Geologists use a variety of techniques to divide the Earth's history up into manageable units The boundaries between these units mark large changes to the Earth or the Earth's biosphere: extinction, faunal turnover etc. The "official" (this whole process is remarkably unofficial) time scale for North America is put out by the Geological Society of America and is called the DNAG scale (Decade of North American Geology). As we get more and more refined radiometric dates and as old tems exit and new terms enter (like I said: remarkably unofficial) the scale needs to be updated. So here's the new one: know it, learn it, live it, love it. (or just click on it to go to the GSA web site and print it. )
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