Period and System

Period is the basic chronostratigraphic unit of the international geologic timescale. It is equivalent to the time span of a system (basic lithostratigraphic unit) and is a subdivision of an era. For example, the Palaeozoic Era can be subdivided into six periods: the Cambrian Period, Ordovician Period, Silurian Period, Devonian Period, Carboniferous Period and Permian Period. Each period covers a different time span, and a period can be further subdivided into epochs. A system is all of the rocks that formed during a certain period; it is a higher level than a series but a lower level than an erathem in the International Chronostratigraphic Chart. Examples are the Cambrian System and the Triassic System. In general, a system can be subdivided into two to four series.