Dating and Chronology
In 450 B.C., Herodotus realized that the sediments left by the annual flooding of the Nile were responsible for the deposits on either side of the river and that the depth of the sediments indicated a probable age of thousands of years. A numerical value was not forthcoming, however, for another two millennia, when in 1854 the foundation of the statue of Ramses II at Memphis was found to be buried under 9 feet of sediment. Historical records proved the statue to be 3000 years old, leading to a rate of accumulation of 3 × 10−3 feet per year. The upper alluvial deposit covering other strata at Memphis was 40 feet deep, from which it was apparent that, if the rate were constant, some 13,000 years would have elapsed during the formation of this upper layer. This ability to derive numbers marked a great advance in our understanding of the scale of time. In the previous decades it had been appreciated from European mining and from other observations in Germany, France, and Switzerland that the earth was often layered. Since rivers cut through these layers while mountains clearly had carried them upwards, the layers must predate the rivers and mountains. The further deduction was made that the layers represented successive periods of time in a scale much larger than human life-spans. Dating, then, by alluvial deposits offered a method of semiquantitation and placed geologic time in the tens of thousands of years at least. In the 1860s the scale was further extended by Lord Kelvin. Based on heat flow from the center of the earth, he estimated geological age as no less than 25–40 million years old. Not knowing exact values for density gradients, etc., he could not have been expected to have made an exact calculation, and his number was still further expanded in 1899 by estimates of the age of the ocean based on the concentration of salt in rivers and the annual rate of flow. It was concluded that the ocean is 100 million years old. While the exact reliability of this number also suffered from inaccuracies, notably in the estimate of the total volume of river flow, constancy of the rate, and the effect of dry seas, the size itself of the number, 100 million years, placed our view of the history of the earth in still a new dimension and one which is only a factor of ten less than the value currently accepted.
KeywordsPair Production Tree Ring Radioactive Nuclide Olduvai Gorge European Mining
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