Sedimentary Rocks Sedimentary rocks (also called deposited rocks) are rocks, which are derived from the decomposition, and weathering of older rocks. The sedimentary rock cover of the continents of the earth's crust is extensive, but the total of that being of sedimentary rocks is estimated to be only five percent of the total. Sedimentary rocks contain important information about the history of the earth. They contain fossils, the preserved remains of ancient plants and animals. The composition of the sediments provides us with clues as to the original rock. Differences between different layers indicate chang