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How are geologic eons, eras, periods, epochs, and ages arranged from longest to shortest?
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Layered strata in an outcropping of the Morrison Formation on the west side of Dinosaur Ridge, near Denver, Colorado.
In the field of geochronology, the geologic time scale serves as a standardized calendar for Earth's history. These units are organized hierarchically based on their duration.
From longest to shortest, the units are arranged as follows:
- Eons: The largest intervals of geologic time.1
- Eras: Subdivisions of eons.21
- Periods: Subdivisions of eras.21
- Epochs: Subdivisions of periods.1
- Ages: The smallest formal units of geologic time, which are subdivisions of epochs.1
These divisions are delineated by stratigraphy and the fossil record, with the appearance and disappearance of specific species often marking the boundaries between these intervals.13