Faunal Succession
The principles of original horizontality and superposition are even more powerful tools for relative dating when combined with the observations of William Smith. Smith’s observations of how the fossil record varies in different rock layers gave us the principle of faunal succession, which states that groups of fossils occur in the geologic record in a definite chronologic order. Thus, a period of geologic time can be recognized by its characteristic fossils.
Smith observed, for example, that in one outcrop of interbedded sandstone and shale, many of the shale layers were similar except for the fossils they contained. (Interbedding is when layers of the same kind of rock are embedded between other layers of rock.) Each shale layer contained its own unique assemblage of fossils. He further observed that the fossils appeared in a predictable and invariable sequence. Note, too, that by faunal succession, we can correlate rock units separated by distance. In the picture above you see that two rock layers can be separated by distance and be at different elevations, but if they contain the same fossils, you can conclude that they are of the same age.
