BALANCED ROCK, near head of Fruita Canyon. Spire and rock are Wingate Sandstone resting on red Chinle Formation; thin caprock is protective layer of resistant Kayenta Formation. (Frontispiece)
The Geologic Story of
COLORADO
NATIONAL MONUMENT
By S. W. Lohman
GEOLOGICAL SURVEY BULLETIN 1508
UNITED STATES DEPARTMENT OF THE INTERIOR
JAMES G. WATT, Secretary
GEOLOGICAL SURVEY
Doyle G. Frederick, Acting Director
Library of Congress Cataloging in Publication Data Lohman, Stanley William, 1907- The geologic story of Colorado National Monument. (Geological Survey Bulletin 1508) Bibliography: p. 131 Includes index. 1. Geology—Colorado National Monument. 2. Colorado National Monument. I. Title. II. Series: United States Geological Survey Bulletin 1508 QE75.B9 no. 1508 [QE92.C6] 557.3s [557.88′17] 80-607952
For sale by the Superintendent of Documents, U.S. Government Printing Office
Washington, D.C. 20402
Contents
Page [Preface] XI [History of the Monument] 1 [Early history of the region] 5 [Prehistoric people] 5 [Late arrivals] 10 [Early settlement] 10 [The Brown-Stanton river expedition] 11 [Kodel’s gold mine] 12 [Recent cave dweller] 13 [Artesian wells] 14 [Geographic setting] 16 [The geologic story begins] 17 [Ancient rocks and events] 24 [A great gap in the rock record] 26 [The age of reptiles] 27 [Early landscape] 28 [Ancient sand dunes] 29 [The rains came] 32 [Another gap in the rock record] 35 [The sea to the west] 39 [Deposits and events east of the sea] 39 [Dinosaurs roam the Monument] 47 [Dinosaurs on the move] 53 [Yet another gap in the rock record] 54 [Peat bogs] 55 [The sea covers the Plateau] 56 [The sea’s final retreat] 56 [End of the dinosaurs] 60 [The age of mammals] 61 [Early deposits and events] 63 [Lake Uinta] 63 [The mountains rise again] 64 [Nearby lava flows] 71 [Ancestral Colorado River] 72 [Piracy on the high plateaus] 72 [The age of man] 76 [The ice age] 77 [Capture of East Creek] 78 [Canyon cutting] 78 [A look into the future] 83 [How to see the Monument] 85 [Trips through and around the Monument] 88 [From Grand Junction through the Redlands to the West Entrance of the Monument] 88 [From Fruita to the West Entrance of the Monument] 96 [Through the Monument from West to East Entrances] 97 [From the East Entrance to Grand Junction] 118 [Through Glade Park from the northwest arm of Ute Canyon to Columbus Canyon] 119 [From Glade Park to Grand Junction via the Little Park Road] 121 [Résumé of geologic history and relation to other National Parks and Monuments in the Colorado Plateau] 125 [Acknowledgments] 130 [References] 131 [Additional reading] 133 [Index] 135
Figures
Page
[Frontispiece. Balanced Rock] II [a]Figure] [1. John Otto] 2 [2. John Otto’s Monument] 4 [3. Map of Colorado National Monument] 6 [4. Petroglyphs] 9 [5. Cave] 14 [6. Independence Monument] 19 [7. Rock Column of Colorado National Monument] 20 [8. Geologic Map] 22 [9. Block diagrams of early Proterozoic events] 25 [10. Petrified sand dunes] 30 [11. The Coke Ovens] 31 [12. Red Canyon] 33 [13. Thin-bedded Kayenta Formation] 34 [14. Kayenta Formation] 36 [15. Gap in the rock record] 37 [16. Entrada Sandstone] 41 [17. Moab Member of Entrada Sandstone] 42 [18. Mottled salmon-and-white Slick Rock Member] 43 [19. White Entrada Sandstone] 44 [20. Summerville Formation] 46 [21. Morrison Formation] 48 [22. Excavating type specimen of Brachiosaurus altithorax Riggs] 51 [23. Skeletons of typical dinosaurs of Morrison Formation] 52 [24. Burro Canyon Formation and Dakota Sandstone] 54 [25. Mount Garfield] 57 [26. Photo index map] 58 [27. Common types of rock folds] 62 [28. Common types of faults] 65 [29. Ladder Creek monocline and Redlands fault] 66 [30. Lizard Canyon monocline] 67 [31. Kodels Canyon fault] 68 [32. Kodels Canyon fault] 69 [33. Geologic structures at Fruita entrance to Colorado National Monument] 70 [34. Probable drainage patterns and land forms near the Monument at four successive stages of development] 74 [35. Fallen Rock] 81 [36. Unaweep Canyon] 82 [37. Redlands fault] 89 [38. Closeup of updragged Wingate Sandstone along Redlands fault] 90 [39. Bronze plaque and monument marking the discovery of Brachiosaurus altithorax Riggs] 91 [40. Reverse part of Redlands fault] 92 [41. Northwest end of Redlands fault] 93 [42. Looking west into Monument Canyon] 94 [43. Looking west from divide on Broadway 2 miles east of West Entrance to Monument] 95 [44. New fill on Rim Rock Drive between two tunnels on west side of Fruita Canyon] 98 [45. Fruita Canyon] 100 [46. Campsites at north end of campground] 101 [47. Picnic area and parking lot] 102 [48. Window Rock] 103 [49. Pipe Organ] 104 [50. Visitor Center and the Saddlehorn] 106 [51. Independence Monument] 107 [52. Ute Canyon] 110 [53. Cold Shivers Point] 112 [54. Top of old Serpents Trail] 113 [55. Looking northeast from old Serpents Trail] 114 [56. South portal of tunnel through Wingate Sandstone] 115 [57. Devils Kitchen] 117 [58. Glade Park fault viewed from the ground] 122 [59. Glade Park fault viewed from the air] 123 [60. Ladder Creek monocline and Redlands fault] 124 [61. Geologic time spiral] 126
Preface
From 1946 until about 1956 I carried out fieldwork intermittently on the geology and artesian water supply of the Grand Junction area, Colorado, the results of which have been published.[1] The area mapped geologically contains about 332 square miles in the west-central part of Mesa County and includes all of Colorado National Monument. During the fieldwork several successive custodians or superintendents and several park naturalists urged that upon completion of my professional paper I prepare a brief account of the geology of the Monument in terms understandable by laymen, and which could be sold at the Visitor Center. This I was happy to do and there resulted “The geologic story of Colorado National Monument,”[2] published by the Colorado and Black Canyon Natural History Association in cooperation with the National Park Service. This report contained colored sketches by John R. Stacy and a colored cover, but the photographs and many of the drawings were reproduced in black and white.
Later, after I had prepared popular style reports containing mostly color photographs on Canyonlands[3] and Arches[4] National Parks, officials of Colorado National Monument and I discussed the possibility of preparing a revised and enlarged edition of my 1965 report containing mainly color photographs, inasmuch as the supply of the black and white edition was nearing exhaustion, and later became out of print. At the meeting in the Monument on June 8, 1976, attended by Robert (Bob) E. Benton, Superintendent, A. J. (Jerry) Banta, Supervising Park Ranger, Margaret Short, Park Naturalist and Secretary of the Natural History Association, and me, it was agreed that: (1) A revised and enlarged edition containing mostly color photographs should be prepared for publication as a bulletin of the U.S. Geological Survey, and (2) that the Colorado and Black Canyon Natural History Association gave its permission for use of any or all of the copyrighted material in the first edition. The present report resulted.
The cover is a duotone of a 9- × 12-cm infrared photograph of Independence Monument taken by me. (See also [fig. 6].) Most of the color photographs were taken by me on 4- × 5-inch or 9- × 12-cm tripod mounted cameras using lenses of several focal lengths, but I took some with 35-mm cameras. Some of the color photographs and all the black and white ones were taken by those credited in the captions, to whom grateful acknowledgment is made. The points from which most of the photographs were taken are shown in [figure 26].
West side of Otto’s Monument
History of the Monument
The story of how Colorado National Monument came into being is as colorful as the canyons and cliffs themselves. The fantastic canyon country had a magical attraction for John Otto[5] ([fig. 1]) who, in 1906, camped near the northeastern mouth of Monument Canyon and began building trails into the canyons and onto the mesas—the high tablelands that separate the deep canyons. He did this back-breaking work simply because he wanted to and so that others could share the beauty of this wild country.
In 1907 Otto got the Grand Junction Chamber of Commerce to petition Secretary of the Interior James A. Garfield to set aside the area as a National Monument. Otto’s dream came true on May 24, 1911, when President William Howard Taft signed the proclamation creating the Monument. On June 14, Otto climbed to the top of Independence Monument ([fig. 6]) where he placed the Stars and Stripes to celebrate Flag Day. For several years thereafter Otto placed the flag atop Independence Monument on July 4th to celebrate Independence Day.
