Geologic Setting

Devils Tower lies on the west flank of the Black Hills/Bear Lodge uplift. The Bear Lodge uplift is a mountainous region in western South Dakota and northeastern Wyoming. This uplift, along with the Powder River Basin to the west, formed approximately 60 million years ago during the Laramide Orogeny. The Laramide Orogeny formed many of the Rocky Mountain ranges we are familiar with today. Some of these are the Bighorn Mountains, the Wind River Range, the Colorado Front Range and the Uinta Mountains in Utah.

 

Sequence of Geologic Events

The general sequence of events leading to the formation of the Tower is as follows:

1) Deposition of sedimentary rocks. The layers of sedimentary rocks in the Black Hills area are nearly two miles thick (Robinson et al., 1964)! Most of these layers were deposited in shallow seas beginning about 500 million years ago.

2) Uplift of the Black Hills. Tectonic processes formed the Black Hills sixty million years ago. During the Black Hills formation, the rocks were uplifted, fractured and faulted. The Black Hills is an elliptical dome. The process of erosion is greatest in the higher, central part of the dome. Therefore, the oldest rocks are exposed in the center of the dome. An example is the pre-Cambrian granites at Mount Rushmore. The younger rocks are exposed along the edges of the dome where there has been less erosion. These Triassic/Jurassic sedimentary rock layers are found near Devils Tower.

3) Intrusion of magma. About 50 million years ago, molten rock (magma) welled up into the older sedimentary rocks. This happened in many places across the northern part of the Black Hills. This was near the end of the Laramide Orogeny, during a time period known as the Tertiary (66 to 2 million years ago). One of these magma bodies would later become Devils Tower. However, the magma that formed the Tower never reached the surface and cooled underground. Thus, the Tower remained buried under more than a mile of sedimentary rock for many millions of years.

4) Regional uplift and erosion. The entire Rocky Mountain region (Montana, Wyoming, Colorado, Arizona, etc.) was lifted high above sea level about five million years ago. As a result, the rivers in the region began to flow much faster. They cut down rapidly into the soft, underlying sedimentary rocks. The material they carried away was deposited in the Gulf of Mexico and Gulf of California. As the Belle Fourche River removed the layers of softer sedimentary rocks in the Devils Tower area, the harder rock of the Tower itself was eventually exposed at the surface.

 

Geologic Tour

You are now ready to travel through geologic time at Devils Tower! The photo below contains links that will take you to the major events in the geologic history of the monument. To travel through time, start with the Sedimentary Rocks link, then the Igneous Rocks link and, finally the Erosion link.

See you in the Triassic!

 

Links

Devils Tower Main Index

Geologic Overview (current section)

Sedimentary Rocks

Igneous Rocks

Erosion

Review

 

Multimedia version

 

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