Answer :
Examples of index fossils, the living forms that existed for brief periods of geologic time scale and are employed as indicators of the age of the rocks in which they are preserved, are keyed to the relative time scale.
The geological time scale is based on the geological rock record, which documents geological occurrences including erosion, mountain formation, and others. Mountains, seas, and continents have traveled great distances in both the vertical and horizontal planes over the course of hundreds to thousands of millions of years. For instance, mountainous desert landscapes have evolved where deep oceans had existed hundreds of millions of years ago. The oldest rocks were placed at the bottom of a sedimentary rock sequence (stratum) in the earliest geological time periods.
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