Divisions of Geologic Time: Position Versus Time, and the Words for Each

Every USGS report and map dates its rocks against one chart: the "Divisions of Geologic Time," published periodically by the survey's Geologic Names Committee, which has set the standards for geologic names at the USGS since 1899. Authors may use another published time scale — the Geological Society of America's or the International Commission on Stratigraphy's (ICS) — as long as they say which one. All three are linked from the USGS Geolex lexicon.

The USGS chart of major chronostratigraphic and geochronologic units, with numerical ages in millions of years before present. Units are not shown to scale. Chart: U.S. Geological Survey.

Two ways to name the same slice of Earth history

The time scale does two jobs at once. It names a body of rock by its position in the stack (chronostratigraphic units), and it names the time in which it formed (geochronologic units). Each rank has a word for each:

• Position suits rocks that obey the law of superposition — younger on top. "In this area, the Upper Jurassic Morrison Formation is unconformably overlain by the Lower Cretaceous Kootenai Formation."
• Time suits deposits that do not stack in order. The oldest glacial deposit of the Pleistocene may lie highest on a mountain, so it is the "oldest till," not the "upper till."

The committee's own everyday example: heavy things go on the lowest shelves, not the earliest ones.

Capital letters

Lower/Early, Middle and Upper/Late are capitalised only when they are the formal names of series or epochs within a system or period, as shown on the chart. Otherwise they are ordinary lowercase words: early Silurian, upper Paleozoic.

Where the dates come from

The numerical ages of boundaries come from the ICS's register of Global Boundary Stratotype Sections and Points (GSSPs) and its international chart (Cohen and others, 2025). Ages updated since the previous USGS edition (Orndorff and others, 2023) are marked with an asterisk.

Most units are defined by a physical reference point in rock. Only the Hadean, Archean and Proterozoic units — except the Cryogenian and Ediacaran — are defined by numerical ages instead (Global Standard Stratigraphic Ages). Those for the Hadean and Eoarchean have been revised, and are marked with a diamond.

Where the USGS chart differs

The committee adopts units deliberately, and keeps a few of its own choices:

• Precambrian is kept as an unranked, informal (though capitalised) name for everything older than the Phanerozoic; the formal names are preferred where they fit.
• Lower/Early, Middle and Upper/Late Cambrian stay formal until the ICS ratifies all the Cambrian's series — they correspond to the Terreneuvian, series 2 plus the Miaolingian, and the Furongian.
• The base of the Lopingian (upper Permian) has been revised.
• Tertiary stays a formal system/period because it is so widely used, though Paleogene and Neogene are encouraged where they apply.
• The Anthropocene has no status at all — recognised by neither the ICS nor the committee.
• Regional (provincial) terms are acceptable if clearly specified and referenced.

Sources

Written from Divisions of Geologic Time—Major Chronostratigraphic and Geochronologic Units, U.S. Geological Survey Fact Sheet 2026–3023, by the USGS Geologic Names Committee — Randall C. Orndorff, Nancy R. Stamm, David R. Soller, Lucy E. Edwards, Jeremiah B. Workman, Phil A. Frederick and Christopher S. Swezey of the USGS, with Berry H. Tew, Jr. (State Geologist of Alabama) and Peter P. McLaughlin (State Geologist of Delaware); a work of the United States government in the public domain. It cites Cohen and others (2025), Gradstein and others (2020), and Orndorff and others (2023). The chart is reproduced from the fact sheet.