Until about 1921 the only routes into the Monument proper were John Otto’s trails, but in that year the ranchers of Glade Park joined with Otto in building the steep, twisting Serpents Trail from No Thoroughfare Canyon to the mesa above—a much shorter route to Grand Junction. It had 54 switchbacks and climbed about 1,500 feet in 2½ miles. The Serpents Trail was included in the Monument in 1933 and was used until 1950 when an easier route was built up the west side of No Thoroughfare Canyon and through a tunnel to the top of the mesa (figs. [3], [56]). The Serpents Trail has been preserved as an interesting foot trail ([fig. 55]), which can be hiked downhill in an hour or so. A parking area near the foot of the trail allows one member of a group to drive ahead to await the others.
JOHN OTTO, fantastic father of Colorado National Monument, and his helpers. Photograph courtesy Grand Junction Chamber of Commerce. (Fig. 1)
In 1924 John Otto got the idea that the Monument should include a herd of big game, so he talked the Colorado Game and Fish Department into shipping six young elk, and he got the local Elks Lodge to pay the transportation costs. The elk were turned loose in Monument Canyon, but they found the sparse vegetation and scant water supply ill suited to their needs, so after a few years they found a way out over the rim and migrated about 20 miles south of the Monument to lush high country where they joined with native elk and multiplied to become the ancestors of the present fine herd on Piñon Mesa (shown in [fig. 34]D). Occasionally a few return to the Monument and may be seen mainly in Ute Canyon. Native mule deer are frequently seen in and near the Monument.
Far from being discouraged, Otto then hatched the idea to start a buffalo[6] herd to be purchased by donations of buffalo nickels from school children and by contributions from the Odd Fellows and others. He finally raised enough money to get the patient Game and Fish Department to send him two cows and one bull. Unfortunately the bull died, so Otto talked the National Park Service into shipping him a bull from Yellowstone National Park. This time success crowned his efforts, and the small herd eventually multiplied to as many as 45 animals, but generally the herd has been kept at about 20-25 head ever since. You may spot some of them when you gaze down into Monument or Ute Canyons or when you drive past the northeastern boundary. Rarely, you may spot one in Red Canyon.
At the northeast corner of Fourth Street and Ute Avenue in Grand Junction is a most unusual object, which illustrates yet another peculiarity of John Otto—fantastic father of Colorado National Monument ([fig. 2]). Its history is best told by quoting from Al Look,[7] though its purpose still remains a mystery.
One day a horse drawn dray backed up to a vacant lot on Grand Junction’s Main Street [corner 6th] and unloaded a granite cube four feet square, carved on two sides. It weighed more than a ton and Otto supervised the setting.
One side [now facing west and not visible in [fig. 2]] showed a three foot circle containing a swastika with a five pointed star in each quarter. Above the emblem was carved “Rock of Ages” and below read “Cross of Ages.” The second side [now facing south, and shown in [fig. 2]] was beyond normal comprehension. Two large W’s on either side of a small swastika were over the letters or initials P.P., then four chain links with the letters T, H, L, J. inscribed, followed by the initials I.E. Below on the left was “1918,” over “Year 1”. On the right was “Old Count” and under it “New Count.” Between them stands the word ‘MARCH.’ Below this are abbreviations for the seven days of the week with the figure 1 under MON ending with a 6 under SAT. The bottom line [most of which is barely visible in the photograph] contained the figure 7 in a circle, a carpenter’s square, a small rectangle, probably representing a level, a plumb bob, a carpenter’s compass and a circle showing the western hemisphere. That is all. It made sense to John Otto because from somewhere he gathered considerable money to have this monument carved by the local gravestone merchant. It stood for several years to mystify pedestrians, and was finally moved beside the Redlands road to the [east entrance of] Colorado Monument where it is now hidden by weeds.[8]
It was still there in the 50’s when my family and I were startled to find it. We were afraid it might be lost forever so are glad it finally found a safe resting place on a concrete slab at the museum. I shall greatly appreciate hearing from any reader who can decipher this enigma.
JOHN OTTO’S MONUMENT, at southwest corner of the Historical Museum and Institute of Western Colorado, at northeast corner of Fourth Street and Ute Avenue, Grand Junction. View looking north. Face is 4 feet square. (Fig. 2)
Otto’s rock is at the southwest corner of The Historical Museum and Institute of Western Colorado. The main attraction inside is a life-size skeleton of Allosaurus ([fig. 23]), whose bones are exact plastic replicas of real ones at the museum of Brigham Young University, at Provo, Utah. The painstaking casting of the “bones” and assembly of the self-supporting skeleton was done by Al T. Look, son of author Al Look listed under “References.” The museum also houses other items of interest from the Grand Junction area.
Construction of the scenic Rim Rock Drive through the Monument was begun by the National Park Service about 1931 using workers from the Civilian Conservation Corps, and the drive eventually was completed to join roads from Fruita and Grand Junction. The route from Fruita includes a winding road up Fruita Canyon and through two tunnels to the mesa (figs. [3], [44], [45]).
A modern Visitor Center, new housing facilities for park personnel, additions to the campgrounds, the Devils Kitchen Picnic Area near the East Entrance, several self-guiding nature trails, and additional overlooks and roadside exhibits were completed in 1964 as part of the Mission 66 program of the National Park Service.
The Monument originally included 13,749 acres, but boundary changes in 1933 and 1939 increased the total to 17,660 acres, and the inclusion of all of No Thoroughfare Canyon and other boundary adjustments in 1978 increased the size to about 20,457 acres, or about 32 square miles (see map, [fig. 3]).
Early History of the Region
Prehistoric People
John Otto, early explorers, and even the Ute Indians who once hunted in the area were by no means the first people to view the Monument, in fact they were “Johnnies-come-lately.” Considerable evidence indicates that prehistoric people inhabited the area thousands of years ago.
MAP OF COLORADO NATIONAL MONUMENT, showing location in Colorado, boundaries, streams, highways and roads, principal trails, named features, overlooks, and—in triangles—trip guides localities. The trip guides numbers correspond to the numbers in the right margins of the section entitled “Trips through and around the Monument.” Visitors are given pamphlets at the two entrance stations and may purchase other reports and maps at the Visitor Center. (Fig. 3)
[High-resolution Map]
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Many years ago Al Look, of Grand Junction, discovered and excavated two caves in the part of No Thoroughfare Canyon formerly outside the Monument. He found stone projectile points, knives, awls, milling stones, parts of a sandal and coiled basket, reed matting, corn, corncobs, acorns, and animal bones, but no pottery—indicating that the people had not progressed beyond basket making. Similar artifacts were found in several other nearby places on the Uncompahgre Plateau. Archaeologists have named this old culture the Uncompahgre Complex, and date it back to a few thousand years before the time of Christ.[9] It should be pointed out that it is unlawful to remove artifacts, fossils, rocks, or minerals from a National Park or Monument.
In the summer of 1963 an archaeological survey of Colorado National Monument was carried out, under the terms of an agreement between the National Park Service and the University of Colorado, by Stroh and Ewing and their field assistants.[10] A total of 75 aboriginal sites were found of which 71 were within the Monument boundaries of that date, and 4 were closely adjacent. These comprised 41 open campsites, 24 rock shelters, 2 small caves, and 8 chipping stations. Artifacts recovered included 62 projectile points, 21 metates (grinding stones), 40 manos (hand stones), 111 whole or fragments of blades or scrapers, 6 choppers, several fragments of baskets, potsherds (bits of broken pottery) at two sites, 2 wood awls, several strands of yucca fibers, 3 corncobs, 6 kernels of corn, several bone fragments, storage cists at five sites, and petroglyphs at three locations.
Stroh and Ewing concluded that the majority of the sites appear to have been the campsites of a hunting and gathering people, and they speculated that there may have been aboriginal activity in the area from as long as several thousand years ago to relatively recent times.
The largest of the petroglyphs,[11] or rock drawings, are on a fallen slab of Wingate Sandstone in No Thoroughfare Canyon, and are shown in [figure 4]. Archaeologist John Crouch ([footnote 10]), who kindly reexamined these petroglyphs in February 1980, told me that most of the figures appear to be Shoshonian (Ute), but that some may be of the Fremont culture[12] or even older.
PETROGLYPHS, on fallen slab of Wingate Sandstone in No Thoroughfare Canyon. Figure of man at lower right is about 6 inches high. The fading designs were traced with chalk before photographing them. Photograph by T. R. Giles, U.S. Geological Survey. (Fig. 4)
Late Arrivals
Early Settlement[13]
Prior to 1881 the Monument area was inhabited only by Ute Indians, but it was visited from time to time by a few fur trappers, explorers, and geologists. In 1776 an expedition led by Fathers Dominguez and Escalante passed northward across Grand Mesa, the high plateau just east of the area, which is pointed out in many of the photographs. A trading post was built by Joseph Roubidoux about 1838 just above the present site of Grand Junction. In 1853 Captain John W. Gunnison, seeking a new route for a transcontinental railroad, led an exploring party down what is now the Gunnison River Valley, past the confluence with the Grand River (now called the Colorado, [p. 16]), and on down the valley. Geologists and topographers of the Hayden Survey found only Ute Indians in the area in 1875 and 1876, and their field season of 1875 was abruptly cut short because of skirmishes with hostile Utes. After the Meeker (Colorado) Massacre of 1879, believed by many to have been caused mainly by the ignorance and shortsightedness of Meeker himself, treaties were signed forcing the Utes out of western Colorado onto reservations in eastern Utah, and the last of the Utes was reportedly out of the area by September 1881. The Grand Valley was immediately opened to settlement, and the first ranch was staked out on September 7, 1881. Nineteen days later George A. Crawford founded Grand Junction as a townsite and formed the Grand Junction Town Company the next month. The success of the new town was assured on November 21, 1882, when the narrow-gage line of the Denver and Rio Grande Railroad (now Denver and Rio Grande Western Railroad) reached it via the Gunnison River valley. The town of Fruita was founded by William E. Pabor in 1883 and incorporated the following year.
