Series are subdivisions of rock layers made based on the age of the rock and corresponding to the dating system unit called an epoch, both being formally defined international conventions of the geological timescale. A series is therefore a sequence of rock depositions defining a chronostratigraphic unit. Series are subdivisions of systems and are themselves divided into stages.
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.
Series in the geological timescale
|<span style="float:right;"><span class="plainlinksneverexpand">e  h</span></span><center><big>Units in geochronology and stratigraphy</big><ref>Cohen, K.M., Finney, S., Gibbard, P.L. (2013), International Chronostratigraphic Chart, International Commission on Stratigraphy.</ref></center>|
|Segments of rock (strata) in chronostratigraphy||Time spans in geochronology||Notes to<br/>geochronological units|
|<center>Eonothem</center>||<center>Eon</center>||4 total, half a billion years or more|
|<center>Erathem</center>||<center>Era</center>||10 total, several hundred million years|
|<center>Series</center>||<center>Epoch</center>||tens of millions of years|
|<center>Stage</center>||<center>Age</center>||millions of years|
|<center>Chronozone</center>||<center>Chron</center>||subdivision of an age, not used by the ICS timescale|
</noinclude> The geological timescale has all systems in the Phanerozoic eonothem subdivided into series. Some of these have their own names, in other cases a system is simply divided into a Lower, Middle and Upper series. The Cretaceous system is for example divided into the Upper Cretaceous and Lower Cretaceous series; while the Carboniferous system is divided in the Pennsylvanian and Mississippian series. In 2008, the International Commission on Stratigraphy had not yet named all four series of the Cambrian. Currently series are limited to the Phanerozoic, but the ICS has stated its intention of subdividing the three systems of the Neoproterozoic (Ediacaran, Cryogenian and Tonian) into stages too.
Systems and lithostratigraphy
Systems can include many lithostratigraphic units (for example formations, beds, members, etc.) of differing rock types that were being laid down in different environments at the same time. In the same way, a lithostratigraphic unit can include a number of systems or parts of them.