Epoch and Series
An epoch is a time unit in a higher level than an age but lower than a period. An epoch is the rocks that formed during a series. An epoch is a subdivision of a period. A period can be divided into two or three categories of epochs: early and late epochs and early, middle and late epochs, respectively. An epoch can be further subdivided into ages. A series is a formal stratigraphic unit, which in the traditional stratigraphic scale is higher than a stage but lower than a system. A series is a subdivision of a system, and a system can be divided into two or three series, which are labelled with the same nomenclature as systems (i.e., lower, middle and upper or lower and upper). Series can also be assigned to geographical locations, such as the Furongian Series and the Lepingian Series.