Tertiary period

EtymologyName formalityInformalUsage informationCelestial bodyEarthRegional usageRegional(?)Time scale(s) usedICS Time Scale (formerly)Formerly used byICSDefinitionChronological unitPeriodStratigraphic unitSystemTime span formalityInformalLower boundary definitionK-Pg extinction eventLower boundary GSSPNoneLower GSSP ratifiedN/AUpper boundary definitionBeginning of the Quaternary glaciationUpper boundary GSSPNoneUpper GSSP ratifiedN/A

The Tertiary ( , US also ) is an obsolete geologic period spanning 66 million to 2.6 or 1.8 million years ago. The period began with the extinction of the non-aviandinosaurs in the Cretaceous–Paleogene extinction event, at the start of the Cenozoic Era, and extended to the beginning of the Quaternary glaciation at the end of the Pliocene Epoch. The Tertiary has not been recognised by the International Commission on Stratigraphy (ICS) since the late 1980s, with the timespan of the Tertiary now being split into the earlier Paleogene and the more recent Neogene periods, though the term continues to be used in some scientific publications.