Sedimentary rock, one of three main rock classes of the earth's crust; the others are igneous rock and metamorphic rock. Sedimentary rocks consist of weathered fragments of rock transported usually by water and deposited in distinct strata. They may also be of organic origin, as in coal and some organic limestone, or they may be formed by chemical processes, as in the evaporites. About three-quarters of the earth's land area and most of the ocean floor are covered by sedimentary rock. Most common are shale, sandstone, and limestone. Sedimentary rocks frequently contain fossils as well as most of the earth's mineral resources.