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Both techniques are important in different geologic situations.

And both techniques are used together to develop the geologic time scale and to discern the ages of rocks exposed in Grand Canyon. In flat-lying sedimentary rocks it is simply the Principle of Superposition [see Photo 3] (or the “rule of pancakes”): the rocks at the bottom are oldest, and the rocks on top are the youngest. When rocks are folded, faulted, overturned, or intruded by igneous rocks, it is still possible to determine the relative age of the rocks.

Again, the principle is simple: for example, all rocks with trilobite fossils [see Photo 7] in them are Paleozoic in age, whereas rocks containing dinosaur fossils are Mesozoic.

In fact, the names of the eras in the Geologic Time Scale reflect the relative ages of fossil assemblages: Paleozoic means “ancient life,” Mesozoic means “middle life,” and Cenozoic means “new life.” Further subdivisions of the time scale, such as Periods and Epochs are also based on changes within fossil assemblages with time (for example, dinosaurs are in rocks of the Mesozoic Era, Tyrannosaurus rex is only found in the Cretaceous Period of the Mesozoic).

Index fossils are usually from microscopic organisms that lived in widespread environments for a short period of time.

Relative dating determines the order in which a sequence of past events occurred, but does not determine exactly when the geologic events happened.

Absolute age determinations are numeric and identify when, in years, specific events happened.

Yet, it is the canyon’s rock walls that allow people to develop their greatest perspective on geologic time, because of these rocks’ immense age, their fossil record, and because these rocks formed in environments far different than those found in northern Arizona today.