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Western Oregon University: Earth System Science: Geologic Time [Pdf]
An 81-page slideshow that looks at the principles of geology with respect to relative dating, at different concepts describing geological processes, at absolute age and radiometric dating, at the eons and eras in the geologic time scale,...
US Geological Survey
Index Fossils
If you need examples of index fossils for each of the geologic eras and periods, than this resource is for you. Diagrams of the index fossils and their scientific names are also provided.