Abstract
Because no one was around to chronicle most of the geological events which occurred during the earth's history, it is necessary to rely upon somewhat less direct (but no less useful) methods to place these events in the proper time frame. The basic assumptions used are 1) we live in an ordered universe, and 2) physical laws have not changed through time. The assigning of an age (in years) to a particular event requires that some process having a predictable rate be utilized. Various radioactive nuclides are commonly used but a number of other methods provide supplementary or complementary chronologic information. These other methods include tree-ring counting, varve counting, thermoluminescence, amino acid racemization, obsidian hydration, and magnetic reversals. Because of the cross testing of methods and the interlocking nature of the age data, the geologic time scale rests upon a very firm foundation.
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