Geologic periods are divided based on the chronology of one complete transgressive-regressive cycle.

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Geologic periods are actually divided based on significant geological events, such as the appearance or disappearance of certain species or major changes in the Earth's climate or landscape. Transgressive-regressive cycles, which are cycles of sea level rise and fall, can be used to help determine the timing and duration of these events, but they are not the sole basis for dividing geologic periods.

Geologic Periods Are Divided Based on the Chronology of One