slate

slate In popular usage the word ‘slate’ refers to any sort of natural roofing material, including those composed of thin-bedded, flaggy, sandstones and limestones such as the Cotswold Slates of Jurassic age that cover so many buildings in Oxford. However, to a geologist these ‘slates’ are simply flagstones, unmetamorphosed sedimentary rocks that can be split into sheets thin enough to be used for roofing buildings.

True slates do not necessarily split along bedding planes. Instead, most of them split along planes that are inclined at an angle, sometimes a very high angle, to the bedding planes. These planes, referred to as slaty cleavage, are caused by the rearrangement of minerals such as mica, chlorite, and clay within the rock. It is along these planes that the rock splits into the characteristic roofing slates that we know from so many parts of Europe. These rocks were affected by the Caledonian orogeny, in which fine-grained shales composed of mud and volcanic ash were transformed by low-grade metamorphism and pressure into the variously coloured grey, purple, and green patterned slates. The colour banding indicates the original bedding of the rocks. As the intensity of metamorphism increases, so slates are transformed first into phyllites and then into schists.

Harold G. Reading