Divisions of Geologic Time—Major Chronostratigraphic and Geochronologic Units

Fact Sheet 2018-3054
Prepared in cooperation with the Association of American State Geologists



Effective communication in the geosciences requires a consistent nomenclature for stratigraphic units and, especially, for divisions of geologic time. A geologic time scale is composed of standard stratigraphic divisions based on rock sequences and is calibrated in years.

Geologists from the U.S. Geological Survey (USGS), State geological surveys, academia, and other organizations require a consistent time scale to be used in communicating ages of geo­logic units in the United States. Many international debates have occurred over names and boundaries of units, and various time scales have been used by the geoscience community.

For consistent usage of time terms, the USGS Geologic Names Committee and the Association of American State Geologists devel­oped the Divisions of Geologic Time; the 2018 update in this fact sheet contains the unit names and boundary age estimates ratified by the International Commission on Stratigraphy in 2018. Scientists may use other published time scales, provided that these are specified and referenced.

Suggested Citation

U.S. Geological Survey Geologic Names Committee, 2018, Divisions of geologic time—Major chronostratigraphic and geochronologic units: U.S. Geological Survey Fact Sheet 2018–3054, 2 p., https://doi.org/10.3133/fs20183054.

ISSN: 2327-6932 (online)

ISSN: 2327-6916 (print)

Table of Contents

Publication type Report
Publication Subtype USGS Numbered Series
Title Divisions of geologic time—Major chronostratigraphic and geochronologic units
Series title Fact Sheet
Series number 2018-3054
DOI 10.3133/fs20183054
Year Published 2018
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Eastern Geology and Paleoclimate Science Center
Description 2 p.
Online Only (Y/N) N
Additional Online Files (Y/N) N
Google Analytic Metrics Metrics page
Additional publication details