Series synonomyous with epoch are subdivisions of rock layers based on the age of the rock and formally defined by international conventions of the geological timescale. A series is therefore a sequence of rock depositions defining a chronostratigraphic unit. Series are subdivisions of systems (periods) and are themselves divided into stages (ages). Series is a term defining a unit of rock layers formed in a certain interval in time; it is equivalent to the term geological epoch epoch criteria defining the interval of time itself, although the two words are sometimes confused in informal literature.