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2017-07-29T15:22:01Z

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|description={{|1=Specimen of slate from Ruhumman Quarry, Glenshee, Glen Almond, Perthshireand, Scotland. The Glenshee slate quarries lie between Craiglea and Birnam quarries, the largest of the Logiealmond-Dunkeld group. These quarries exploit the same geological rock formation as the Aberfoyle and Luss quarries further south-west, all forming part of the Highland Border Slate Belt. The specimen is of Precambrian, Dalradian age. British Geological Survey Petrology Collection sample number EMC 1510. True slate is formed from very fine-grained sediments of bedded clay (mud) and silt, probably laid down on a deep ocean floor. Slight differences in the proportions of clay and silt in different beds are reflected in the quality and appearance of the slate-rock into which the clays and silts are eventually transformed, and sediments deposited in different areas or at different geological times show variations allowing one group to be distinguished from another. Slate is formed by intense pressure deep in the Earth's crust which bends the bedding into crumples or folds. As the folds become tighter profound physical changes take place under heat and pressure in the shales. A new set of planes is formed, termed cleavage planes, and the degree of compactness and the strength of the rock is greatly increased. The shale has been transformed into a true slate, the fissility of which depends entirely upon the presence of the cleavage planes. Specimen size: 12x9cm. P519653.}}
|date=2017-07-29 15:59:58
|source=British Geological Survey
|author=British Geological Survey
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