Strata of limestone and shale on a road cutting. Strata (horizontal bands) are parallel layers of different coloured or differently structured rock or soil. Individual layers may vary from millimetres to a kilometre or more. Limestone (thick bands) and shale (thinner bands) are both sedimentary rocks, formed by the compaction of water deposits. Limestone is composed mainly of the mineral calcite, usually from the shells of marine organisms. Shale is the most common form of sedimentary rock and is a fine-grained rock composed of clay minerals or muds. Photographed near Ales, south of France.
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