These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth's surface is moving and changing.As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils.
Tree ring dating offers over 1,000 years of clues in dates of artifacts from the American Southwest.Radiocarbon dating provides additional clues necessary for absolute dating.The study of strata is called stratigraphy, and using a few basic principles, it is possible to work out the relative ages of rocks.Just as when they were deposited, the strata are mostly horizontal (principle of original horizontality).Geologists have established a set of principles that can be applied to sedimentary and volcanic rocks that are exposed at the Earth's surface to determine the relative ages of geological events preserved in the rock record.
For example, in the rocks exposed in the walls of the Grand Canyon (Figure 1) there are many horizontal layers, which are called strata.
Second, it is possible to determine the numerical age for fossils or earth materials.
Numerical ages estimate the date of a geological event and can sometimes reveal quite precisely when a fossil species existed in time.
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The layers of rock at the base of the canyon were deposited first, and are thus older than the layers of rock exposed at the top (principle of superposition).