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GEOLOGICAL SURVEY BULLETIN 1021-1

A CONTRIBUTION TO GENERAL GEOLOGY

By CHARLES S. ROBINSON

ABSTRACT

Devils Tower is a steep-sided mass of igneous rock that rises above tne sur-

^ ronnding 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.

i The hills in the vicinity and at the base of the Tower are composed of red, 

1 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 

A Gypsum Spring formation, of Middle Jurassic age, and the Sundance formation, 

of Late Jurassic age. The Sundance formation consists of the Stockade Beaver 

~t, 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

k . 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 

i 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 Creole County in northeastern Wyoming (fig. 52). A paved road from the entrance of the National

289
290 CONTRIBUTIONS TO GENERAL GEOLOGY

FIGURE 52. Index map showing location of Devils Tower National Monument.

Monument goes south 7 miles to join U. S. Highway 14 at a point 

29 miles nortliAvest of Sundance, Wyo., and 33 miles northeast of Moorcroft, Wyo. The entrance to the National Monument may also be 

reached by a road (paved in Wyoming) that goes northeastward 

from the entrance, via Hulett and Aladdin, Wyo., to Belle Fourche, 

S. Dak., a distance of about 54 miles, where it joins U. S. Highways 

212 and 85. '

Public campgrounds and a natural history museum are maintained 

by the National Park Service at the base of the Tower about 3 miles 

by paved road from the Monument entrance.
GEOLOGY OF DEVILS TOWER NATIONAL MONUMENT 291

The geology of the Devils Tower National Monument was mapped 

during the summer of 1954 by theU. S. Geological Survey in collaboration with the National Park Service. The work was part of a study 

of the geology of the northern and western parts of the Black Hills 

region conducted by the Survey on behalf of the Division of Raw 

Materials of the U. S. Atomic Energy Commission. The author 

wishes to acknowledge the assistance of the National Park Service 

and, in particular, Mr. Raymond Mclntyre, Superintendent of Devils 

ToAver National Monument.

GEOLOGY

The rocks exposed in the Devils Tower National Monument may bo 

divided on the basis of their origin into two general types; igneous 

arid sedimentary. The Tower itself is composed of igneous rock; 

that is, rock formed directly by cooling and crystallization of once 

molten materials. The rocks exposed in the remainder of the Monument are sedimentary; that is, they were formed by the consolidation 

of fragmental materials derived from other rocks or accumulations 

of chemical precipitates that were deposited either on the floors 'of 

prehistoric seas or near the shores of such seas. These rocks, which 

crop out around the igneous mass, are layers of shale, sandstone, siltstone, nuidstone, gypsum, and limestone. Devils Tower owes its impressiveness to the differing rates of erosion of these rock types 

the soft sedimentary rocks erode more easily than the hard igneous 

rock and to the contrast of the somber color of the igneous column to 

the brightly colored bands of sedimentary rock that surround its base.

DEVILS TOWER

Devils Tower rises steeply for about 600 feet from a broad talus 

slope at its base. The top of the Tower, at an altitude of 5,117 

feet, is about 1,270 feet above the Belle Fourche River. The Tower is 

about 800 feet in diameter at the base. The sides rise almost vertically 

from the base for a distance of from 40 to 100 feet and then slope in 

more gently vto form a narrow bench. Above this bench, the sides 

again rise steeply, at tingles of 75° to over 85°, to within about 100 

feet of the top where the angle becomes less steep and the top edge of 

the Tower is somewhat rounded. The top of the ToAver is almost 

flat and measures about 180 feet from east to west and about 300 feet 

from north to south.

