Part 2

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 1½ 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.”

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.

Transcriber’s Notes

—Publication information is from the original print copy—this e-text is public domain in the country of publication.

—Silently corrected palpable typos; left non-standard spellings and dialect unchanged.

—Split single captions describing multiple images, into multiple captions.

—In the text versions, delimited italics text in _underscores_ (the HTML version reproduces the font form of the printed book.)