Andelusite crystals (white) in slate (grey). The crystals formed underground by thermal metamorphosis due to an intrusion of granite. The rust-coloured staining is due to the presence of iron pyrites. This is a sample of Skiddaw slate from Roughten Gill, Glendaterra Valley, Cumbria, UK. This slate is among the oldest rocks in Cumbria, dating from the late Cambrian (460-500 million years ago). Slate can provide a useful building and roofing material, but Skiddaw slate has poor and unpredictable cleavage, and is not suitable for roofing. Sample size: around 7 by 4.7 centimetres.
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