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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