One of the most striking features of the Tower is its polygonal 

columns (fig. 53). Most of the columns are 5 sided, but some are 

4 and 6 sided. The larger columns measure 6 to 8 feet in diameter at 

their base and taper gradually upward to about 4 feet at the top. The 

columns are bounded by well-developed smooth joints in the middle
292 CONTRIBUTIONS TO GENERAL GEOLOGY

FiGL'HE 53. A. Northwest skip (if Devils Tower showing how the columns taper or converge 

and in places unite near the top and are cut by numerous cross-fractures. J}. South side 

of Devils Tower showing the columns flaring out and merging to form the massive base.
GEOLOGY OF DEVILS TOWER NATIONAL MONUMENT 293

part of the Tower, but as the columns taper upward, the joints between them, rather than being smooth, may be wavy and some of the 

columns may unite. Numerous cross-fractures in the upper part of the 

Tower divide the column into many small irregularly shaped blocks 

(fig. 534).

The columns in the central and upper parts of the Tower are almost 

vertical but flare out at the bench about 100 feet above the base (fig. 

53Z?). On the southwest side the columns are nearly horizontal. 

Where the columns flare out, several columns may join to form a larger, 

less distinct column that merges with the massive base.

At the base of the tower, below the bench, the rock is massive 

and jointing, poorly developed. Here the joints form large irregularly shaped blocks rather than columns.

Columnar joints form as the result of contraction within a rock mass. 

In igneous rock the contraction is the result of cooling; that is, the 

cold solidified rock requires less volume than the same rock when 

molten. As a rock cools it contracts, and the resulting tension is in a 

plane parallel to the cooling surface. When rupture takes place, 

three fractures radiate from numerous centers in the plane parallel 

to the cooling surface. Ideally, the fractures are at 120° to each other. 

If the centers were evenly distributed, the fractures from different 

centers would join forming hexagonal (6 sided) columns. These 

fractures will go deeper and deeper into the rock as cooling progresses. 

This condition because of many factors, is seldom attained in nature, 

so the columns may have 4, 5, 6, or even more sides.

The rock making up Devils Tower is classified as phonolite porphyry 

(Darton and O'Harra, 1907, p. 6) and is of Tertiary age. The fresh 

specimens have a light- to dark-gray or greenish-gray very finegrained groundmass with conspicuous crystals of white feldspar 

commonly about one-fourth to one-half inch in diameter and 

smaller very dark-green crystals of pyroxene. On the weathered 

surfaces the phonolite porphyry is a light gray or brownish gray. 

Lichens growing on the rock may give it a green, yellowish-green, or 

brown color.

Using a microscope, Albert Johannsen (Darton and O'Harra, 1907, 

p. 6) identified the feldspar crystals as a soda-rich orthoclase and the 

pyroxene crystals as augite with an outer zone of aegiritei In addition, phenocrysts of apatite and magnetite, were identified. The 

groundmass, according to Johannsen, consists of orthoclase laths in 

subparallel arrangement, needles of aegirite, possibly some nephelite, 

small cubes of magnetite, and secondary minerals of calcite, kaolin, 

chlorite, analcite, and a anisotropic zeolite.

370722 56- 2
294 CONTRIBUTIONS TO GENERAL GEOLOGY

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JURASSIC

TRIASSIC

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Upper Jurassic

Middle Jurassic

Formation and 

member

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Red water shale 

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Lak member

Hulett sandstone 

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Stockade Beaver 

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Spearfish formation

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Description

Gray and gray-green shale. Thin fine-grained 

sandstones in lower part; thin fossiliferous 

limestones in upper part

Yellow soft fine-grained calcareous sandstone

 Yellow massive fine-grained calcareous sandstone

Gray and gray-green shales with thin calcareous 

sandstones

i White massive gypsum interbedded with thin 

red mudstone

Red to maroon siltstone and sandstone inter - 

bedded with some thin shale

FIGURE 54. Generalized section of the sedimentary rocks of the Devils Tower 

National Monument.

SEDIMENTARY BOCKS

The sedimentary rocks that surround Devils Tower have a total 

exposed thickness of about 400 feet. They are divided, from oldest 

to youngest, into the Spearfish formation of Triassic age, the Gypsum 

Spring formation of Middle Jurassic age, and the Sundance formation of Late Jurassic age (fig. 54).

