The relative ages of sedimentary rock can be determined by the sequence of layers. Index fossils allow rock layers to be correlated by age, although there are vast distances between the layers.

To date rocks geologists first give a relative age to a layer of rock at one location. Then they can give the same age to matching layers of rock at other locations. Certain fossils, called index fossils, help geologists match rock to layers.

Explanation:

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