geologic-time unit

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geologic-time unit (geochronologic unit) A subdivision of geologic time, based on the rock record of the corresponding chronostratigraphic unit. Each time unit coincides with a particular chronostratigraphic unit and, like them, time units are ranked in order of decreasing duration, each unit comprising a number of units of shorter time interval (e.g. two or more chrons comprise an age, two or more *chrons comprise an *age, two or more ages comprise an epoch, etc.).

Chronostratigraphic Unit

Geologic-time Unit

eonothem

eon (longest)

erathem

era

system

period

series

epoch

stage

age

chronozone

chron



Geologic-time units generally bear the same name as their chronostratigraphic counterparts, but the terms ‘lower’, ‘middle’, and ‘upper’ are changed to ‘early’, ‘middle’, and ‘late’.