Limestone cobbles on an eroded shale platform. The cobbles (light grey) are blocks that have eroded away from a limestone platform (at top), which forms a layer on top of the shales (dark grey). The limestone and shale strata alternate. Limestone is a sedimentary rock formed from calcium minerals such as calcium carbonate. Shales contain clay minerals and quartz. Oil can be obtained from shales by methods such as fracking. Photographed in Bridgewater Bay, Somerset, UK.