SPEARFISH FORMATION

The Spearfish formation crops out in the southern and northeastern 

parts of the Devils Tower National Monument along the valley of the
GEOLOGY OF DEVILS TOWER NATIONAL MONUMENT 295

Belle Fourche River and its tributaries and forms conspicuous brownish-red to maroon cliffs that border the Belle Fourche valley for 

several miles in the Devils Tower region. The formation is 4.50 to 

600 feet thick in the northern Black Hills area (Darton, 1909, p. 28); 

however, only the uppermost 100 feet are exposed within the National 

Monument.

The Speariish formation consists of red to maroon siltstone and 

sandstone interbedded with mudstone or shale. Locally, greenishblue shale partings are found in the siltstone and sandstone. The 

formation is poorly cemented and weathers very easily forming, for 

the most part, gentle slopes, as on the northeast and southwest sides 

of the monument. Where it does form cliffs, as south of the Tower, 

the cli ffs are cut by many sharp gullies.

No fossils have, been found in the Spearfish formation in the 

Devils Tower region, but elsewhere in Wyoming, stratigraphically 

equivalent rocks contain land vertebrates of Triassic age.

GYPSUM SPRING FORMATION

The Gypsum Spring formation is exposed in a thin but almost 

continuous band around the Tower on the southwest to northeast 

sides. It also crops out near the top of the small hill at the eastern 

boundary of the National Monument, a few hundred feet north of the 

Registration Building. This formation is composed mostly of white 

gypsum, which stands out conspicuously between the red beds of 

the underlying 'Spearfish formation and beds of gray-green shale 

at the base of the overlying Sundance formation.

The Gypsum Spring formation ranges in thickness from about 15 

to about 35 feet. It is thickest on the hill at the eastern boundary 

of the Monument. Here the formation is made up of a lower unit 

consisting of a bed of white massive gypsum 20 feet thick overlain 

by 14 feet of interbedded white gypsum and dark-maroon mudstone. 

The formation is 15 feet thick along the cliff directly south of Devils 

Tower. At this place, the formation consists of 12 feet of white massive gypsum interbedded with 1-6 inch thick beds of dark-maroon 

mudstone overlain by 3 feet of dark-brownish-red mudstone. The 

differences in thickness are primarily the result of erosion of the 

Gypsum Spring formation prior to the deposition of the Stockade 

Beaver shale member of the Sundance formation (Imlay, 1947, p. 243).

SUNDANCE FORMATION

The Sundance formation consists of an alternating sequense of 

greenish-gray shale, light-gray to yellowish-brown sandstone and 

siltstone, and gray limestone. The formation crops out above the 

gypsum and red shale of the Gypsum Spring formation on the bluffs 

and low rolling hills that surround the Tower. The formation con-
296 CONTRIBUTIONS TO GENERAL GEOLOGY

sists of four members that are, in order of age from oldest to youngest, 

the Stockade Beaver shale member, the Hulett sandstone member, the 

Lak member, and the Kedwater shale member (fig. 54) (Imlay, 194f, 

p. 227-273).

Stockade Beaver shale member. In general, this member, because 

it is composed mostly of shale, is poorly exposed. The best exposures 

of the lower part are on the hill at the east boundary of the Monument 

and along the steep slope south of the Tower. The upper part is 

fairly well exposed on the south side of the ridge north of the Tower, 

near the north boundary of the Monument. The member has a 

thickness of 85 to 100 feet.

The composition differs considerably in detail from one exposure 

to another, but in general it consists of gray-green shale with interbedded fine-grained calcareous sandstone. At the base of the member, 

at nearly all exposures, is a thin sandstone, 1 to 24 inches thick, containing black or dark-gray water-worn chert pebbles that have a 

maximum dimension of about 2 inches. Above the basal sand, the 

lower half of the member is composed mostly of gray-green shale, 

which locally contains some interbedded fine-grained calcareous 

sandstone, thin sandy and shaly limestone or dolomitic limestone, and 

rarely thin beds of red mudstone. The upper half of the member 

consists of dark-gray to gray-green shale with interbedded finegrained calcareous sandstone that range from less than 1 foot to 6 

feet in thickness.