The Brown-Stanton River Expedition[14]
Of the many early expeditions down the Colorado River, only one went past what is now Colorado National Monument—the ill-fated Brown-Stanton expedition. After the pioneering expeditions of 1869 and 1871 down the Green and Colorado Rivers by Major John Wesley Powell and his men, the many ensuing river expeditions started in Utah or Wyoming; but the first phase of the Brown-Stanton expedition started in Colorado—at Grand Junction. In 1889 Frank M. Brown organized a company for the construction of the proposed Denver, Colorado Canyon, and Pacific Railway, which was to carry coal from mines in Colorado over a “water-level” line through the mighty canyons of the Colorado River to the Gulf of California some 1,200 miles away, from which coal would be shipped to various ports in California. On March 26, 1889, president Brown, chief engineer F. C. Kendricks, and assistant engineer T. P. Rigney drove the first stake at Grand Junction for a survey of the new line. Then Brown left for the East to obtain financing, and the other two men plus some hired hands took off in a boat down the Grand River. After reaching the confluence, they towed the boat up the Green River to the town of Green River, Utah, thus becoming the first to make this trip upstream, albeit on foot and dragging their boats. Brown, who had returned from the East, his newly appointed chief engineer Robert Brewster Stanton, and 14 others in six ill-designed boats of cedar rather than oak, left Green River, Utah, on May 25, 1889. Against the advice of Major Powell and others, they carried no life preservers. After many mishaps, Brown and two others were drowned near the head of Marble Canyon, and the ill-fated expedition temporarily ground to a halt. However, the indefatigable Stanton had new boats built of oak, and with a reorganized party of 12 left the mouth of the Dirty Devil River on November 25. After many additional mishaps the party finally reached the Gulf of California on April 26, 1890. In spite of Stanton’s heroic efforts, the railroad was never built, and the Grand Canyon was spared the huffing and puffing of locomotives.
Kodel’s Gold Mine[15]
As shown in figures [3], [8], and [26], the first major canyon west of the West Entrance of the monument is called Kodels Canyon (pronounced \‘kōdǝls\). It was named after an early-day stonemason turned prospector, a hermit, who came to the Fruita area before 1900 and prospected for gold until at least 1930 in the canyon that now bears his name. He seemingly built a cabin or house near the mouth of the canyon, spent most of the rest of his life in a vain quest for gold in the canyon, barricaded his house against would-be intruders, and took potshots at anyone approaching his home for fear they were after his “gold.” Some thought him only half crazy, but when he took repeated shots at an Indian named Henry Kadig, he was adjudged wholly insane and sent to the mental hospital at Pueblo, Colorado, for several years. When he got out he sold the grazing rights in his canyon to the late Irving Beard of Fruita, and seemingly was not heard from again. According to various estimates, Kodel dug an adit between 18 and 150 feet into the dark Proterozoic rock in the side of the canyon (shown in [fig. 3]), then sunk a shaft somewhere between 30 and 50 feet deep. He was always “on the verge of a big strike,” but there is no record of any actual production.
Later, a prospector from the midwest spent several summers digging in Devils Canyon, the next major canyon to the west, but he was equally unsuccessful. The unsuccessful attempts of Kodel and others is not surprising, for the two canyons are some 100 miles north of the Colorado mineral belt—a band extending roughly from Boulder to the western part of the San Juan Mountains, in which ore-bearing Upper Cretaceous or lower Tertiary rocks were intruded into all overlying rocks of whatever age.
Recent Cave Dweller
About 3 miles west of the Glade Park Store and Post Office are three large caves in a cliff of the Wingate Sandstone on the north wall of a canyon containing a tributary of Clark’s Wash. The middle cave, which formerly contained a small one-room framehouse and other improvements, was occupied for about 40 years prior to 1958 by Mrs. Laura Hazel Miller ([fig. 5]). A large cave just to the west (left) was used for storage, and another large cave just to the east formerly was fenced to shelter domestic animals. Mrs. Miller lived alone most of this time but had a dog for companionship the last few years she lived in the cave. When my wife and I visited her in the mid-fifties we had a very pleasant conversation with this very intelligent woman and could hardly believe she was 87 years old. She could not understand why anyone could live in crowded cities as she much preferred the peace and quiet of her cave. Once a week she walked the 6 miles round trip to and from the Glade Park Store and Post Office, bought what few necessities she needed, and telephoned her daughter in Grand Junction. Maybe she had something the rest of us have missed! She became sick in her nineties and moved to Grand Junction to live with her daughter. After she died, the property was sold, and I have since observed that vandals had burned her one room house and had destroyed most of the other improvements.
CAVE in Wingate Sandstone inhabited by Mrs. Laura Hazel Miller (visible between gate posts) until 1958. One-room house was entirely within cave, and smaller storehouses extended back of the house. Note blackening of cave roof by soot. (Fig. 5)
Artesian Wells
It may surprise you to learn that several sandstone formations supply water to artesian wells northeast of the Monument in The Redlands, Orchard Mesa, and the southwestern side of the Grand Valley, most of which are 500 to more than 1,000 feet deep. When first drilled and for some years later these wells flowed at the land surface, but eventually after too many wells had been drilled too close together, each well reduced the output of neighboring wells until most wells ceased to flow naturally. This made it necessary for most well owners to install pumps, which further aggravated the problem by reducing the artesian head (the height to which the water rises above the formation from which it issues). This created a situation not unlike too many children sucking on straws in the same ice cream soda, and led to a detailed investigation by the U.S. Geological Survey and the Colorado Water Conservation Board,[16] outgrowths of which were the present report and its predecessors.
The water system of the Ute Conservancy District was virtually completed by late 1964 and began to supply water to rural residents of Grand Valley between the towns of Palisade and Mack through a vast network of pipelines. The water is obtained from surface sources on the north flank of Grand Mesa east of the valley and is brought to the valley via a pipeline down the valley of Plateau Creek. Use of the new water has reduced the draft on many of the artesian wells. The reduced draft has locally arrested the decline in the artesian head or has actually allowed some recovery in head.
In order of their importance and productivity the water-bearing sandstones are the Entrada, the Wingate, and local sandstone lenses in the lower part (Salt Wash Member) of the Morrison Formation ([fig. 7]). In a few places small flows or yields are obtained from wells that tap the Dakota Sandstone and underlying Burro Canyon Formation, but inasmuch as the Dakota contains some marine sandstones from which all the salt seemingly has not yet been flushed out, the water from most of these wells is brackish or salty.
As we will see on the trip “From Grand Junction through The Redlands to the West Entrance of the Monument,” pages [88]-95, in and near the Monument these sandstones look bone dry, so how can they supply water to artesian wells? They are indeed dry in all the cliff exposures, but as will be noted later when the bending and breaking of the rocks are discussed (p. [64]-71), erosion has exposed the upturned sandstones so that they may take in water from the many small streams that drain the Monument and adjacent areas for short periods after summer thundershowers or during spring thaws. The water moves slowly down the dipping sandstones and becomes trapped under pressure beneath overlying beds of siltstone or mudstone—materials that are nearly impervious.
Geographic Setting
Geologists and geographers have divided the United States into many provinces, each of which has distinctive geologic and topographic characteristics that set it apart from the others. Colorado National Monument is in the northeastern part of the Canyon Lands section of the Colorado Plateau Province—a province that contains 15 national parks and monuments, about 3 times as many as any other province. This province, hereinafter referred to simply as the Colorado Plateau, or the Plateau, covers some 150,000 square miles and extends from Rifle, Colo., at the northeast to a little beyond Flagstaff, Ariz., at the southwest, and from Cedar City, Utah, at the west nearly to Albuquerque, N. Mex., at the southeast. This scenic province consists of high plateaus generally ranging in altitude from 4,500 feet to more than 7,000 feet, which are deeply and intricately dissected by literally thousands of canyons.
Colorado National Monument is drained entirely by the Colorado River, which flows to the northwest in the wide Grand Valley just a few miles from the northeastern border ([fig. 3]). The small streams that drain the Monument contain water only after summer thundershowers or after rapid snowmelt.
