Greywacke

Greywacke comes in grey-and-black layers. Each of the tough, grey layers is a single underwater avalanche of mud and sludge, sliding down off 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.