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How do fossils help geologists correlate rock layers in different places?

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Grand Canyon rock layering — geochronology The steep walls of the Grand Canyon contain a number of layers of sedimentary rock laid down over millions of years. The lower formations belong to the early Precambrian age, while the upper layers are of the Paleozoic age. The line between the two sets of formations is called the Great Unconformity.

Geologists use fossils to correlate rock layers in different locations by applying the principle of faunal succession, which observes that fossilized plants and animals appear in the rock record in a predictable, non-repetitive order over time. Because specific organisms lived during distinct intervals of geologic time, finding the same fossils in separated rock outcrops suggests those layers were deposited contemporaneously.123

The primary tools for this process are index fossils, which are ideal for correlation because they were geographically widespread, abundant, easily identifiable, and existed for only a short span of time. By identifying these characteristic species, geologists can match strata across different regions, effectively creating a "calendar" of Earth's history. This practice, known as biostratigraphy, allows scientists to reconstruct the geologic history of various regions and the planet as a whole. While fossils are essential for this relative dating, geologists often verify these correlations using absolute radiometric dating methods to ensure accuracy.1435