A system in stratigraphy is an idealized composite unit of the geologic record made up of a succession of rock layers that were laid down together within a certain corresponding geological time span, and are used in turn to date things to a certain corresponding geologic period. The system is thus a unit of the geologic record or rock column, pieced together using the Law of Superposition and mapped to its corresponding period— the associated continuous chronostratigraphical time unit, a relative metric that science committees have determined solid dating for as organized on the geologic time scale. A system is therefore a unit of chronostratigraphy, unrelated to lithostratigraphy, which subdivides rock layers on their lithology. Systems are subdivisions of erathems and are themselves divi