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Geologic Time Geologists and paleontologists usually represent geologic time vertically. This arrangement is derived from the vertical succession of rock strata in Earth itself. Because of the Principle of Superposition, geologists know that new layers can be laid down only on top of pre-existing older strata, and therefore older rocks lie below younger ones. The vertical time scale mimics this stratigraphic arrangement by placing older time periods below younger ones, with the present day at the very top. By itself, this vertical arrangement reveals only the relative ages of rocks, not how old the layers are. Geologists and paleontologists, however, also want to know the specific ages of rock layers and of the fossils within them. They use a variety of absolute dating methods to determine precisely when particular rocks were formed. Many different rocks from different strata around the world have been dated this way. Once numerical ages have been obtained, they can be combined with the vertical time succession to create a true geologic time scale. This works because some of the strata that have been dated are the same ones that were used to create the relative vertical time scale. The result is a global, comprehensive time scale that lists all the ages of Earth’s history, with the oldest ages placed at the bottom and the youngest at the top. Any rock layer can be correlated with this global time scale and given a real, rather than a relative, age. Scientists have also developed a system of standard names and colors for all the time periods. Geologists are especially interested in the ages of the boundaries between different eras, periods, and epochs because these often mark important events in Earth’s history. As geologists refine the numerical dates and acquire new fossil evidence, these boundary dates can shift. In fact, the combined absolute/relative time scale is always being revised in order to produce an ever more precise picture of Earth history. View the Time Scales Geologic Time Scale - American Geologic Time Scale - European Back to the beginning of the Timeline Dating Methods | Earth Processes | Life Processes Department of Paleobiology Home | National Museum of Natural History Home Smithsonian Institution Home | HTML Version