Slate, Cornwall

Cornish slates have been quarried for roofing material for at least 600 years, although they are not as well-known as the roofing slates of North Wales. They were originally deposited as muds on the sea-floor around 400 million years ago. They were later buried, folded and metamorphosed to form slates about 300 million years ago, when south-west England became joined to the rest of Britain.

The fossils in these slates show that the muds were originally deposited in the sea, since, although squashed and deformed, they are clearly sea-shells despite their local name: “Delabole Butterflies”.