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Greywacke comes in grey-and-black layers. Each of the tough, grey layers is a single avalanche of mud and sludge, sliding down the continental shelf at 50mph, to settle on the seabed in the course of a couple of minutes. Each of the thinner, darker, softer layers between is formed of fine silt drifting to the sea bed over tens of thousands of years. Girvan foreshore, Ayrshire

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