Fossils and relative dating

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Nonetheless, how geologists determine the age of rocks is a mystery to many members of the public, and even to many park rangers [see Photo 2], guides, and others who share the canyon’s geologic story with others. ” when geologists say a Grand Canyon rock formed 270 million years ago.

Further confusion arises when one publication or geologist says, for example, that the Kaibab Formation is 270 m.y. The same questions arise for the other rock units at Grand Canyon. This article will answer these questions by providing a short primer on geologic dating methods and how they were applied to Grand Canyon rocks.

The most obvious feature of sedimentary rock is its layering.

This feature is produced by changes in deposition over time.

The Grand Canyon landscape is geologically young, being carved within just the last 6 m.y.

There are two basic approaches: relative age dating, and absolute age dating.

Here is an easy-to understand analogy for your students: relative age dating is like saying that your grandfather is older than you.

Absolute age dating is like saying you are 15 years old and your grandfather is 77 years old.

We then describe Grand Canyon rocks as belonging to three “sets,” or packages of rocks, each with unique geologic histories, and present a compilation of “best” numeric ages for Grand Canyon rocks to use when explaining Grand Canyon geology to the public.

Photo 1: Many people consider Grand Canyon National Park the world’s premiere geologic landscape and a “geologic wonder”.

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