In geology, the rocks have a way of messing with our pretty schemes. One instance I'm thinking of involves the base of the geologic time scale. The Earth itself is about 4.5 billion years old--but the time scale starts at the base of the Archean Eon with a time unit called the Eoarchean Era, running from 4.0 billion years ago (4 Ga) to 3.6 Ga. Like most of the Precambrian time periods...
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