Geology of
Devils Tower
National Monument
Wyoming

By CHARLES S. ROBINSON

A CONTRIBUTION TO GENERAL GEOLOGY

The National Park Service and the Devils Tower Natural History Association wishes to thank the United States Geological Survey for their kind permission to have this Bulletin reprinted with minor changes.

CONTENTS

Page

Abstract 1

Introduction 1

Geology 3

Devils Tower 3

Sedimentary rocks 6

Spearfish formation 6

Gypsum Spring formation 7

Sundance formation 7

Stockade Beaver shale member 8

Hulett sandstone member 8

Lak member 9

Redwater shale member 9

Stream terrace deposits and alluvium 10

Talus and landslide material 10

Structure 11

Geologic history 11

Origin of Devils Tower 12

Selected bibliography 13

ILLUSTRATIONS

FIGURE Page

52.—Index map showing location of Devils Tower National Monument 2

53.A.—Northwest side of Devils Tower showing how the columns taper or converge and in places unite near the top and are cut by numerous cross-fractures 4

B.—Southwest corner of Devils Tower showing the columns flaring out and merging to form the massive base 4

54.—Generalized section of the sedimentary rocks of the Devils Tower National Monument 6

A CONTRIBUTION TO GENERAL GEOLOGY
GEOLOGY OF DEVILS TOWER NATIONAL MONUMENT, WYOMING

By Charles S. Robinson

ABSTRACT

Devils Tower is a steep-sided mass of igneous rock that rises above the surrounding hills and the valley of the Belle Fourche River in Crook County, Wyo. It is composed of a crystalline rock, classified as phonolite porphyry, that when fresh is gray but which weathers to green or brown. Vertical joints divide the rock mass into polygonal columns that extend from just above the base to the top of the Tower.

The hills in the vicinity and at the base of the Tower are composed of red, yellow, green, or gray sedimentary rocks that consist of sandstone, shale, or gypsum. These rocks, in aggregate about 400 feet thick, include, from oldest to youngest, the upper part of the Spearfish formation, of Triassic age, the Gypsum Spring formation, of Middle Jurassic age, and the Sundance formation, of Late Jurassic age. The Sundance formation consists of the Stockade Beaver shale member, the Hulett sandstone member, the Lak member, and the Redwater shale member.

The formations have been only slightly deformed by faulting and folding. Within 2,000 to 3,000 feet of the Tower, the strata for the most part dip at 3°-5° towards the Tower. Beyond this distance, they dip at 2°-5° from the Tower.

The Tower is believed to have been formed by the intrusion of magma into the sedimentary rocks, and the shape of the igneous mass formed by the cooled magma is believed to have been essentially the same as the Tower today. Devils Tower owes its impressiveness to its resistance to erosion as compared with the surrounding sedimentary rocks, and to the contrast of the somber color of the igneous column to the brightly colored bands of sedimentary rocks.

INTRODUCTION

Devils Tower, a mass of bare rock that rises abruptly from the surrounding grasslands and pine forests, is one of the most conspicuous geologic features of the Black Hills region. Because of its scenic beauty and scientific interest, President Theodore Roosevelt in 1906 established Devils Tower and a small surrounding area as the first National Monument.

The Devils Tower National Monument covers an area of about 2 square miles near the center of Crook County in northeastern Wyoming (fig. 52). A paved road from the entrance of the National Monument goes south 7 miles