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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.

The principle of superposition states that in an undeformed sequence of sedimentary rocks, each layer of rock is older than the one above it and younger than the one below it (Figures 1 and 2).

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Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms.

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.

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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