Oyster beds in cliff. Prehistoric remains of oysters in cliffs formed from sedimentary rock. Such rock forms by the deposition of successive layers of water-borne sediment that are later compressed into rock. Changes in sea level and movement of the Earth's crust can raise the rock to where the strata and additional deposits, such as the oyster shells present here, are revealed by erosion. Photographed at at Playa del Puerco, Conild de la Frontera, Cadiz Province, Andalucia, Spain.