Geologists divide the earth's history into a sequence of periods: Permian, Jurassic, etc. But seldom do they agree on the age of each period, and a particularly annoying question mark is the Pleistocene, an epoch of intermittent ice ages during which man became true man. The geological dating system that uses the decay of uranium and other radioactive elements to tell the age of very ancient rocks is much too vague for the comparatively short Pleistocene. Dating by carbon 14, which is fine for recent times, reaches back only 60,000 years—not nearly enough....
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