The contact of the Stockade Beaver shale member with the overlying Hulett sandstone member is gradational. The sandstone becomes more abundant in the upper part of the Stockade Beaver shale, 

and the contact between those two members is placed at that point 

where the sandstone makes up more than 50 percent of the rocks.

Hulett sandstone member. The Hulett sandstone member is resistant to weathering and forms a conspicuous, almost vertical, cliff 

that nearly encircles the Tower. This member ranges in thickness 

from about 60 to 70 feet.

The Hulett sandstone member consists, in general, of massive finegrained glauconitic calcareous sandstone. It is typically yellow or 

brownish yellow but locally may be pink or red. In the lower 5 to 

10 feet the sandstone is in beds from less than 1 inch to 2 feet thick 

separated by gray or greenish-gray shale partings of from less than 

1 inch to 6 inches thick. Many of the sandstone beds at the base of 

the member are ripple marked.

The 50 to 60 feet in the middle of the member consists of massive 

beds that range in thickness from 5 to 20 feet. This portion is well 

cemented and forms the conspicuous cliff seen throughout the area. 

The upper 5 to 10 feet is thin bedded (beds from less than 1 inch-to 6
GEOLOGY OF DEVILS TOWER NATIONAL MONUMENT 297

inches in thickness) locally shaly, and poorly cemented. This grades 

upward into the overlying sandstone and siltstone of the Lak member.

Lak member. The Lak .member crops out above the cliff of Hulett 

sandstone that almost encircles the Tower, and it underlies a broad 

rolling area in the northwestern part of the Monument. The member 

is rarely exposed because it is composed of soft sandstone and siltstone that usually weather to gentle slopes and become covered with 

vegetation. The best exposure is on the steep hill east of the Tower 

and northwest of the bridge across the Belle Fourche river.

This member is 65 feet thick a few hundred feet east of the 

Tower, but mapping within the Monument and measured sections 

within a few miles of the Monument indicate that the average thickness is about 45 feet.

The Lak member is typically poorly bedded soft very fine-grained 

calcareous sandstone and siltstone with a few thin gray-green sandy 

shale partings. At the base and near the top of the member may be a 

few thin (less than 1 inch to 6 inches thick) Avell-cemented sandstone 

beds that form small ridges. The sandstone and siltstone grade almost 

imperceptibly from one to the other. The color ranges from light 

yellow brown and yellow to red. In the Devils Tower area, shades 

of yellow and yellowish brown are most common.

The contact of the Lak with the overlying Redwater shale member 

can be observed only in the exposure east of the Tower. Here, the 

upper 3 feet of the Lak is a yellowish-brown calcareous silty sandstone with a few discontinuous sandy shale partings (less than 1 inch 

thick), and the lower 3 feet of the overlying Redwater shale consists 

of dark-gray-green shale with interbedded, thin silty sandstone.

Redwater shale member. This member encircles Devils Tower, but 

at most places it is covered by talus from the Tower. Even where it 

is not covered by talus, it is poorly exposed. It consists mostly of 

shale that weathers into gentle slopes, which are usually covered by 

vegetation. The Redwater shale is partly exposed on Fossil Hill, 

northwest of Devils Tower, and on the hill in the northwest corner of 

the Monument. The best exposures are on Fossil Hill.

The top of the Redwater shale member is not exposed within the 

limits of the Monument; consequently, the thickness could not be 

determined. In surrounding areas the Redwater shale ranges in 

thickness from 150 to 190 feet. It is at least 100 feet thick on the hill 

in the northwest corner of the Monument.

