Geologic Column: Definition & Example

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lesson, you will be able to explain what a geologic column is, how it forms, and give an example of a geologic column. A short quiz will follow.

What is a Geologic Column?

Have you ever been to the Grand Canyon? Or seen exposed layers of rock on a bare column in Arizona? The rock there is interesting, and often very attractive. This is because it contains striped layers which can be clearly seen. These striped layers are called a geologic (or stratigraphic) column.

Example of Visible Layers of Rock
Example of Visible Layers of Rock

A geologic column is a series of layers of different types of rock, placed by the settling of sediments over millions of years, when the rock was a sea bed. If that rock is now on dry land, it can be seen and studied by humans. These layers are often colorful and dramatically different to each other, and each of the layers represents the kinds of sediments that were most common in that era. Rocks take millions of years to form, and so by looking at the layers of rock and measuring their age, we can figure out what was happening in that area of the earth all those years ago. These layers can go back as far as billions of years in some cases, and provide valuable information about the earth's history.

Example of a Geologic Column: The Grand Canyon

One of the best and most visited examples of a geologic column is the Grand Canyon. Since the Grand Canyon is so deep, there are a huge number of layers that have become exposed and visible. These layers go back as far as 1.84 billion years.

This diagram shows the layers that can be seen at the Grand Canyon:

The Geologic Column at the Grand Canyon
The Geologic Column at the Grand Canyon

The blue part of the diagram shows rocks from the Paleozoic era, a time which stretches from about 542 million years ago to about 251 million years ago. During that time one of the earth's supercontinents was forming and then breaking apart again, so it was a time of great change on the earth. At the Grand Canyon a big part of the Paleozoic era can be seen, with a total of 11 individual layers---a mixture of sandstone, limestone and shale.

Underneath the Paleozoic rocks should be Proterozoic (or Precambrian) rocks from 542 million or more years ago, since the Proterozoic era preceded the Paleozoic time period. But instead, if you look at the lower Grand Canyon, in many places the rock seems to jump straight to 1.68 billion years old---still part of the Proterozoic era, but much further back in time than you might expect. This jump is called the Great Unconformity.

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