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Title: Geology of Devils Tower National Monument, Wyoming
A Contribution to General Geology
Author: Charles S. Robinson
Release Date: September 14, 2015 [EBook #49966]
Language: English
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Geology of
Devils Tower
National Monument
Wyoming
_By_ CHARLES S. ROBINSON
A CONTRIBUTION TO GENERAL GEOLOGY
[Illustration: DEPARTMENT OF THE INTERIOR · March 3, 1949]
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 to join U. S. Highway 14 at a point 29 miles northwest 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.
[Illustration: Figure 52.--Index map showing location of Devils
Tower National Monument.]
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.
The geology of the Devils Tower National Monument was mapped during the
summer of 1954 by the U. 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
McIntyre, Superintendent of Devils Tower National Monument.
GEOLOGY
The rocks exposed in the Devils Tower National Monument may be divided
on the basis of their origin into two general types; igneous and
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, mudstone, 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
to form a narrow bench. Above this bench, the sides again rise steeply,
at angles 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 Tower 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 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. 53_A_).
[Illustration: Figure 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.]
[Illustration: Figure 53.--B. Southwest corner of Devils Tower
showing the columns flaring out and merging to form the massive
base.]
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.
53_B_). 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 fine-grained
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 aegirite. 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.
[Illustration: Figure 54.--Generalized section of the sedimentary
rocks of the Devils Tower National Monument.]
System
Series
Formation and member; Thickness, in feet; Columnar section;
Description
JURASSIC
Upper Jurassic
Sundance formation
Redwater shale member; 100+; Gray and gray-green shale. Thin
fine-grained sandstones in lower part; thin fossiliferous
limestones in upper part
Lak member; 40-65; Yellow soft fine-grained calcareous sandstone
Hulett sandstone member; 60-70; Yellow massive fine-grained
calcareous sandstone
Stockade Beaver shale member; 85-100; Gray and gray-green shales
with thin calcareous sandstones
UNCONFORMITY
Middle Jurassic
Gypsum Spring formation; 15-35; White massive gypsum interbedded
with thin red mudstone
UNCONFORMITY
TRIASSIC
Spearfish formation; 100; Red to maroon siltstone and sandstone
interbedded with some thin shale
SEDIMENTARY ROCKS
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 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 450 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 Spearfish formation consists of red to maroon siltstone and
sandstone interbedded with mudstone or shale. Locally, greenish-blue
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 cliffs 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 sequence 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 consists 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 Redwater shale member (fig. 54) (Imlay, 1947, 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 fine-grained 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
fine-grained 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 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) well-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 gray-green 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.
The Sundance formation contains clams, oysters, belemnites (squids), and
other marine fossils that establish its age as Late Jurassic (Imlay,
1947, p. 244-264).
STREAM TERRACE DEPOSITS AND ALLUVIUM
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. 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
Tower 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 down 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 off 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 medium-grained brownish-white
sandstone and, what is apparently, a highly silicified 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.
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 one-half 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 down-fold 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 (pl. 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 is 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 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, which 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 when it was above sea level, the higher land
was eroded, and the sediments deposited at a lower level. When the area
was covered by the sea, marine sediments, principally shales, were
deposited. Near the end of the Cretaceous period, the sea made its final
withdrawal, and the sediments from late Cretaceous time to the present
were deposited in fresh water.
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 were
developed, and, probably, the igneous rocks, such as Devils Tower, were
intruded (Jaggar, 1901, p. 266). Following this, the Black Hills area
was 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 Tower 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 volcano
(Carpenter, 1888; Dutton and Schwartz, 1936). Another theory is that
Devils Tower and Missouri Buttes (a mass of the same type of rock 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 Jaggar, 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 laccolith or sill.
5. In a well drilled about 1-1/2 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 the 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.
_The following new material has been added to this booklet by the
National Park Service (Devils Tower National Monument, 1985)_
The most recent in depth, geologic study of Devils Tower was done by Don
L. Halverson (1980) and presented in a dissertation, to the Graduate
Faculty of the University of North Dakota.
He stated that, "The Missouri Buttes and Devils Tower, however, are
necks of extinct volcanoes which have been exposed by erosion. This
theory was first proposed by Carpenter (1888) and later expanded by
Dutton and Schwartz (1936). The material which fed these volcanoes came
from a minimum depth of 18 km. Evidence for this conclusion is listed in
the following statements:
1. The alloclastic breccia in the vicinity of Devils Tower and the
Missouri Buttes is definitely igneous in origin and probably
represents periods of violent eruption.
2. A very definite similarity exists between these two features and
the volcanic necks in the Taylor Mountain area of New Mexico.
3. The distinctive columns with basal flare are also found in the
volcanic necks of the Taylor Mountains (Dutton and Schwartz
1936), but have not been reported in columnar-jointed
laccoliths.
4. The Missouri Buttes and Devils Tower were intruded directly through
horizontal sediments without disrupting them, even in the
immediate vicinity of the igneous bodies.
5. Recent research indicates that many of the laccolithic intrusions
in the Black Hills region may have been less passive than
previously considered. Sundance Mountain may be a mixed
volcanic cone consisting of welded ash fall, massive quartz
latite, and ash flow tuffs. Nearby Sugarloaf Mountain is
composed of layered tuffs (Fashbough 1979).
6. Collapse of materials into partly evacuated reservoir chambers
accounts for the depressions surrounding the Missouri Buttes
and Devils Tower. The 90 m of depression at the southern end
of the Buttes is difficult to explain with a laccolithic
model.
7. Flow directions deduced from oriented thin-sections and field
observations indicate mostly vertical flow. It must be noted
that in both igneous bodies orientation of some grains is
horizontal; this could, however, simply indicate turbulent
flow.
8. The stability field for the analcime-liquid system is 5 kbar
minimum (Roux and Hamilton 1976), which indicates that the
original melt of Devils Tower and Missouri Buttes rock had to
originate at a minimum depth of 18 km.
9. It is unlikely that magma which had ascended from great depths and
had just penetrated the resistant Hulett Member of the
Sundance Formation, as well as the Lakota and Fall River
Formations, would be stopped abruptly by the less resistant
shales above. When the magma reached the shale beds, the
weight of the column of igneous rock could have exceeded the
strength of the shale, causing the magma to flow horizontally.
No indication of horizontal spread, however, is observed. The
continuously cylindrical shape of the intrusions indicates
that the magma moved steadily upward and probably reached the
surface.
10. Carbonatites have been found, and formally reported, in the nearby
Bear Lodge Mountains, and also as fragments in the alloclastic
breccias of the Missouri Buttes. Their presence suggest a high
volatile content for the magma and the possibility of
explosive volcanism."
[Illustration: 1.]
[Illustration: 2.]
[Illustration: 3.]
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.
Imlay, 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 phonolite 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, Wyo.): Earth Science Digest, v. 3, no. 1, p. 3-7.
Halverson, D. L., 1980, Geology and petrology of the Devils Tower,
Missouri Buttes and Barlow Canyon Area, Crook County Wyoming,
Dissertation.
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