The Redwater shale consists mostly of light-gray to dark graygreen soft shale. In the lower 20 or 30 feet are beds of yellow soft 

sandstone, 3 inches to 2 feet thick. In the upper part, ranging from 

50 feet above the base to the top, are lenticular beds of fossiliferous 

limestone 1 inch to 4 feet thick. Two such beds of fossiliferous limestone are exposed on Fossil Hill.
298 CONTRIBUTIONS TO GENERAL GEOLOGY '

The Sundance formation contains clams, oysters, belemnites 

(squids), and other marine fossils that establish its age as Late Jurassic **" 

(Imlay, 1947, p. 244-264).

STEEAM TERRACE DEPOSITS AND ALLUVIUM »-f*

Stream deposits (alluvium) are found in the valleys of the small 

streams around the Tower and, in particular, in the valley of the Belle 

Fourche River, that cuts across the southeast corner of the Monument. f 

The deposits consist of unconsolidated gravel, sand, silt, and mud. ,

Along the Belle Fourche River, northwest of the river and between 

it and the main road, the river cut a terrace in the Spearfish formation. -* 

On the terrace were deposited gravel and sand.

TALUS AND LANDSLIDE MATERIAL

The talus and landslides are composed primarily of the material 

from the ToAver and the Hulett sandstone.

Talus from the Tower forms a broad apron that completely surrounds the Tower. The talus extends from high on the shoulders of 

the Tower down to and across the sedimentary rock. Locally, landslides of this talus have extended through valleys in the sedimentary «, 

rock clown almost to the level of the surrounding streams. The talus 

from the Tower is composed of fragments of the columns that range 

from a few inches in diameter to large sections of the columns as much 

as 8 feet in diameter and 25 feet long.

The cliff of Hulett sandstone that surrounds the Tower breaks oil * 

into rectangular blocks that form talus slopes at the base of the cliffs , and locally large landslides down the hill below the cliffs. These 

blocks of Hulett sandstone range in size from a few inches to many 

feet in diameter. The talus material from the Tower has at several 

places overlapped the cliff of Hulett sandstone and become mixed 

with the material from the cliff. *

About 1,400 feet north of the Tower are two patches of what is 

believed to be talus formed from sedimentary rocks that once surrounded the Tower. The talus consists of fragments of mediumgrained brownish-white sandstone and, what is apparently, a highly 

silicih'ed gray or white fine-grained quartzite. The sandstone resembles that found in the Lakota (Darton and O'Hara, 1907, p. 3) that 

lies about 200 feet stratigraphically above the Redwater shale in the 

area west of the Monument.

The sandstone and quartzite occur in angular blocks that range 

from less than 1 inch to several feet in diameter. The spaces between 

the blocks are filled with a yellowish or brownish-white sand.

The Lakota sandstone at one time surrounded the Tower and it is 

believed that-these blocks are residual blocks that have not been 

removed by erosion.
GEOLOGY OF DEVILS TOWER NATIONAL MONUMENT 299

STRUCTURE

The sedimentary rocks in the National Monument, and in the surrounding area, are gently folded into many small rolls, basins and 

domes, which .locally are cut by faults of small displacement. These 

small folds are superimposed on a large dome that is collapsed in the 

middle.

Devils Tower is near the middle of the collapsed dome. From onehalf to about a mile from the Tower the sedimentary rocks dip gently 

from 2° to 5° away from the Tower to form a broad dome. Within a 

radius of about 2,000 to 3,000 feet of the Tower, the dips change, and 

the rocks dip, in general, from 3° to 5° towards the Tower to form a 

shallow structural basin. In the basin itself and on the dome are 

several small folds. As an example, Spring No. 1 southwest of the 

Tower is in the center of a comparatively sharp syncline or downfold at the edge of the basin. Fossil Hill northwest of the Tower is 

another small structural basin. The beds along the top and on the 

north side of Fossil Hill dip from 12° to 52° S. Those on the south 

side of the hill, north of the road, apparently dip very gently northward.

