The geologic time scale is primarily divided into geochronological units. The following lists the longest to the shortest divisions:

Eon – On most geologic time scales, eons are the longest time intervals represented.

Era – One of the longest eras was the Precambrian Era, meaning “before old life,” a time that shows little fossil evidence. Thus, when talking about eras, most scientists refer to the eras of the Phanerozoic, a time when life became more abundant on the Earth. There are three divisions: the Paleozoic, Mesozoic, and Cenozoic Eras.

Period – Each era is broken down into periods. Most of the division names are from the geographic localities in which the period was first identified, Latin roots for a deposit’s characteristic, or even ancient peoples who lived nearby.

Epoch – Some periods are further broken down into epochs. Por example, the Tertiary Period (Cenozoic Era) is broken down into the Paleocene, Eocene, Oligocene, Miocene, and Pliocene Epochs.

Age – This unit (although not used in most general geologic time scales) is even shorter than the epoch, with most epochs divided into ages that usually number in the thousands of years only. The term “age” is not to be confused with the nicknames for certain geologic times. For example, the “Age of the Dinosaurs” is often the nickname for the Paleozoic Era.

Chron – ehron is an even shorter unit of time than age and is not used as frequently as the larger units on the geologic time scale. It is usually used to describe a local division of rock in an area.