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Answer:

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. As well as are used as guides to the age of the rocks in which they are preserved.

Explanation:

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A useful index fossil must be distinctive or easily recognizable, abundant, and have a wide geographic distribution and a short range through time. Index fossils are the basis for defining boundaries in the geologic time scale and for the correlation of strata.

A good index fossil is one with four characteristics: it is distinctive, widespread, abundant, and limited in geologic time. Because most fossil-bearing rocks formed in the ocean, the major index fossils are marine organisms. That being said, certain land organisms are useful in young rocks and specific regions.

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