Slate, North Wales

The slates of North Wales have been quarried for at least 500 years and, during the 19th and early 20th centuries, provided most of the roofing slate used in Britain. They were originally formed as deep-water mudstones on an ancient sea-floor around 500 million years ago. These rocks were later uplifted, folded and metamorphosed to form slates about 400 million years ago.

Slate splits easily into thin sheets because of the alignment, or foliation, of tiny mica crystals in the rock. It is also, hard, durable and impermeable (waterproof).

 
The quarry shown above is now a hydro-electric power station.