text: Cobblestones on a flat sand beach. Cobble stones are polished round and smooth by years of wave energy. They are alternately exposed and covered by sand depending on the tides, waves and seasons of the year. Cobblestones are common on the beaches of southern California, contained in the sandstone bluffs along the beach and released onto the beach as the bluffs erode. Carlsbad, USA, natural history stock photograph, photo id 19828