Why is the large valley of the Colorado River called the Grand Valley? The Colorado River northeast from its confluence with the Green River in the middle of Canyonlands National Park[17] formerly was called the Grand River, and the Green and Grand joined at the confluence to form the Colorado River. The Grand River was renamed Colorado River by act of the Colorado State Legislature approved March 24, 1921, and approved by act of Congress July 25, 1921. But the old term still remains in names such as Grand County, Colo., the headwaters region; Grand Valley, a town 16 miles west of Rifle, Colo.; Grand Valley between Palisade and Mack, Colo.; Grand Mesa, an extensive plateau which towers more than a mile above the Grand and Gunnison River Valleys; Grand Junction, Colo., a city appropriately situated at the confluence of the Grand and Gunnison Rivers; and Grand County, Utah, which the river traverses after entering Utah.
The Geologic Story Begins
Colorado National Monument is a land of brightly colored cliff-walled canyons and towering monoliths—a majestic sample of mysterious canyonlands that stretch hundreds of miles to the west and south. Now a desert region more than a mile above the sea, it was not always so. More than a billion years ago the site of the Monument was deep beneath the sea. Later, lofty mountains were pushed up only to be obliterated eventually by the slow but relentless forces of erosion. Millions of years later the earth shook to the stride of 10-ton dinosaurs—then the sea returned again and sharks swam over the region looking for food.
These are but a few samples of the interesting—even exciting—events in the long geologic history of the Monument. Many pages, indeed several whole chapters, of its history are missing and must be inferred from nearby regions where the story is more complete. Thus, the cliffs and canyons you are looking at did not get that way overnight. An understanding of the geologic processes and events that led to the scenic features of today should help you toward a clearer picture and greater appreciation of nature’s handiworks ([fig. 6]).
Geologists recognize rocks of three distinctly different modes of origin—sedimentary, igneous, and metamorphic, and there are many variations of each type. The sedimentary rocks of the Monument are composed of clay, silt, sand, and gravel carried and deposited by moving water; silt and fine sand transported by wind; and some limestone, composed mainly of the mineral calcium carbonate, which was precipitated from water solutions in freshwater lakes. In areas not far to the northeast and southwest are many sedimentary rocks of marine origin, that is, materials that were deposited in the ocean or shallow inland seas, but in the Monument marine sedimentary rocks occur only in parts of the Dakota Sandstone; however, the overlying marine Mancos Shale underlies the adjacent Grand Valley and forms the lower slopes of the Book Cliffs across the valley ([fig. 25]).
Igneous rocks were solidified from liquid molten rock intruded upward into any preexisting rocks along cracks, joints, and faults. Molten rock that reaches the land surface and forms volcanos or lava flows is called extrusive igneous rock. Joints are cracks or breaks in rocks along which no movement has taken place. Faults are cracks or joints along which one side has moved relative to the other. Different types of faults are shown in [figure 28]. Metamorphic rocks were formed from either of the other types by great heat and pressure at extreme depths in the Earth’s crust. Metamorphic rocks and some intrusive igneous rocks make up the hard, dark rock that floors all the deep canyons in and near the Monument. The nearest extrusive igneous rocks are the thick, dark lava flows that cap towering Grand Mesa to the east and Battlement Mesa to the northeast.
INDEPENDENCE MONUMENT, separating the two entrances of Monument Canyon. Looking north from Grand View; Colorado River, Grand Valley, and Book Cliffs in distance. Roan Cliffs are white cliffs at extreme distance on right skyline. Dark rock flooring canyon is Proterozoic metamorphic rock, red material in slope at base of cliffs is the Chinle Formation, vertical cliffs are Wingate Sandstone, thin protective caprock on top of cliffs is lower sandstone of the resistant Kayenta Formation. The top of Independence Monument is nearly 450 feet above the floor of the canyon. (Fig. 6)
ROCK COLUMN OF COLORADO NATIONAL MONUMENT. 1 foot = 0.305 meter. (Fig. 7)
AGE (millions of years) GEOLOGIC AGE NAME OF ROCK FORMATION KIND OF ROCK AND HOW IT IS SCULPTURED BY EROSION THICKNESS (feet) NAMED FOR OCCURRENCE AT OR NEAR 80 Late Cretaceous Mancos Shale Gray and black shale, and thin beds of sandstone and limestone. Contains sea shells. Eroded from Monument, but underlies Grand Valley and forms lower part of Book Cliffs. 3,800 Mancos, Colo. Dakota Sandstone Coaly shale, sandstone, conglomerate, and lignite coal. Contains plant remains. Forms benches and slopes. Caps highest hill in Monument. 150 Dakota, Nebr. 115 Early Cretaceous Burro Canyon Formation Green siltstone and shale, and sandstone and conglomerate. Forms benches and slopes. Crops out on highest hill in Monument. 60 Burro Canyon San Miguel Co., Colo. EROSIONAL UNCONFORMITY 150 Late Jurassic Morrison Formation Brightly colored siltstone and mudstone, and sandstone and limestone. Contains dinosaur bones and fresh-water shells. Forms slopes and badlands. Lower third with sandstone lenses is Salt Wash Member, upper two thirds is Brushy Basin Member. 600 Morrison, Colo. 170 Middle Jurassic Summerville Formation Brightly colored siltstone and mudstone, and thin sandstones. Forms slopes. 54 Summerville Point San Rafael Swell, Utah Entrada Sandstone White and salmon-red sandstone. Upper level-bedded Moab Member forms stair steps, lower mostly cross-bedded Slick Rock Member forms cliffs. 150 Entrada Point Moab, Utah Slick Rock, Colo. 195 Jurassic and Triassic(?) (missing) EROSIONAL UNCONFORMITY 210 Late Triassic(?) Kayenta Formation Red and purple siltstone and shale, and sandstone and conglomerate. Forms bench between two cliffs and mesas between canyons. 45-80 Kayenta, Ariz. Late Triassic Wingate Sandstone Buff and light red sandstone. Cross-bedded and level-bedded. Forms highest cliffs and most of named rock features in Monument. 350 Fort Wingate, New Mex. Chinle Formation Red siltstone and shale, and some limestone conglomerate. Forms steep slopes at foot of cliffs. 80-100 Chinle Valley N.E. Ariz. GREAT UNCONFORMITY 240-1000 Triassic, Paleozoic, Younger Proterozoic (missing) Unnamed Schist, gneiss, granite, and pegmatite dikes. Floors main canyons and forms high bluff above The Redlands. Unknown 1500 Older Proterozoic
After the materials of the sedimentary rocks were deposited and covered by younger layers, they generally became saturated or partly saturated with ground water containing small amounts of dissolved minerals. Some of these minerals precipitated from solution and cemented the loose particles into rocks of varying hardness. Thus, most of the sandstones are partly cemented with the mineral calcite, composed of calcium carbonate (CaCO₃), but some are cemented also with silica (SiO₂) or hematite (Fe₂O₃).
Look almost anywhere in the Monument and you will see that the rocks are piled up in layers of different color, thickness, and hardness—much like a vast layer cake. In most of the Monument, these layers are flat or slope gently down to the northeast, but along the northeastern boundary they are sharply bent or broken as though the cake had been carelessly placed over the edge of a table and had sagged.
Let us consider these layers one by one, beginning with the oldest at the bottom, for each is a partial record of events long past. Layers of rock that can be easily recognized and distinguished from other layers are called formations and are named after a place where they are well exposed. For the name to be accepted for general usage it must be the first published description in a technical report of a particular sequence of rock layers. The places after which the formations of the Monument were named are given in the rock column ([fig. 7]), and the outcrops of the formations are shown on the geologic map ([fig. 8]). In the pages that follow, the geologic events that shaped the Monument we see today are discussed in chronological order, beginning with the oldest rocks that floor the deep canyons.
GEOLOGIC MAP of Colorado National Monument and vicinity, simplified and greatly reduced from part of maps at scale 1:31,680 by Lohman (1963, 1965a). For additional surficial deposits in the Grand Valley and Orchard Mesa see Cashion (1973). (Fig. 8)
[High-resolution Map]
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EXPLANATION QUATERNARY Qal—ALLUVIUM Qls—LANDSLIDE DEPOSITS CRETACEOUS Km—MANCOS SHALE Kdb—DAKOTA SANDSTONE AND BURRO CANYON FORMATION, UNDIVIDED JURASSIC Jms—MORRISON AND SUMMERVILLE FORMATIONS, UNDIVIDED Je—ENTRADA SANDSTONE TRIASSIC TRk—KAYENTA FORMATION TRwc—WINGATE SANDSTONE AND CHINLE FORMATION, UNDIVIDED PROTEROZOIC PL—SCHIST, GNEISS, GRANITE, AND PEGMATITE CONTACT FAULT—Dashed where approximately located; dotted where concealed. U, upthrown side; D, downthrown side ANTICLINE SYNCLINE CENTRAL AXIS OF SYMMETRICAL MONOCLINE—Showing direction of plunge UPPER BEND OF MONOCLINE—Showing direction of plunge STRIKE AND DIP OF BEDS ABANDONED MINE Geology simplified from Lohman, 1965a (Showing location of—) DEVILS CANYON MONOCLINE KODELS CANYON FAULT LIZARD CANYON MONOCLINE FRUITA CANYON MONOCLINE LADDER CREEK FAULT
Ancient Rocks and Events
The dark rocks that floor all the large canyons of the Monument ([fig. 6]) and form the high bluffs along the northeastern boundary (figs. [37], [38], [40], [41]) are of early Proterozoic[18] age—among the oldest known rocks of the Earth. Most were once sand and mud that spread out on the bottom of the sea and later hardened into sedimentary rocks ([fig. 9]-1). After thousands of feet of such rocks had accumulated, they were squeezed, bent, and lifted up by slow but mighty movements of the Earth’s crust to form high mountains perhaps like the Rockies. Heat and pressure that developed at great depth in the roots of these mountains changed the sediments into metamorphic rocks known as schist (finely banded) and gneiss (coarsely banded) ([fig. 9]-2). The rocks are about 1½ billion years old ([fig. 7]).
