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.