Stratigraphy is the science of naming and relating the layers of rock that form the earth's crust. You might compare it to the study of geography and geographical place names. Continents are divided into countries, countries into states or provinces, states into counties, etc.
The rock layers of the earth's crust are classified in a similar way. A "group" is made up of "formations," and formations are comprised of "members." Found within the members are volcanic ashbeds that can be dated using mass spectrometry. When fossils are discovered, their distance from the nearest ash layer is noted and their approximate age can be determined.
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