The geologic time scale is based on

The geological time scale is based on the major events that has taken place on earth in the geological past. This includes the great mass extinction events, appearance and extinction of specific living organisms, abundance of oxygen and depletion of oxygen in the atmosphere and many others. These events are arranged in a sequential order, right after the formation of the earth about 4.6 billion years back. These events are dated with the help of radiometric dating that gives an absolute age and also these data helped in the construction of the past events with respect to time.

This geological time scale is commonly used by the geologists.