Where does slate come from?

Where does slate come from?

Slate is a metamorphic rock. The slate in North Wales began as mud that settled at the bottom of the sea some 500 million years ago. As new layers settled on top, the lower ones eventually get compressed to form shale. Over millions of years the tectonic plates shifted and wrinkled to form the Welsh mountains and the shale became heated and compressed. This caused the minerals in the shale to partially melt and reform as thin sheets of mica and quartz. Because of the particular way the crystals grow, the layers of slate are at right angles to the original shale layers.