** The Ediacaran is the only formal system in the Proterozoic with a global boundary stratotype section and point (GSSP).
It reflects ratified unit names and boundary estimates from the International Commission on Stratigraphy (Ogg, 2009). * Changes to the time scale since March 2007 (see text).
Another change to the time scale is the age of the base of the Holocene Series/Epoch.
The boundary is now defined on the basis of an abrupt climate change recorded by indicators in a Greenland ice core (Walker and others, 2009). For many years, the term "Precambrian" was used for the division of time older than the Phanerozoic.
The GNC decided in 2006 that the USGS colors should be used for large-scale and regional geologic maps of the United States. Gradstein, Felix, Ogg, James, and Smith, Alan, eds., 2004, A geologic time scale 2004: Cambridge, U.
For international maps or small-scale maps (for instance, 1:5 million) of the United States or North America, the GNC recommends use of the international (CGMW) colors. C., and the Subcommission on Quaternary Stratigraphy, 2010, Formal ratification of the Quaternary System/Period and the Pleistocene Series/Epoch with a base at 2.58 Ma: Journal of Quaternary Science, v.
O., and others, 2009, Formal definition and dating of the GSSP (global stratotype section and point) for the base of the Holocene using the Greenland NGRIP ice core, and selected auxiliary records: Journal of Quaternary Science, v.
Since publication of a chart showing divisions of geologic time in the seventh edition of the USGS guide Suggestions to Authors (Hansen, 1991), no other time scale has been officially endorsed by the USGS. M., 2008, The concise geologic time scale: Cambridge, U. Specifications for the USGS colors are in the Federal Geographic Data Committee, Geologic Data Subcommittee (2006) guide, and those for the CGMW colors are in Gradstein and others (2004). The "North American Stratigraphic Code" (North American Commission on Stratigraphic Nomenclature, 2005) recommends abbreviations for ages in SI (International System of Units) prefixes coupled with "a" for "annum": ka for kilo-annum (103 years); Ma for mega-annum (106 years); and Ga for giga-annum (109 years).