Explain how geologists use fossils to determine the relative ages of rock layers within a large region

Geologists use Relative Dating to determine the age of rocks through the use of fossils.

Explanation:

Relative dating relies on the study of fossils and strata to stimulate rock age. However, only fossils that are distributed in short time intervals in the history of the earth, having a wide geographical distribution, allow this dating. These fossils are called old fossils. The method of relative dating is based not only on the information provided by the fossils but also on two other fundamental principles: the principle of initial horizontality, whereby, if there is no disturbance, sedimentation takes place regularly, forming layers generally parallel and horizontal strata, and the principle of strata overlap, according to which each stratum is older than the one that covers it.

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weathering is the answer.