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Geologic Time Chart
This is a typical example of the Geologic Time Chart. The first image shows the Eons and Eras of Time. The second image shows the Eras, Periods, and Epochs of Time. Both charts provide an indication of Time in Millions of Years Ago.
The estimation of Time is not necessarily exact. For example, the Phanerozoic Eon did not begin precisely 540 Million Years Ago. The actual event when the Precambrian / Proterozoic Eons became the Phanerozoic Eon might have been either thousands of years earlier or later. In the past few years geologists have identified rock layers deposited at that Time which provide a more exact estimation of the change from Precambrian / Proterozoic Eons to the Phanerozoic Eon. Very little geologic evidence remains to indicate the Time exactly, although geologists continue to refine the Time through fieldwork and research.
As the time chart comes closer to the present, the estimation of the passage of Time becomes more exact due to the fact that more geologic evidence remains and provides better information. The intervals and units of Time are subject to change as geologists find more geologic evidence that refines our understanding of the events that delineate the respective units.