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How do geologists date rocks?
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 | | Trilobites lived 500 million years ago, so the shale bed that the trilobite fossil was found in must also be this old. |
Sedimentary rocks form in layers or beds. If the beds remain undisturbed, the oldest rocks lie at the bottom and the youngest at the top. A geologist can usually tell the age of a bed of rock by examining the fossils that it contains, because many fossils are of types of animals that only lived for a short time before becoming extinct.
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Previous:
Back
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Book:
1001
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Section:
Earth
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Chapter:
Rocks and Minerals
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