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How do geologists date rocks?
Trilobites lived 500 million years ago, so the shale bed that the trilobite fossil was found in must also be this old.
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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Rocks and Minerals