The subdiscipline of paleontology concerned with correlation of strata or of determining their ages is called "biostratigraphy".
Biostratigraphers have noticed that not all fossils are equally useful for their work. There are four main reasons for this:
1) Some species persisted through geologic time for much longer than others making them less useful for resolving age as tightly as possible.
2) Some species are too restricted in their geographic range, making it impossible to use them to correlate outside that range.
3) Some species are too thoroughly tied to a single environment, making it impossible to use them for the multitude of rocks that did not form in that specific environment.
4) Some species are so rare or so difficult to identify properly that other species allow biostratigraphers to accomplish the job much more easily.
Species that have been found useful for correlation or age determination are called "guide fossils". The following page allows you to select a certain age from a simplified geologic time scale and view the rocks of that age present in southwest Georgia. Links from there allow you to see a few of the characteristic guide fossils for those rocks.
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