Sedimentary rocks are derived from the mechanical or chemical breakdown of igneous, metamorphic, or sedimentary rocks. Rock fragments are carried by water or wind and redeposited as sediments. If the sediments are buried, the grains are compacted and may be cemented by minerals in groundwater to form a sedimentary rock. This process is known as lithification. Clastic sedimentary rocks, consisting of rock fragments, are classified primarily by grain size. Clay-sized grains become shale, silt-sized grains become siltstone, sand-size grains become sandstone, and pebble or boulder-sized grains become conglomerate. Another class of sedimentary rocks are those precipitated from water, either by biological processes or concentration through evaporation. These sedimentary rocks include limestone, anhydrite, gypsum, and salt (halite).