Later in Proterozoic time, about a billion years ago, molten material from below was forced upward along cracks or faults and cooled slowly to form thin seams or dikes and irregular bodies of granite ([fig. 9]-3). Dikes are called pegmatite when they contain large crystals of pink feldspar, white or clear quartz, black tourmaline, and large flakes of white mica. Small pegmatite dikes that pass through the older schist and gneiss may be seen along roadcuts in Fruita and No Thoroughfare Canyons.
BLOCK DIAGRAMS OF EARLY PROTEROZOIC EVENTS (after Edwin D. McKee). (Fig. 9)
① Layers of sand, mud, and other sediment accumulated in the sea and later were hardened into sedimentary rocks.
② The strata were compressed, bent, and uplifted into high mountains. Heat and pressure at great depth changed the sediments into banded schist and gneiss.
③ Molten rock flowed upward along cracks or faults. Upon cooling it formed lava at the surface and granite or pegmatite beneath.
④ During eons of time the forces of erosion wore down the mountains to a nearly level plain.
A Great Gap in the Rock Record
If you look down into any of the large canyons in the Monument, you will notice a brick-red formation, the Chinle, which forms steep slopes at the foot of the high cliffs and lies upon the dark Proterozoic rocks along nearly straight lines of contact. Such a straight-line contact is particularly well shown about midway up the high bluffs along the northeastern boundary of the Monument ([fig. 37]). If the red layer and all overlying rocks were stripped away, these straight lines would be the exposed edges of a remarkably smooth, nearly flat erosion surface on the top of the dark Proterozoic rocks, as shown in the last diagram of [figure 9]. A vast amount of time passed between the carving of this surface and the deposition of the red Chinle, and no record of the events during this time is preserved in the Monument.
During the latter part of the Proterozoic Eon and parts of the long Paleozoic Era that followed, the dark rocks were submerged beneath the sea several times and received sediments now found in areas to the northeast and southwest. Beginning in the Pennsylvanian Period some 330 million years ago ([fig. 61]), a large upfold of the rocks, or anticline ([fig. 27]), known to geologists as the Uncompahgre Highland, rose high above sea level, probably reaching its highest level in Late Pennsylvanian or Permian time. This old highland formed an imposing chain of mountains in about the position of the present Uncompahgre Plateau.
After the old rocks were pushed up into these high mountains what became of them? From the moment the mountains began to rise, their rocks were buffeted by wind, pounded by rain, pried open by frost, scoured by debris-laden streams and, perhaps by glaciers, and the loosened rock particles were dissolved or carried to the sea. Most rocks are brittle enough to crack when bent by Earth forces. Such cracks, called joints, are easy targets for erosion. The freezing of water in joints tends to pry the rocks apart. The breakup of the rocks was hastened by the chemical attack on rock minerals by water charged with oxygen and carbon dioxide. When land plants became established in later geologic eras, soil acids formed from decaying vegetation also assisted materially in breaking up the rocks.
These same erosion processes are going on today, but their effects are scarcely noticeable from year to year except in soft earth after storms or floods. During eons of time, however, the mountains were again worn down to a nearly level plain. Missing between the red Chinle and the dark rocks are many thousands of feet of rocks, some of which once covered this surface and still occur in other regions less affected by erosion. This gap in the rock record, which represents more than a billion years, is known to geologists as a great unconformity. Missing are part of the lower Proterozoic rocks, all the upper Proterozoic rocks, all those of the Paleozoic Era, and part of those of the Triassic Period of the Mesozoic Era. (See figs. [7] and [61].)
Traces of primitive life have been found in some Proterozoic rocks in the form of lime-secreting algae and casts of worms, but no fossils of more advanced types have been found because at that time the primitive animals seemingly had not yet developed shells or skeletons. The ensuing Paleozoic Era saw the appearance and great development of shellfish, fish, amphibians, reptiles, and primitive plants. Some of the rock layers of ages missing at the Monument may be seen as near as Glenwood Springs to the northeast and Gateway to the southwest.
The Age of Reptiles
All the layers of sedimentary rocks preserved in the Monument above the dark Proterozoic ones were deposited by wind and water during the Mesozoic Era. This long era has been called the age of reptiles, for reptiles, including dinosaurs (meaning terrible lizards), were then the dominant land animals. The Mesozoic Era has been divided into three parts—the Triassic, Jurassic, and Cretaceous Periods. Rocks of each of these periods crop out in the Monument.
Early Landscape
By late Triassic time the Monument was part of a nearly flat plain cut on the dark Proterozoic rocks, but there were hills or low mountains to the northeast. Streams from these hills dropped mud, silt, sand, and some gravel on this plain and into many small lakes that occupied the gentle depressions. Later, these deposits hardened mainly into red siltstone and sandstone, but thin beds of gravel were cemented to form conglomerate, and thin beds of limestone formed in some of the shallow lakes by the precipitation of the mineral calcium carbonate. These rocks, which comprise the Chinle (pronounced Chin-lee) Formation, are only 80 to 100 feet thick in the Monument but are as much as 700 feet thick near Moab, Utah, southwest of the Uncompahgre Plateau, where the entire formation is present. There, the Chinle rests on still older Triassic and Paleozoic rocks—all absent in the Monument for the reasons noted previously. In some parts of the Plateau, sandstone or conglomerate beds in the lower part of the Chinle yield uranium ore, but these beds were not deposited in or near the Monument.
The red color of the Chinle and some of the overlying rocks is caused by minute amounts of iron oxide—the same pigment used in rouge and red barn paint. Various oxides of iron, some including water, produce not only brick red but also pink, salmon, brown, buff, yellow, and even green or bluish green. This does not imply that the rocks could be considered as sources of iron ore, for the merest trace of iron, generally only 1 to 3 percent, is enough to produce even the darkest shades of red.
Because it is soft, the Chinle is easily eroded into steep slopes at the foot of high sandstone cliffs in all canyons of the Monument and on top of the high bluffs that face The Redlands. It also forms the broad base of Independence Monument. Rim Rock Drive crosses the Chinle three times in the lower part of Fruita Canyon and twice in No Thoroughfare Canyon.
Fossil reptile bones, petrified wood, and freshwater shells come from the Chinle in parts of Arizona and Utah. Reptiles probably roamed the Monument in Chinle time, but their remains have not been located.
Ancient Sand Dunes
Still later in the Triassic Period the Monument became part of a vast desert. Winds blowing from the northwest brought great quantities of fine sand and piled them up into large dunes like those in the Sahara or in Great Sand Dunes National Monument in Colorado. But like all deserts, it was not always dry—occasional rainstorms produced many small lakes and ponds. Some of the sand was washed into these lakes or ponds and settled in level layers. This huge sandpile eventually hardened into the buff and light-red sandstone that we now know as the Wingate. The shapes of the old dunes are indicated by the steep dips of sand layers, called crossbeds, which stand out in sharp contrast to the nearly level layers formed in the lakes and ponds ([fig. 10]).
The spectacular scenery of Colorado National Monument owes its existence largely to the 350-foot cliffs of the Wingate Sandstone ([fig. 6]) and to the desert climate, which allows us to see virtually every foot of the vividly colored rocks and has made possible the creation and preservation of such a wide variety of fantastic sculptures. A wetter climate would have produced a far different and smoother landscape in which most of the rocks and land forms would have been hidden by vegetation.
Eroded remnants of the Wingate form most of the named rock features of the Monument and are shown in many of the photographs. Independence Monument—a towering slab of sandstone that resembles a bridge pier ([fig. 6])—is all that is left of a high narrow wall that once connected the point east of Independence View with the high mesa north of the slab and which once separated the two entrances of Monument Canyon. In a few thousand years this remnant, too, may be gone.
Vertical cliffs and shafts of the Wingate Sandstone endure only where the top of the formation is capped by beds of the next younger rock unit—the Kayenta Formation. The Kayenta is much more resistant to erosion than the Wingate, so even a few feet of the Kayenta, such as the cap on top of Independence Monument, protect the rock beneath. Once this cap has been eroded away, the underlying Wingate weathers into rounded domes, such as the Coke Ovens.
Cold Shivers Point ([fig. 53])—a toadstool shaped cap of sandstone of the Kayenta above a vertical cliff of the Wingate—is perhaps the most aptly titled feature of the Monument.
