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Contents of 7.4 How was the geologic time scale constructed?

7.4 How was the geologic time scale constructed?

• The names of periods (the fundamental time interval) derive from places where rocks of that age were first described.
• Periods are grouped into eras, which are further grouped into eons.
• Periods are subdivided into smaller units called epochs, and the periods in the Cenozoic era as well as the Carboniferous of the Paleozoic are subdivided once prior to the epoch.