Series (stratigraphy)

A series in geology is a way scientists group layers of rock based on how old they are. These rock layers, or strata, tell us about Earth's history, like pages in a very old book. Each series represents a specific time period and is part of a larger grouping called a system. Series help scientists understand the order of events that shaped our planet.

Series are defined by international agreements to make sure everyone uses the same terms. They are smaller than systems but bigger than stages, which are even more detailed time bits. Think of series like chapters in a story—each one covers a chunk of time when certain rocks were formed.

Even though we call them series, they match up closely with something called a geological epoch, which is just a slice of time. Sometimes people use these words interchangeably, but they really mean slightly different things. Series help us organize Earth's long and complicated history into pieces we can study and understand.

Series in the geological timescale

The geological timescale divides rock layers into groups called systems, which are then split into smaller parts known as series. Some series have special names, while others are simply called Lower, Middle, and Upper. For example, the Cretaceous system is split into the Upper Cretaceous and Lower Cretaceous Series, and the Carboniferous System is divided into the Pennsylvanian and Mississippian Series.

As of 2008, not all series of the Cambrian had been named by the International Commission on Stratigraphy. Currently, series are only used for the Phanerozoic, but there are plans to divide the systems of the Neoproterozoic into series too.

Systems and lithostratigraphy

Systems can include many different types of rock layers, called lithostratigraphic units, such as formations, beds, and members. These units are made of different kinds of rocks that formed in various environments at the same time. Similarly, a single lithostratigraphic unit might contain several systems or parts of systems.