Rock composed of layered bands of sediment is called sedimentary rock. Erosion creates sand, clay, silt and other sediment. Sediment builds up, and with the passage of time, the layers are compacted and cemented together to form sedimentary rock.
Sedimentary rock is categorized as either clastic, chemical or organic. Clastic sedimentary rock is composed of small pieces of broken rock, while chemical sedimentary rock is composed of dissolved minerals. Organic sedimentary rock is composed of calcium deposited by organic processes.
Sedimentary rock lies in a thin layer over igneous rock on most of the earth's crust. Fossils can be sorted by age according to where they are located in sedimentary rock Older fossils occur in the bottom layer of sedimentary rock.