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Geologic period
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In geology, a period or age is a time span of many millions of years that are assumed to have had similar characteristics.
See geologic timescale for a list of geologic Periods and Ages.
In general, the duration of Periods/Ages tends to get longer the further one travels into the past.
Geologic Period/Age boundaries are generally set at some distinct worldwide event such as a mass extinction. For example, the boundary between the Cretaceous Period and the Palaeogene Period is set at an extinction event that marked the demise of the dinosaurs and of many marine species.
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