Some fossils, known as index fossils, help geologists balance rock layers. A fossil must be broadly dispersed to be a valuable index fossil and indicate a form of organism that has lived for a short time. Index fossils are valuable when the relative ages of the rock layers are told.
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1) Index fossils are of organisms that lived over a wide area. They lived for a fairly short period of time. An index fossil allows a scientist to determine the age of the rock it is in.
2) Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages.