PETRIFIED SAND DUNES in Wingate Sandstone along old Serpents Trail. Looking north across The Redlands and Grand Valley to the Book Cliffs. Battlement Mesa on right skyline. (Fig. 10)
The Coke Ovens ([fig. 11]) and Squaw Fingers were formed partly because most of the caprock of Kayenta has been weathered away and also because the brittle rocks were cracked along an evenly spaced set of vertical joints. These joints trend northward between the two named features. More rapid weathering along these joints helped form the separate rounded domes or spires between them. Similarly, northwestward-trending vertical joints connect and helped shape Kissing Couple, Pipe Organ, and Sentinal Spire.
THE COKE OVENS, looking north from overlook beneath Artists Point. A set of north-south joints has allowed erosion of the Wingate Sandstone to proceed more rapidly along these zones of weakness and has helped create the four ovens shown. Weathering away of the protective caprock of the overlying Kayenta Formation has produced rounded tops on all but the left-hand shaft, which is still protected by the Kayenta. Note alcoves and arches in cliff of the Wingate beyond, the formation of the one on the right having been aided by removal of the underlying soft Chinle Formation. Bench covered by piñon and juniper above Wingate is resistant thin-bedded Kayenta Formation. Cliff above the bench is the Slick Rock Member of the Entrada Sandstone. The Coke Ovens were named from their resemblance to the beehive-shaped brick ovens formerly used to convert bituminous coal into coke for smelting iron. (Fig. 11)
Many of the cliff walls of the Wingate are vertical, some even overhang, yet in some places the slopes are gentle enough to hold talus and to be climbed ([fig. 12]). Why is this? The answer to this question is given in a later section on “Canyon Cutting.”
Arches or shallow caves weathered out of some cliff faces of the Wingate, particularly where the underlying Chinle Formation has been partly scoured away. Although there are no large caves within the Monument, there are three in a row along the road 3 miles west of the Glade Park Post Office. One of these was inhabited until 1958 ([fig. 5]).
Many of the cliff faces of the Wingate are darkened or blackened by desert varnish—a natural pigment of iron and manganese oxides, silica, and clay.[19]
Dinosaurs left their footprints in the sands of the Wingate in parts of the Colorado Plateau, but no tracks or fossils have yet been found in this formation in or near the Monument.
The Rains Came
The arid climate of Wingate time was followed by a wet period, when streams from the northeast gradually covered the sand dunes with mud, sand, and some gravel. The sand and gravel of the stream channels were cemented into hard sandstone and conglomerate, and the mud of the flood plains hardened into red and purple siltstone and mudstone. The resulting Kayenta Formation makes up the bench between the two cliffs upon which the Visitor Center, campgrounds, and most of scenic Rim Rock Drive were built. Here, nature was kind, for this gently sloping bench was an ideal place to build the road from which to look down into the deep chasms. The Kayenta also caps the broad mesas between the canyons. It is about 350 feet thick in eastern Utah, only 45 to 80 feet thick in the Monument, and it is absent altogether not far east of the Monument. The reasons for the eastward thinning and ultimate disappearance of the Kayenta and some younger rocks are given in the [next section].
RED CANYON, looking northeast toward Grand Junction from Red Canyon Overlook. Dark notch at the bottom of the northeast end of the canyon is known as the Gunsight. Linear feature in the Grand Valley beyond is the Denver and Rio Grande Western Railroad. Prominent point near middle of Book Cliffs is Mount Garfield ([Fig. 25]). Battlement and Grand Mesas form left and right skylines, respectively. Dark green bush in right foreground is Mormon Tea. (Fig. 12)
THIN BEDDED KAYENTA FORMATION protecting underlying cliff of softer Wingate Sandstone. Rim Rock Drive is on bench of the Kayenta close to thinner cliff of Entrada Sandstone in background. Looking northwest from a point northeast of Monument Canyon View. (Fig. 13)
As noted earlier, the sandstone beds and lenses of the Kayenta generally are coarser grained (some even contain small pebbles) and much harder than the underlying Wingate Sandstone—particularly the lower beds of the Kayenta, which serve as a protective capping, as shown in [figure 13] and in many of the other photographs. Unlike the dominantly fine grained, well sorted, windblown sands of the Wingate, the coarser stream-laid sands of the Kayenta are angular and poorly sorted, so that small grains fill spaces between larger ones. Moreover, in addition to the calcite cement (which also holds together the sand grains in the Wingate and Entrada Sandstones), most of the sand grains and pebbles in the Kayenta are covered by interconnected “overgrowths” of silica (SiO₂), which make up about 10 percent of the rocks and serve as a nearly insoluble hard cement.[20]
The combination of the coarse and fine grains and interlocking silica “overgrowths” makes the Kayenta one of the most resistant rocks in the Colorado Plateau.
In distant views of weathered outcrops the Kayenta appears to consist mainly of thin beds or lenses of sandstone, which indeed it does, but in some fresh exposures, such as roadcuts, the highly lenticular red flood-plain deposits form striking features which may wedge out from 3 or 4 feet thick to a featheredge within horizontal distances of only a few feet ([fig. 14]).
The Kayenta has yielded fossil bones of dinosaurs and other reptiles in northeastern Arizona and freshwater shells in eastern Utah. As yet, however, no fossils have been reported from it in or near the Monument.
Another Gap in the Rock Record
Following the wet interval when the Kayenta Formation was deposited over wide areas of the Colorado Plateau by streams, the Plateau once again became a vast desert, and this time the dry climate persisted from the Late Triassic into the Jurassic. The howling winds piled up enormous sand dunes, layer upon layer, to a total thickness of more than 2,200 feet at Zion National Park, and as much as 500 feet remains in eastern Utah and parts of southwestern Colorado. This immense sandpile eventually was cemented by calcite into the Navajo Sandstone.
KAYENTA FORMATION, showing lenses of hard channel sandstones and wedge of red siltstone and mudstone. Along road cut of Rim Rock Drive near head of main stem of Ute Canyon. Vertical grooves remain from drill holes used in blasting roadcut. (Fig. 14)
Beautifully sculptured remains of the Navajo are featured attractions at Zion, Capitol Reef, and Arches National Parks, Rainbow Bridge, Navajo, and Dinosaur National Monuments; border many miles of beautiful Lake Powell; and form the eastern flank of the San Rafael Swell. For reasons to be explained, this sandstone thins to the northeast, and is absent entirely at about the Utah-Colorado State line, some 35 miles southwest of the Monument. Thus, in the Monument, the Navajo, most of the Kayenta, and the lower part of the Entrada Sandstone are missing at another gap in the rock record, as shown in [figure 15].
GAP IN THE ROCK RECORD, between Kayenta Formation below 3½- × 6-inch green notebook and Slick Rock Member of Entrada Sandstone above. The reasons for this gap are given in the text on [page 38]. That this is an erosional unconformity is clearly indicated by the uneven top of the Kayenta, particularly to the left of the notebook. Note solution pits and openings in the Entrada near top of photograph. (Fig. 15)
How is it possible that the Navajo Sandstone is more than 2,200 feet thick in Zion National Park, is several hundred feet thick in much of the Plateau in Utah and parts of southwestern Colorado, yet is absent entirely, together with a considerable thickness of younger rocks in and near Colorado National Monument? How much of the missing strata once were present in the Monument is not known, but it seems clear that at least part was present but was eroded away before the Entrada Sandstone was deposited. There is evidence[21] that following the deposition and consolidation of the Navajo Sandstone the Plateau and adjacent areas were uplifted, tilted gently westward, and eroded for a considerable period of time. Erosion naturally was most pronounced in the eastern areas, including the Monument, where the uplift was greatest. Thus, in the northeastern part of the Plateau all the Navajo and most of the Kayenta were eroded away, and erosion continued there while the lowest member of the Entrada, the Dewey Bridge Member, and the lower part of the overlying Slick Rock Member were being laid down in the Moab, Utah, area.[22] This old erosion surface is clearly visible in many places along the cliff wall on the southwest side of Rim Rock Drive between the Visitor Center and Kissing Couple.
The reduction in thickness of the Navajo Sandstone from southwest to northeast and absence of the Navajo and some younger rocks in and near the Monument are shown on an isometric (three dimensional) block diagram prepared by artist John R. Stacy and me, which is displayed in the Museum of the Visitor Center. This block diagram portrays the surface and subsurface rocks from Zion National Park, Utah, to Black Canyon of the Gunnison National Monument, Colo., via Capitol Reef National Park, the Henry Mountains, and Colorado National Monument. Throughout the Plateau and parts of adjacent areas, the erosion surface on top of the Navajo Sandstone is covered by scattered pebbles of chert—a hard variety of silica (SiO₂) derived from cherty beds of freshwater limestone in the Navajo.[23] Where the Navajo has been completely eroded away and the ancient erosion surface is on the Kayenta Formation, as in Colorado National Monument, scattered pebbles (some of which are chert) derived from the conglomerate lenses in the Kayenta are found locally on the old surface.[24]
Because of this gap in the rock record we will continue part of our story farther west, where the rock record is more nearly complete.
