Geologic Period

A period is the geochronologic time unit equivalent to a chronostratigraphic system. The cited article, Geologic time scale, which also defines the other three ranks because their own titles redirect to it, says there are 22 defined periods, with the current one being the Quaternary period. It explains that the time scale has been developed through the study of rock layers and the observation of their relationships and identifying features such as lithologies, paleomagnetic properties and fossils, and that the definition of the standardised international units of geological time is the responsibility of the International Commission on Stratigraphy. Time units of this rank in the atlas are grouped under this entry.