Layered rocks.docx - Layered rocks Geologic time is nearly...

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Layered rocks Geologic time is nearly incomprehensible to the human mind. We usually think in terms of a few tens of years, since that measures a lifetime. Hundreds or thousands of years are considered ancient history. But in geology, we must think in time spans of millions of years, hundreds of millions of years, even of billions of years. Geologists measure such tremendous spans of geologic time in a relative sense, that is, one event occurred after another and prior to a third event. In that sense, a geologic time scale was developed in the 1800's to make handy "boxes" of relative time periods in order to think of events in an understandable manner. "Periods" and "eras" were named to designate varying intervals of time that then seemed to be useful. The geologic time scale is shown in fig. 3. Figure 3 --Geologic time scale showing geological eras and periods recognized in North America from oldest to youngest in ascending order. Length of time of each period and million years before present are rough estimates based on radiometric dating of rocks ad relative rock relationships. Earliest humans did not appear until about the beginning of the Pleistocene Epoch some two million years ago. (Figure from the KGS's GeoKansas web site.)
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The actual time in years, "absolute time," is more difficult to determine. Geologists are now able to
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