The Geologic Story

Introduction: Written in the Rocks

Like pages in a book, the rock layers of Black Canyon tell a story of past environments, ancient animals and dynamic processes of change.
drawing of rock layers in Black Canyon with labels
The rocks in the canyon tell a story of deep geologic time.

Modified stratigraphic column from Wallace R. Hansen's "The Black Canyon of the Gunnison," USGS Bulletin 1191, published in 1965.

Geologic Time

One spin of the Earth on its axis is a day, and one trip around the sun is a year.

While this concept seems rather straightforward, for centuries, scholars have sought to understand time and its relationship to the age of the Earth. Today, geologists estimate that the Earth is 4.6 billion years old! Who can fathom such an expanse of time?

Geologists have designed a very special type of calendar in order to grasp Earth's long history. This geologic time scale is very different from the familiar calendar we use to keep track of our lives. In some ways, the geologic time scale is more like a book, with the rocks as pages. Geology gives us the tools to help decipher and read this incredible book!

Geologic time scale has its own unique set of time divisions. The largest division is called an eon. Eons, which can span billions of years, are subdivided into eras, which are subdivided into periods, which are subdivided into epochs, which are subdivided into ages, etc.

The names used to designate the divisions of geologic time may seem bewildering at first glance, but nearly every name represents an historic breakthrough in geologic thought.

Additional Resources

Geologic Time, Online Edition by William L. Newman

This USGS online publication provides a thorough introduction to relative verses, absolute dating and the developmental history of the geologic time scale.

Fossils, Rocks and Time, Lucy E. Edwards and John Pojeta, Jr.

In this USGS online publication, learn the important role that fossils play in shaping the geologic time scale.


Last updated: April 9, 2022

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