Sedimentary rocks are formed by processes that occur at Earths surface. They can be made of other sedimentary rocks, igneous rocks and metamorphic rocks. Some sedimentary rocks are deposited chemically from a solution. Others are biochemical or organic in origin and are made of the calcium carbonate from the shells of bygone organisms or are made from the breakdown of organic tissues of plants and animals. Most sedimentary rocks are made of the fragments of eroded rocks that are cemented together. When rocks of all kinds are exposed to the surface they undergo an erosion process called weathering.