Geologic Time and Rock Formation in Pinal County

Geologists divide the Earth’s history into nested chunks called Eons, Eras, Periods, and Epochs. Unlike our calendar, when these begin and end depends not on a set number of days or years, but on what’s happening in the geologic record of the Earth. Eons are divided at Big changes, Epochs on smaller changes. The other chunks are divided at different kinds of changes. The exact change date can’t usually be determined, and the time each chunk starts and ends is approximate and our understanding sometimes changes. Table does not reflect time scale.

This Geologic Time Scale is presented with most recent at the top, and most ancient at the bottom. Periodically there are corrections, additions, and updates to the information given by time scales.  The information on dates for this time scale is based on GSA Geologic Time Scale v. 5.0 (2018).