Time periods of dating
Eons are divided into smaller time intervals known as eras.
In the time scale above you can see that the Phanerozoic is divided into three eras: Cenozoic, Mesozoic and Paleozoic.
There is large body of evidence associating this extinction with the large impact crater at Chicxulub, Yucatan Peninsula, Mexico.
Very important events in the history of the world are used to determine the boundaries of the age.
Detailed geologic time scale: The United States Geological Survey has published “Divisions of Geologic Time: Major Chronostratigraphic and Geochronologic Units.” It is a much more detailed time scale than the simplified scale shown above.
Finer subdivisions of time are possible, and the periods of the Cenozoic are frequently subdivided into epochs.
Subdivision of periods into epochs can be done only for the most recent portion of the geologic time scale.Ions are divided into smaller time periods known as age.