Index fossil

Index fossils are those used to identify a particular layer of the geological column. Many fossils are found above or below other fossils. This sorting of fossils poses a particular challenge for creationists to explain through flood geology. Index fossils are those organisms that are presumed to have spread throughout much of the world and lived for only a short period of time.

Marine fossils are a preferred index fossil as can be seen in the following U.S. Geological Survey diagram. Marine fossils are abundant in the fossil record and can be found in every single layer of the geological column. For instance, if a core sample brought contains remains of the sea urchin Micraster then geologists can conclude with some certainty that it is unlikely to find oil in this stratum due to the fact that it is a zone index fossil found in chalk where the oil is absent.