geologic time
The geological time scale has two parallel vocabularies, and confusing them is the commonest error in writing about Earth history. One names bodies of rock; the other names lengths of time.
About: Phanerozoic · Cenozoic · Quaternary · International Chronostratigraphic Chart · Holocene
Chronostratigraphic units are bodies of rock, defined between specified stratigraphic horizons that represent particular intervals of geological time. The intervals of time themselves are geochronologic units. A system is rock; a period is time.[1]
The hierarchy pairs them rank for rank: system with period, series with epoch, subseries with subepoch, stage with age. The purpose of the whole scheme is to give Earth history a shared reference frame.[1,2]
A stage contains all the rock deposited between two horizons defined by Global Boundary Stratotype Sections and Points — physical reference points in real sections. The stage is the lowest rank recognisable worldwide, and its boundary stratotypes also fix the boundaries of every higher rank built from it.[1]
This is why the chart itself warns that numerical ages are subject to ongoing revision and do not define units. The number attached to a boundary is a measurement of a rock section; the definition is the section.[1,2]
The Phanerozoic — the eon of abundant visible life — began 538.8 million years ago, with a stated uncertainty of 0.6 million years. Everything before it, from the base of the Archean at 4,031 million years to that boundary, is more than seven times as long.[1,2,3]
Within the Phanerozoic, the Cenozoic covers the last 66 million years, the Quaternary the last 2.58 million, and the Holocene the last 11,700 years. Each nests inside the one before it, and the chart records exactly where.[1,2,3,4]
Numerical ages come from radiometric dating: parent isotopes decaying to daughters at known rates, read as geological clocks.[1]
What such a date records is the last time the sample was melted or disturbed enough to reset it — not, in general, the age of the material. That limitation is why the boundaries are defined by rock sections and only calibrated by numbers.[1]
Often said
“Periods and systems are two words for the same thing.”
What the evidence supports
A system is a body of rock; a period is the interval of time during which that rock formed. Every chronostratigraphic rank has a geochronologic counterpart, and keeping them apart is what allows a boundary to be defined by a physical section rather than by a number.
Supporting records: chrono-vs-geochron · time-hierarchy-ranks
Often said
“The geological time scale is defined by numerical ages.”
What the evidence supports
Units are defined by Global Boundary Stratotype Sections and Points — physical reference points in rock. The chart states explicitly that numerical ages are subject to revision and do not define units.
Supporting records: gssp-defines-boundary · holocene-base-age
Bracketed references link to the structured entity records whose claims support each paragraph. Last updated 2026-09-06.