Index fossils and relative dating
Other subdivisions reflect the evolution of life; the Archean and Proterozoic are both eons, the Palaeozoic, Mesozoic and Cenozoic are eras of the Phanerozoic eon.The three million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale.
Older time spans, which predate the reliable fossil record (before the Proterozoic eon), are defined by their absolute age.
The geologic time scale (GTS) is a system of chronological dating that relates geological strata (stratigraphy) to time.
It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events that have occurred during Earth's history.
The first three of these can be referred to collectively as the Precambrian supereon.
Eons are divided into eras, which are in turn divided into periods, epochs and ages.