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A geologic period is a time unit subdivision of geologic time defined as a span of years into which the larger era time units are divided into smaller timeframes, as era's divide the eon. In the Earth Sciences rocks and especially the sequences of rocks called stratum (plural: strata) arrayed in a ordered "rock column" occurring during a timespan are the focus of study so the time units are paired with corresponding Rock strata units whose characteristics define such points elsewhere that occurred concurrently as the local rock layers were laid down as sediments. For the Geological Period the paired rock strata term, a geologic stage is used to denote the corresponding rock layers of both the geologic record and the fossil record; thus the rocks of the Devonian System were laid down during the Devonian Period, and such equivalent units exist at each level of refinement of geological chronology and biogeological or stratigraphic classification.

Each unit, of strata no matter how interrupted the record recorded in the local rock column is mapped into the overall geologic record and classified carefully into chronological units of geologic time based on world wide efforts of ISC working to correlate the world's local stratigraphic record into one uniform planet wide benchmarked system, in an steady effort ongoing since 1974. While paleontologists often refer to faunal stages rather than geologic periods, they are often used in popular presentations of paleontology or plate reconstructions. An example of a paleontological reference is the book and movie Jurassic Park.

As illustrated in the article on the geologic time scale, most periods are subdivided into smaller units called epochs. In 2004 the International Union of Geological Sciences (I.U.G.S.) recognized the Ediacaran period of the Neoproterozoic era, the first such newly-designated period in 130 years.

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