A geologic period is a timeunit 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 movieJurassic Park.
* Most and the most time specific time divisions[1]
†Some Geologists still designate a third period, or "sub-era", of the Cenozoic Era called the Quaternary, comprising the last two epochs of the Neogene Period and extending up to the present.
