This list is divided first into the geologic eras of the Phanerozoic (the
Cenozoic, the
Mesozoic, and the
Paleozoic) and then into the geologic periods of each era. Each period is marked below the era bar on top of its subdivided epochs and stages. Each stage is assigned an age in
mya, an
acronym for million years ago, which is the age at which it began. Most of these ages are derived from astronomical cycles in sediments, magnetic data,
biostratigraphic data, and
radiometric dating methods. The GSSP assigned to each stage is that stage's lower boundary and oldest point.
Ages are given in "million year ago" (mya). They are obtained with different
radiometric dating methods depending on the type of rock and its age. Ages that have a tilde (~) prefix are approximate ages for GSSPs that have not been defined or not been accurately dated. The Status column has a "golden spike" for every GSSP which has been formally agreed by the ICS. Those without have only candidate sections which have not yet been formally ratified. The clock stands for times that are currently defined only by an age. The "Defining markers" column lists the evidence in the rock used to define the boundary. (Ideally, these are applicable in rock sections worldwide.) Most of the boundaries rely on the
fossil record (biologic),
paleomagnetic data (magnetic), and/or
climate data determined by
carbon and
oxygen isotopes. == List ==