Quaternary, in thegeologic history ofEarth, a unit of time within theCenozoic Era, beginning 2,588,000 years ago and continuing to the present day. The Quaternary has been characterized by several periods of glaciation (the “ice ages” of common lore), when ice sheets many kilometres thick have covered vast areas of thecontinents in temperate areas. During and between these glacial periods, rapid changes inclimate andsea level have occurred, andenvironments worldwide have been altered. These variations in turn have driven rapid changes in life-forms, bothflora andfauna. Beginning some 200,000 years ago, they were responsible for the rise of modernhumans.

The Quaternary is one of the best-studied parts of the geologic record. In part this is because it is well preserved in comparison with the other periods ofgeologic time. Less of it has been lost to erosion, and the sediments are not usually altered by rock-forming processes. Quaternary rocks and sediments, being the most recently laid geologic strata, can be found at or near the surface of the Earth in valleys and on plains, seashores, and even the seafloor. These deposits are important forunraveling geologic history because they are most easily compared to modern sedimentary deposits. The environments and geologic processes earlier in the period were similar to those of today; a large proportion of Quaternaryfossils are related to living organisms; and numerous dating techniques can be used to provide relatively precise timing of events and rates of change.

The term Quaternary originated early in the 19th century when it was applied to the youngest deposits in theParis Basin inFrance by French geologistJules Desnoyers, who followed an antiquated method of referring to geologic eras as “Primary,” “Secondary,” “Tertiary,” and so on. Beginning with the work of Scottish geologistCharles Lyell in the 1830s, the Quaternary Period was divided into twoepochs, thePleistocene and theHolocene, with the Pleistocene (and therefore the Quaternary) understood to have begun some 1.8 million years ago. In 1948 a decision was made at the 18thInternational Geological Congress (IGC) in London that the base of thePleistocene Series should be fixed in marine rocks exposed in the coastal areas of Calabria in southern Italy. As ratified by theInternational Commission on Stratigraphy (ICS) in 1985, the type section for boundary between the Pleistocene and the earlier Pliocene occurs in a sequence of 1.8-million-year-old marine strata at Vrica inCalabria. However, no decision was made to equate the beginning of the PleistoceneEpoch to the beginning of the Quaternary Period, and indeed the very status of the Quaternary as a period within the geologic time scale had come into question. Various gatherings of the IGC in the 19th and 20th centuries had agreed to retain both the Tertiary and Quaternary as useful time units, particularly for climatic- and continent-based studies, but a growing number of geologists came to favour dividing the Cenozoic Era into two other periods, thePaleogene and theNeogene. In 2005 the ICS decided to recommend keeping the Tertiary and Quaternary in the time scale, but only as informal sub-eras of the Cenozoic.

The ICS abandoned the sub-era structure in 2008, deciding instead to formallydesignate the Quaternary as the uppermost period of the Cenozoic Era, following the aforementioned Paleogene and Neogene periods. In 2009 theInternational Union of Geological Sciences (IUGS) officially ratified the decision to set the beginning of the Quaternary at 2,588,000 years ago, a time when rock strata show extensive evidence of widespread expansion of ice sheets over the northern continents and the beginning of an era of dramatic climatic and oceanographic change. This time is coincident with the beginning of the Gelasian Age, which was officially designated by the IUGS and the ICS in 2009 as the lowermost stage of the Pleistocene Epoch. The type section for theGelasian Stage, the rock layer laid down during the Gelasian Age, is found at Monte San Nicola near Gela, Sicily.