Rocks composed of sediments, usually with a layered appearance. The sediments are composed of particles that come mostly from the weathering of pre-existing rocks, but often include material of organic origin; they are then transported and deposited by wind, water, or glacial ice. Sedimentary rocks are deposited mainly under water, usually in approximately horizontal layers (beds). Clastic sedimentary rocks are formed from the erosion and deposition of pre-existing rocks and are classified according to the size of the particles. Organically formed sedimentary rocks are derived from the remains of plants and animals (for example, limestone and coal). Chemically formed sedimentary rocks result from natural chemical processes and include sedimentary iron ores. Many sedimentary rocks contain fossils.