression. This gave the rock a "grain" called "slate cleavage" that allows it to be split easily along one plane but not another.