Revision History for GIP 141

Divisions of Geologic Time (Bookmark)

By U.S. Geological Survey

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First posted online May 3, 2012

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Reprinted October 2016 and reposted December 30, 2016

For the October 2016 reprint, the content of the time scale was not changed, although minor edits were made to the bookmark.

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Revised and reposted March 27, 2020, as Version 2.0

For version 2.0 of the bookmark, the boundary age estimates were updated to match those in USGS Fact Sheet 2018-3054.

To explain differing box heights in the time scale, the text on the back of the bookmark was expanded slightly as follows:

"Ages shown for divisions of geologic time are general representations. Ages in mega-annum mean millions of years before present. Box heights are related to how long time periods lasted; different scaling factors are used for the column on the front of the bookmark and for the column on the back, which represents a much longer time period. Map symbols are in parentheses. For more specific age information, see U.S. Geological Survey Fact Sheet 2018–3054 (https://doi.org/10.3133/fs20183054)."

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Revised and reposted March 29, 2024, as Version 3.0

For version 3.0 of the bookmark (GIP 141), the error ranges (plus or minus values) were removed for simplicity. The age estimates for four boundaries were updated to match those in figure 1 of chapter C of USGS Professional Paper 1879-1, which was published in 2023. The affected boundaries were the bases of the Oligocene (from 34.09 to 33.9 Ma), Eocene (from 55.9 to 56.0 Ma), Jurassic (from 201.3 to 201.4 Ma), and Cambrian (from 541.0 to 538.8 Ma).

To avoid duplication of colors, a few cells changed color to match figure 1 of chapter C of USGS Professional Paper 1879-1.

The text on the back of the bookmark was modified to explain the variations in box heights and to credit Professional Paper 1879-1 (https://doi.org/10.3133/pp1879v1) instead of Fact Sheet 2018-3054.