Three faults were observed in the area of the National Monument. 

Two of the faults are in the Hulett sandstone west of the main road 

and west of the Tower, and the third is in the northwestern side of the 

Tower near its base (pi. 30). The faults in the Hulett sandstone are 

probably vertical, and the displacement along them is believed to be 

less than 10 feet. The fault at the base of the Tower is a low-angle 

fault that trends northwesterly. The attitude of this fault at the 

point where it disappears below the talus is: strike, N. 41° W.; dip, 

21° NE. The fault zone is 4 to 12 inches wide and is filled with a 

yellowish-green clay and sheared fragments of altered phonolite 

porphyry. The rock of the Tower below this fault in somewhat 

altered; the groundmass is a light greenish gray, and the normally 

shiny crystals of feldspar have a dull earthy luster.

GEOLOGIC HISTORY

The geologic history of the Devils Tower area is part of that of the 

Black Hills region. The uplift of the Black Hills and the subsequent 

erosion have exposed the rocks, from which the geologic history of the 

area may be interpreted.

Most of the rocks within the area around the Black Hills are composed of sediments deposited by water. These sedimentary rocks, 

which overlie much older rocks (Precambrian), were deposited in a 

series of successive layers during time intervals from the Cambrian 

period to well into the Tertiary period. Deposits in the ancient seas 

are represented by limestone, shale and some sandstone; deposits on
300 CONTRIBUTIONS TO GENERAL GEOLOGY

low lands adjacent to seas, as flood plains and deltas, by sandstone, 

siltstone, and some mudstone; and deposits along streams by conglomerate, sandstone and siltstone. Between the periods of deposition 

were intervals when the land was relatively high, and in certain areas 

all of the sediments of an earlier period were eroded away.

The oldest formation exposed in the National Monument, the Spearfish formation, was deposited during Triassic time along flat lands 

bordering the sea. Arms of the sea locally invaded the area to leave 

deposits of gypsum, Avhich are found near the base of the Spearfish 

in areas outside the National Monument. The Gypsum Spring formation was deposited in the sea in Middle Jurassic time following a 

period of uplift and erosion that occurred after the deposition of the 

Spearfish formation. After the Gypsum Spring formation was deposited, the sea retreated, and another period of erosion followed 

before Late Jurassic time when the sea invaded the area again and 

the Sundance formation was deposited. The depth and conditions for 

deposition in this sea changed from time to time, as shown by the 

alternating beds of shale and sandstone in the Sundance formation.

Following the deposition of the Sundance formation, there were 

several periods when the area was above sea level and when the sea 

covered the area. During the periods Avhen it Avas above sea level, 

the higher land Avas eroded, and the sediments deposited at a loAver 

level. When the area Avas covered by the sea, marine sediments, principally shales, Avere deposited. Near the end of the Cretaceous period, 

the sea made its final AvithdraAval, and the sediments from late 

Cretaceous time to the present Avere deposited in fresh Avater.

The Black Hills uplift developed primarily during early Tertiary 

time, although it may have started in very late Cretaceous time. At 

this time the present general structural features of the Black Hills 

area Avere developed, and, probably, the igneous rocks, such as DevilsToAver, were intruded (Jaggar, 1901, p. 266). FolloAving this, the 

Black Hills area Avas repeatedly uplifted, and erosion exposed the 

older sedimentary and intrusive rocks. Even today streams continue 

to strip more and more rock from the country, leaving only local deposits, such as alluvium and terrace deposits, along the valleys.

ORIGIN OF DEVILS TOWER

The origin of Devils ToAver has been a matter of speculation for 

many years, and even today after detailed geologic mapping of the 

area, no conclusive proof of its mode of origin can be presented.

