Dating earth radioactivity

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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).

In the Grand Canyon, the layers of strata are nearly horizontal.

Layers that cut across other layers are younger than the layers they cut through (principle of cross-cutting relationships).

The principle of superposition builds on the principle of original horizontality.

Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms.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).In addition to being tilted horizontally, the layers have been faulted (dashed lines on figure).Applying the principle of cross-cutting relationships, this fault that offsets the layers of rock must have occurred after the strata were deposited.

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