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Field Division of Education
The Geology of Devils Tower National Monument
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This report presents the results of a search of geological literature for important information concerning the character, structure, field relations, petrography, geological history, and theories as to the origin of Devils Tower. Much of the material presented is in the form of abstracts. Frequent references are made to original sources of information, and a bibliography of the more important literature dealing with the subject accompanies the report. Special emphasis has been placed on information which might be used in developing a museum story of this monument.

Extensive use was made of the Matthew Library, the Geology Library, and the Main Library of the University of California. Members of the faculty of the Geology Department of the University of California have been consulted concerning the more dubious phases of the subject.


Devils Tower is located in Northeastern Crook County, Wyoming, on the Belle Fourche River. It is situated in that group of hills, lying principally in South Dakota but extending westwardly into Wyoming, known as the Black Hills. This group of hills, rising several thousands of feet above the surrounding northern Great Plains, constitutes an outlying member of the Rocky Mountain System. The hills have been carved from a dome-shaped uplift of the earth's crust, and consist largely of rocks which are older than those forming the surface of the Great Plains. The length of this elevated area is about one hundred miles and its greatest width is fifty miles. It is believed that about the same time the uplifting of these beds occurred, igneous activity became operative, resulting in the intrusion of numerous igneous bodies assuming various shapes. Where they cut the older rocks (Algonkian) consisting of schists and slates tilted on end, and where the lines of least resistance lie in an approximately vertical direction, a great profusion of dikes conforming with the strike and dip of the slate are formed. When these eruptives reached the Cambrian and later formations, the lines of least resistance lay in a horizontal direction, and eruptives, on encountering the heavier members of these formations, found it easier to insinuate themselves between the easily cleavable shales and sandstones than to break through the heavy overlying rocks. Therefore the predominant type of intrusion in the Cambrian is the intrusive sheet. In the higher formations especially, if the intruded mass has been large and the force of intrusion great, not only has the igneous rock been spread out between the sediments, but it has domed up the overlying formations, producing a laccolith.

The old dome-shaped uplift of the Black Hills has been truncated by erosion so that the salient features of the present topography consist of concentric ridges and valleys carved from strata of varying degrees of hardness. Protruding above these ridges and valleys are the denuded igneous bodies which, being more resistant to erosion than the surrounding sediments, stand out as prominent features of the landscape. The most spectacular of these igneous masses, even though smaller in size, is Devils Tower, which rises 600 feet above its base of rounded hills, being visible for many miles. Because of its unique beauty and its prominence as a landmark during the days of early exploration, it was set aside as a national monument by Presidential Proclamation, September 24, 1906.

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