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| Segments of rock (strata) in chronostratigraphy | Periods of time in geochronology | Notes |
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4 total, half a billion years or more |
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12 total, several hundred million years |
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tens of millions of years |
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millions of years |
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smaller than an age/stage, not used by the ICS timescale |
The International Commission on Stratigraphy (ICS), sometimes referred to by the unofficial "International Stratigraphic Commission" is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratigraphy, geological, and geochronological matters on a global scale.
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It is a subordinate body of the International Union of Geological Sciences—of which it is the largest scientific body within the organisation—and of which it is essentially a permanent working subcommittee that meets far more regularly than the quadrennial meetings scheduled by the IUGS, when it meets as a congress or membership of the whole.
One of its main aims, a project begun in 1974, is to establish a multidisciplinary standard and global geologic time scale that will ease paleontogical and geobiological comparisons region to region by benchmarks with stringent and rigorous strata criteria called Global Boundary Stratotype Section and Points (GSSPs) within the fossil record. (i.e section of the rock record as of a core sample section or accessible exposed strata, which when a core sample are usually "trayed" in long pieces, also called "sections" about a meter in length.)
Additionally the ICS defines an alternative type of benchmark and criteria called Global Standard Stratigraphic Ages (GSSAs) where the characteristics and dating criteria set solely by physical sciences methods (such as magnetic alignment sequences, radiological criteria, etcetera.) as well as encouraging an international and open debate amongst Earth scientists in the paleontology, geology, geobiology and chronostratigraphy fields, among others.
The International Commission on Stratigraphy has spawned numerous subcommittee level organizations organized and mobilized on a local country-wide or regional basis that are the true working committees of the IUGS, and these do the field work, basis comparisons in conference or co-ordination research committee meetings of local or wide-scale scope.
The ICS publishes various reports and findings as well as revised references periodically, summarized in the International Stratigraphic Chart, a combined working proposal and guideline-to-date released after the last ICS deliberations prior to the upcoming (next) meeting of the IUGS (PDF file, updated January 2008, compare with the [ICS-I.S. Chart of 2004]). Until the IUGS accepts the recommendations, they are unofficial since the IUGS parent approves or dismisses the individual deliberation reports of the ICS, which are presented as recommendations, and span dating and strata selection criteria, and related issues including nomenclatures. In de facto everyday matters, the deliberative results reported out of any meetings of the ICS are widely accepted and immediately enter everyday use, except in the rare cases where they result in a strong body of dissenting opinion, which matters are resolved before the full IUGS.
One such controversy arose this past year when the ICS deliberated and decided the Pliocene series of the current but unofficially named Quaternary period should be shifted into the Neogene system and period. The term Quaternary has yet to be officially adopted by the IUGS, but has widespread support as acceptable nomenclature for the current geologic period beginning at the GSSP accepted at 5,332,000 years ago at the transition between the Messinian age to the Zanclean (3.6 mya) age. The ICS voted, perhaps because the time units span human paleo-archaeological strata, to begin the Quaternary at the end GSSP of the Piacenzian age (2.588 mya) or possibly the end of the Gelasian (1.806 mya), any of which are in a different epoch.
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