The Sea to the West
In Middle Jurassic time the land now called central Utah, which then was the eroded surface of the Navajo Sandstone, sank beneath an arm of a shallow sea that came in from the north, and most of the area remained beneath this sea until Late Jurassic time. Sediment carried into this sea and into bordering lagoons and estuaries later hardened into the sedimentary rocks of the Carmel Formation, Entrada Sandstone, and Curtis and Summerville Formations. The Carmel and Curtis contain abundant marine fossils of Middle Jurassic age, and in central Utah the intervening unfossiliferous Entrada also is believed to have been deposited in or near the sea, and the unfossiliferous Summerville Formation probably was deposited upon a tidal flat that was submerged part of the time.
Deposits and Events East of the Sea
In eastern Utah, east of the ancient Jurassic sea, the Entrada Sandstone is entirely unfossiliferous, was partly water laid and partly wind blown, and has been divided into three distinctive parts, which in ascending order are the Dewey Bridge, Slick Rock, and Moab Members.[25] In and near the Colorado National Monument, the long period of erosion discussed in a preceding section probably continued well into the Jurassic, so only the upper part of the Slick Rock Member and the overlying Moab Member were deposited on the eroded surface of what little remained of the Kayenta Formation ([fig. 14]).
The Slick Rock Member was named from its occurrence at and near the mining town of Slick Rock, Colo., which originally was named after the appearance of the rock because it generally forms slick, smooth cliffs. It reminds one of the chicken and egg conundrum. The Slick Rock is composed mainly of sand dunes that were piled up on the eastern shore of the Jurassic sea by winds blowing from the northeast. Occasional rainy spells created lakes and ponds in which some of the sand was laid down in level beds. This pile of sand later hardened into the cliff-forming Slick Rock Member, which looks something like the Wingate but is generally only half as thick, weathers into less abrupt cliffs, is mostly salmon red, and is almost free of joints. The joints in the Wingate ([fig. 11]) probably resulted from the uplift and tilting of the Plateau before the long period of pre-Entrada erosion; whereas the land seems to have been more stable during Entrada time. The Slick Rock is cemented with calcium carbonate (CaCO₃), which is soluble even in weak acid, such as rain or snow water containing dissolved carbon dioxide. For this reason solution openings or pits occur in some of the cliff faces, the most striking of which are those shown near the top of [figure 15].
The Slick Rock Member of the Entrada Sandstone forms a line of cliffs and isolated monoliths that are second in height and grandeur only to those of the Wingate. The Member is best displayed southwest of Rim Rock Drive between the Visitor Center and the Coke Ovens and along the western arm of Ute Canyon ([fig. 16]). It also forms the Saddlehorn just south of the camp and picnic grounds near the Visitor Center ([fig. 50]). Most of the smooth cliff faces show both the steeply dipping crossbeds of the old sand dunes and the flat-lying beds of the lake or pond deposits.
ENTRADA SANDSTONE, just above normally dry waterfall in west arm of Ute Canyon. Note smooth unjointed cliff of Slick Rock Member protected at left by overhanging basal bed of Moab Member, which forms about lower half of slope in distance. Upper part of distant slope is the Summerville Formation overlain by Salt Wash Member of the Morrison Formation. Note Slick Rock at left resting upon eroded crossbedded sandstone in Kayenta Formation, in which the canyon was cut. (Fig. 16)
The overlying Moab Member of the Entrada is a white level-bedded sandstone that generally weathers into stairsteps or ledges. One of the best exposures of the Moab Member is shown in [figure 17], but good exposures also are seen along the west side of Rim Rock Drive just northeast of the Coke Ovens Overlook. In some places the Moab Member forms cliffs continuous with those of the underlying Slick Rock Member. It appears to consist of hardened beach or lagoon sand that was deposited along the eastern shore of the sea, which suggests that the sea extended farther east during Moab and Summerville times than it did during Dewey Bridge and Slick Rock times. Like the Slick Rock, the Moab also is cemented by calcium carbonate, but the lower sandstone ledges of the Moab Member are more resistant to erosion than the Slick Rock Member, so the Moab helps preserve the underlying cliffs. The top of the Moab Member forms patches of bare pavement east of the Monument, known as the “Slick Rim,” which may be observed from the Little Park Road.
MOAB MEMBER OF ENTRADA SANDSTONE, showing typical steplike weathering. In west arm of Ute Canyon about a quarter mile above the view shown in [figure 16]. Moab Member caps and protects overhang of Slick Rock Member. Moab is overlain by unexposed slope of Summerville Formation and lower part of Morrison Formation. (Fig. 17)
Although the Slick Rock Member normally is salmon colored or pink, the upper half of an outcrop just north of the highest point on Rim Rock Drive at the head of main Ute Canyon has a distinctly mottled appearance, wherein much of the color has been leached to white, but irregular splotches of color appear in the dominantly white upper part, and white splotches appear in the colored part, as shown in [figure 18]. By way of contrast, in an outcrop of the two members of the Entrada about 2 miles north of the Glade Park Store and Post Office ([fig. 19]), the entire Slick Rock Member is as white as the Moab Member, and the former is white for some distance to the east. Why is the salmon color entirely missing from the Slick Rock near Glade Park, partly missing in [figure 18], but present virtually everywhere else in and near the Monument? The answers to this seeming mystery involve events that occurred long ago, so only the high points will be touched upon here.
MOTTLED SALMON-AND-WHITE SLICK ROCK MEMBER, overlain by white level-bedded Moab Member, on west side of Rim Rock Drive about four-tenths of a mile north of head of main Ute Canyon. (Fig. 18)
It seems reasonable to suppose that the Slick Rock Member at both localities originally was salmon colored or pink, as it is everywhere else, but that later, the coloring agent, red ferric iron oxide (Fe₂O₃), was chemically reduced, or leached to ferrous iron oxide (FeO), by acidic ground water, and was carried away to the northeast by the slowly moving ground water. But as I have already pointed out, the cliff exposures of the sandstones are now bone dry, so what happened to the ground water and why was it acidic here and not elsewhere?
WHITE ENTRADA SANDSTONE, in outcrop just east of gravel road about 2 miles north of Glade Park Store and Post Office. Reasons for absence of salmon color in Slick Rock Member are given in text. (Fig. 19)
Before the cutting of the deep canyons of the Monument, which followed the last major uplifts of the region accompanied by bending and breaking of the rocks, the now dry sandstones were saturated with ground water that moved very slowly northeastward. Somewhere to the southwest the Entrada Sandstone seemingly took in water containing dissolved hydrogen sulfide gas (H₂S), changing the ground water to a weak acid. The H₂S could have been produced by a type of anaerobic bacteria that has the ability to reduce dissolved sulfates (SO₄⁻²) in water to the dissolved hydrogen sulfide gas, thereby obtaining needed oxygen.
The next questions you might logically ask are (1) if the above deductions have any merit, how do we know the acid water was caused by dissolved hydrogen sulfide,[26] (2) what is the source of the sulfate ions (SO₄⁻²) from which the H₂S was obtained, and (3) why is the color of the Slick Rock Member in [figure 19] completely reduced to white whereas that in [figure 18] is only partly reduced in the upper part?
Although the ground waters from artesian wells in the Grand Junction area contain small amounts of sulfate as do most ground waters, the amount needed for the results observed more likely came from solution of the common mineral gypsum (calcium sulfate containing some water, CaSO₄·2H₂O). The overlying Summerville and Morrison Formations contain some gypsum in many places in Utah, so it is not improbable that these formations contain gypsum locally in Colorado. If so, sulfate-bearing water could have percolated downward into the Entrada at some point southwest of Glade Park. But as this must have happened several million years ago, the clues as to just where this occurred have grown quite cold.
Seemingly, the color in the Slick Rock Member near and east of Glade Park was entirely removed by the process described, but the very slow moving ground water had time to leach only the upper part of the Slick Rock (the most permeable part) in Ute Canyon before the process was halted forever by the draining of water from these beds by canyon cutting.
Shortly before the Jurassic sea to the west dried up, silt, mud, and some sand were carried into either a shallow arm of the sea or a broad bay or lagoon near it, and later the silt, mud, and sand hardened to become the Summerville Formation. The Summerville is only 40 to 60 feet thick in the Monument but is much thicker in Utah.
The Summerville Formation is so soft that it weathers very rapidly and hence is exposed at only a few places. It is best displayed in the high roadcut at Artists Point and along the road to the south for the next mile ([fig. 20]), but it is also exposed in roadcuts along the west arm of Ute Canyon. Even the thinnest beds of the Summerville can be traced for hundreds of yards, and individual beds have a nearly constant thickness for such distances. This greatly facilitated the detailed measurement of a section of the Summerville[27] by my son Bill and me from Artists Point to the base of the overlying Morrison Formation about a mile south. Using a 6-foot folding steel rule we measured and described each thin bed from some key bed at about ground level to one at eye level, followed the upper key bed southward to ground level, then repeated the process until the entire 54 feet had been measured and described.
