Geologic Time Scale

Geologic Time Scale

Simplified Geologic Time Scale
Names and dates extracted from the GSA Geologic Time Scale (2009 version) - see reference below.

For a larger version, click here.
For a very large version, click here.

The U.S. Geological Survey released a revised timescale in 2010. The reference for the USGS timescale is:

U.S. Geological Survey Geologic Names Committee, 2010, Divisions of geologic time—major chronostratigraphic and geochronologic units: U.S. Geological Survey Fact Sheet 2010–3059, 2 p. Available on-line at:

For the most accurate and revised time scale for North America, see the Geological Society of America website at:

or a larger .pdf version at:

The most recent citation for the time scale (at the time this website was last modified is):

Walker, J. D., and Geissman, J. W., compilers, 2009, Geologic Time Scale: Geological Society of America, doi: 10.1130/2009.CTS004R2C. The Geological Society of America.

Note that many references in the text of the website use a more historic (or traditional) usage of the terms Mississippian and Pennsylvanian as "periods" and not as "epoch" subdivisions of the Carboniferous Period. Also, usage of the name Tertiary Period is used over the more modern subdivision of the Paleogene and Neogene Periods of the Cenozoic Era. Recent changes in the establishment of the now older (lower or basal) boundary of the Pleistocene Epoch and Quaternary Period is cause to question the age of units how and where the terms Quaternary, Pleistocene, and Pliocene are used in the literature cited on this website.

Also see:

U.S. Geological Survey Geologic Names Committee, 2007, Divisions of geologic time—Major chronostratigraphic and geochronologic units: U.S. Geological Survey Fact Sheet 2007-3015, 2 p. Available online at:

International Commission on Stratigraphy (ICS), 2004, by F. M. Gradstein, J. G. Ogg, A. G. Smith, and others, 2004, A Geologic Time Scale 2004: Cambridge University Press and the ICS under, reproduced with permission at

The URL is:
Last modified: 5/27/2010