Several theories of the origin have been proposed. One of the 

more popular of these is that it is the neck of an extinct A7 olcano (Carpenter, 1888; Dutton and Sch\vartz, 1936). Another theory is that 

Devils Tower and Missouri Buttes (a mass of the same type of rock
GEOLOGY OF DEVILS TOWER NATIONAL MONUMENT 301

about 4 miles northwest of the Tower) are the remnants of a laccolith (a tabular intrusive igneous body, thickest in the middle, and with 

a relatively level floor), the vent for which was under Missouri 

Buttes (Jaggar, 1901, p. 264). Darton (1901, p. 69) believed that 

the Tower is the remnant of a laccolith, smaller than the one proposed 

by Jagger, the feeding vent for which was underneath the Tower.

Much more detailed geologic work will have to be done in the 

surrounding area before the mode of origin of Devils Tower may be 

proved conclusively. The evidence gathered during the present investigation, however, suggests that Devils Tower is a body of intrusive igneous rock, which was never much larger in diameter than the 

present base of the Tower, and which at depth (1,000 feet or more) 

is connected to a sill or laccolith type body. The bases for this theory 

are 

1. The exposed portion of the Tower is the result of recent erosion. At 

the time of its intrusion it was surrounded and probably covered by 

several hundred feet of sedimentary rock.

2. The mineral composition and texture are more typical of shallow intrusive rocks, which are formed at depth, than extrusive rocks, which are 

formed on the surface.

3. No evidence of extrusive igneous activity has been found in the surrounding area.

4. Missouri Buttes, about 4 miles to the northwest, and the Tower have 

the same composition so it is assumed that they were derived from a 

common magma; possibly the magma of a large intrusive body, such as a 

lacccolith or sill.

5. In a well drilled about 1% miles southwest of Missouri Buttes, near 

the center of a structural dome, rock similar to the Tower and Missouri 

Buttes was encountered at about 1,400 feet below the base of Missouri 

Buttes. Inasmuch as the thickness of the sedimentary rocks in this 

area is normally much greater than this depth, the rock in tha drill 

hole probably represents an intrusive body, rather than the Precambrian 

igneous rocks upon which the younger sedimentary rocks were deposited.

6. The relatively small amount of talus, slope wash, or terrace gravel derived 

from the Tower and Missouri Buttes suggests that they have not been 

extensively eroded, and therefore the original igneous bodies were not 

much larger than at present.

7. Columnar .jointing is common in intrusive bodies formed at comparatively 

shallow depths.

SELECTED BIBLIOGRAPHY

Carpenter, F. R., 1888, Notes on the geology of the Black Hills: Preliminary 

report of the South Dakota School of Mines, Rapid City, S. Dak.

Darton, N. H., 1909, Geology and water resources of the northern portion of 

the Black Hills and adjoining regions in South Dakota and Wyoming: U. S. 

Geol. Survey Prof. Paper 65.

Darton, N. H., and O'Harra, C. C., 1907, Description of the Devils Tower quadrangle, Wyoming: U. S. Geol. Survey Geol. Atlas, folio 150.

Dutton, C. E., and Schwartz, G. M., 1936, Notes on the jointing of the Devil's 

Tower, Wyoming: Jour. Geology, v. 44, no. 6, p. 717-728.
302 CONTRIBUTIONS TO GENERAL GEOLOGY

Iinlay, R. W., 1947, Marine Jurassic of the Black Hills area, South Dakota and

Wyoming: Am. Assoc. Petroleum Geologists Bull., v. 31, no. 2, p. 227-273. 

Jaggar, T. A., Jr., 1901, Laccoliths of the Black Hills: U. S. Geol. Survey 21st

Ann. Report, pt. 3, p. 163-290. 

Pirsson, L. V., 1894, On some phouolite rocks from the Black Hills: Am. Jour.

Sci., 3d ser., v. 47, p. 341-346. 

Zuidema, H. P., 1948, The touring public discovers Mato Tipi (Devils Tower,

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U. S. GOVERNMENT PRINTING OFFICE: 1956