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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.

Grand Canyon National Park [see Photo 1] is one of the best places in the world to gain a sense of geologic, or “deep,” time because the canyon exposes a great swath of geologic history.

Rocks exposed in Grand Canyon are truly ancient, ranging from 1840 million years old (m.y.), or 1.84 billion years old (b.y.), to 270 m.y.

For example, in Grand Canyon, there are many examples of sandstone deposition on a beach while limestone was forming off shore.

Geologists usually correlate sedimentary rocks based on their fossil assemblages, particularly on the presence of index fossils.

Rock type (or lithology) is usually not a good basis for correlation.

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