Slate is derived from mudstone and other fine-grained deposits, which have been metamorphosed by high pressure and temperature associated with shortening of the earth’s crust; the same forces which formed mountains. It is for this reason that slate deposits are generally located in mountainous regions. In Britain slate is located in the mountains of Wales, Cumbria and the Highlands of Scotland. Slate is also found in Cornwall, although not a mountainous area, it did experience crustal shortening associated with the Hercynian orogeny. Slates from all of these areas are still in service on British roofs today, in spite of the fact that production has been declining since its peak at the end of the 19th century. British roofing slates are still produced in Wales, Cumbria and Cornwall, although no new Scottish slates have been produced since the 1960s.
Scottish slate is still found on the roofs of traditional buildings in Scotland over 60 years after the last of the Scottish quarries ceased to operate; a testimony to the quality of the material. Not all Scottish slate is the same, different varieties were produced in different parts of the country. The main types of Scottish slate, still found on roofs today, are (1) Ballachulish from Argyll, (2) Easdale and the adjacent slate islands also in Argyll, (3) Highland Boundary from a series of quarries stretching from Arran in the west to Dunkeld in the east and (3) Macduff slate from Aberdeenshire. For more information on Scottish slate see the history of the Scottish slate industry and the geology of Scottish slate