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With a rock record that spans more than 1500 m.y., Grand Canyon is truly a panoramic view into the geologic past.

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.

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

Unlike people, you can’t really guess the age of a rock from looking at it.

How do we know this and how do we know the ages of other events in Earth history?