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To serve as geochronometers, the records must be complete and the accumulation rates known.The fossiliferous part of the geologic column includes perhaps 122,000 metres of sedimentary rock if maximum thicknesses are selected from throughout the world.These so-called Green River Shales also contain abundant freshwater-fish fossils that confirm deposition in a lake. Because the average thickness of a varve is about 0.015 centimetre (0.006 inch), the lake is thought to have existed for more than 5 million years.Each of the examples cited above is of a floating chronology—Sweden, by contrast, it has been possible to tie a glacial varve chronology to present time, and so create a truly absolute dating technique.In spite of this tremendous spread, most geologists felt that time in the hundreds of millions of years was necessary to explain the sedimentary record.If the geologic column (see below) were made up entirely of annual layers, its duration would be easy to determine.
Practical experience indicates that the constant is almost totally dependent on temperature and that humidity is apparently of no significance.
Furthermore, artifacts reused repeatedly do not give ages corresponding to the culture layer in which they were found but instead to an earlier time, when they were fashioned.
Finally, there is the problem that layers may flake off beyond 40 micrometres (0.004 centimetre, or 0.002 inch) of thickness— Sediment in former or present water bodies, salt dissolved in the ocean, and fluorine in bones are three kinds of natural accumulations and possible time indicators.
When certain evidence suggested 25,000 years to be an appropriate figure, factors became years—namely, 75,000, 150,000, and 200,000 years.
And, if glacial time and nonglacial time are assumed approximately equal, the Pleistocene Epoch lasted about 1,000,000 years.
Nonradioactive absolute chronometers may conveniently be classified in terms of the broad areas in which changes occur—namely, geologic and biological processes, which will be treated here.