MICHAEL SZOENYI / SCIENCE PHOTO LIBRARY MICHAEL SZOENYI / SCIENCE PHOTO LIBRARY
Rock strata in a cliff in a quarry. The layers (strata) show how this sedimentary rock formed by the deposition of sediments under an ancient sea or lake. Over thousands and millions of years, the sediments compacted into rock under their own weight, and were then uplifted during movements of the Earth's crust. The composition and structure of such rocks can change as they are subjected to pressure and heat, and the layers can be distorted and warped, as at lower left. These rocks are part of the Jura mountains to the north of the Alps, and are being deformed by the forces building the mountains. The rock strata can be exposed by quarrying and by erosion. Photographed near the River Birse, Switzerland.
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