SUMMERVILLE FORMATION, at Artists Point ([fig. 3]). Base of formation rests upon Moab Member of Entrada just beneath the pavement. Note geologist’s pick resting upon lower ledge of sandstone just to the left of middle. Top of the Summerville here has been removed by erosion. (Fig. 20)
The Summerville at the type locality in the San Rafael Swell, Utah, is much thicker than in the Monument and contains many chocolate-brown beds; but the Summerville exhibits the same lateral continuity of even the thinnest beds. Thin sedimentary beds of such uniform thickness are thought to have accumulated in relatively quiet bodies of water. If you look at the undersides of some of the blocks of hard light-gray sandstone that have broken off, you may see corrugations like those on some metal barn roofs. These are ripplemarks produced by wave or current action while the sand was still loose, which indicates that the water was not always entirely quiet. Although much of the Summerville is red, you will see beds of many other colors including gray, blue gray, greenish gray, chocolate brown, and reddish brown.
Dinosaurs Roam the Monument
In Late Jurassic time the sea to the west eventually dried up, either because it was filled with sediments or because the land rose above sea level, or both. This brought about a change from the parallel bedding in the marginal marine environment of the Summerville to irregular stream-channel sandstones, flood-plain silts and muds, and freshwater lake deposits.
Streams from higher lands to the south brought in mud, silt, and sand that piled up hundreds of feet thick over thousands of square miles, including the Monument. These sediments were later compacted into the brightly colored siltstone, mudstone, sandstone, and limestone now known as the Morrison Formation. The colors are about the same as those of the Summerville. Algae and other microscopic organisms extracted calcium carbonate from the lake waters, and when they died this material settled on the lake bottoms to make limestone.
The soft siltstone and mudstone of the Morrison Formation weather rapidly into steep or fairly steep slopes, but the harder beds of sandstone, most of which are in the lower third of the formation, known as the Salt Wash Member, are sculptured into bold ledges or low cliffs. The generally softer upper two-thirds of the formation is called the Brushy Basin Member. The Morrison is best exposed in and southeast of The Redlands, where the bare rocks are carved into badlands like the famous ones of South Dakota. Both the Fruita Canyon and No Thoroughfare Canyon approaches to the Monument pass typical badlands in the Morrison. The entire 600 feet of this formation is best seen in the high bluff on the east side of the mouth of No Thoroughfare Canyon ([fig. 21]).
MORRISON FORMATION, on east side of mouth of No Thoroughfare Canyon. Forty feet of Summerville Formation at base is concealed by slope wash, but underlying white- and salmon-colored members of Entrada Sandstone are clearly exposed at lower left. Protective caprock at upper right is lowermost sandstone of Cretaceous Burro Canyon Formation. Upper two-thirds of Morrison is typical of the Brushy Basin Member; lower one-third is not typical of the Salt Wash Member, which generally contains more and thicker lenses of sandstone, some of which are just around the corner to the right. Mesa on left skyline is above Serpents Trail in the Monument. Looking west from Little Park Road. See also figures [55] and [60]. (Fig. 21)
In parts of the Colorado Plateau southwest of the Uncompahgre Plateau, the sandstone lenses in the Salt Wash Member of the Morrison contain uranium and vanadium ore associated with carbonaceous matter, including coalified wood. No ores have been found in or near the Monument presumably because such carbonaceous matter, which helped precipitate the ores, is lacking on the northeastern side of the Uncompahgre Plateau.
Some of the beds of siltstone and mudstone in the Brushy Basin Member of the Morrison shown in [figure 21] contain bentonite, a clay derived from the decomposition of volcanic ash, which indicates the presence of active volcanos in or near the Plateau at the time these beds were deposited. Bentonite swells when wetted, so it is widely used in well-drilling muds, sealing canals, etc. Some bentonitic material has been dug from the Brushy Basin along the Little Park Road just south of the point from which the photograph in [figure 21] was taken and was used for sealing irrigation canals in the Grand Valley.
The Morrison is not well exposed in the Monument, as the formation is restricted to the higher parts where most of it is hidden by vegetation. The lower part is seen in roadcuts and outcropping ledges along a high stretch of Rim Rock Drive between Artists Point and the head of the west arm of Ute Canyon, where sandstone lenses in the Salt Wash Member are especially thick.
The climate during Morrison time was wet enough to support abundant vegetation along the many lakes and streams—at least enough to feed the hungry dinosaurs and other reptiles that roamed the area. Many bones and parts of several skeletons of dinosaurs have been found in the Morrison at several places in The Redlands not far northeast and northwest of the Monument.
The most famous dinosaur locality near the Monument is Riggs Hill where, in 1900, the late Elmer S. Riggs of the Field Columbian Museum (now Field Museum of Natural History) dug out part of the first known skeleton of a huge Brachiosaurus ([fig. 22]). This discovery made quite a splash in the scientific world, for it was the first and only type of dinosaur found whose front legs were longer than its hind legs. The fossilized thigh bone (femur) alone is 6 feet 8 inches long and weighs 549 pounds; the arm bone (humerus), though incomplete, is even longer. The ribs are 9 feet long. A bronze plaque now marks the site of the excavation ([fig. 39]).
In 1901, Riggs removed all but the forepart of a skeleton of Apatosaurus from the southeast side of a large hill of the Morrison Formation just south of the old Fruita bridge. Riggs also found remains of Diplodocus, Camarasaurus, and Morosaurus, and, in 1937, Al Look, prominent writer and amateur paleontologist of Grand Junction, and Edwin L. Holt, an instructor in Mesa College at Grand Junction, found the closely associated remains of Allosaurus, Stegosaurus, and Brachiosaurus at the western end of Riggs Hill. Dinosaurs generally are thought of as huge creatures—many were huge indeed ([fig. 23])—but they came in various sizes and some were quite small.
An interesting Late Jurassic vertebrate fossil locality in the Salt Wash Member of the Morrison Formation, about 3 miles northwest of the West Entrance of the Monument and about 3 miles southwest of Fruita, was discovered in June 1975 by George Callison, Associate Professor of Biology and Research Associate in Vertebrate Paleontology at the California State University at Long Beach. During the summers of 1975 and 1976 Dr. Callison and his assistants removed the closely associated skeletal remains of many small, primitive mammals and both small and large dinosaurs and other reptiles. Part of the results were presented in an unpublished manuscript.[28] During the summer of 1977 and later, additional mammalian fossils were removed by Callison and assistants and additional reptilian fossils were removed by Lance Erickson, paleontologist of the Historical Museum and Institute of Western Colorado ([fig. 2]). Hopefully, the work will be continued with the aid of grants from several sources.
EXCAVATING TYPE SPECIMEN OF BRACHIOSAURUS ALTITHORAX RIGGS from south side of Riggs Hill. Photograph taken in 1900, reproduced by permission of the Field Museum of Natural History (Chicago). See also [figure 39]. (Fig. 22)
The locality, which covers parts of about 180 acres of public land administered by the Bureau of Land Management, has been fenced and posted to discourage vandalism, and has been designated the Fruita Paleontological Area.
In order to evaluate the importance of the locality and to make plans for its future development and protection, the Bureau of Land Management held the Fruita Paleontological Workshop on March 28-30, 1977, to which were invited several renowned vertebrate paleontologists and archaeologists together with interested local personnel of the Bureau, the National Park Service, and the Museum. All remarks and prepared speeches were taken down by a shorthand reporter and were reproduced for the attendees in the 83 page unpublished “The Fruita Paleontological Report.”
SKELETONS OF TYPICAL DINOSAURS OF MORRISON FORMATION.[29] A, Camptosaurus, a small dinosaur about 11 feet long; B, Apatosaurus, a gigantic dinosaur about 76 feet long; C, Allosaurus, a large carnivorous dinosaur about 30 feet long; and D, Stegosaurus, a large armored dinosaur about 24 feet long. (Fig. 23)
The close association of Late Jurassic mammalian and reptilian fossils, as found at the Fruita site, is of considerable interest and importance, but is by no means unique, for similar associations occur elsewhere in Colorado, and in Wyoming, Europe, and Africa. Of those in the United States, the quarry at Como Bluff, near Laramie, Wyo., is considered by most of the experts to be the most outstanding. Of the material unearthed at Fruita thus far, which includes bones of some of the large dinosaurs found earlier by Riggs, remains of some of the smaller dinosaurs and a complete skull of the moderately large flesh eater Ceratosaurus are considered the most important.
Freshwater clam and snail shells abound in some beds of the Morrison, particularly in limestones, and occur sparingly in other types of beds. The shells occur mainly in The Redlands, particularly about 1½ miles west of the Fruita bridge. Some of these shells that are filled with agate are sought by rockhounds.
Dinosaurs on the Move
The wet climate of Late Jurassic time was followed by arid or semiarid climate in the Early Cretaceous. Streams continued to deposit gravel, sand, silt, and mud, but at a much slower rate. These deposits eventually hardened into the conglomerate, sandstone, and green shale or siltstone of the Burro Canyon Formation. This formation, together with part of the overlying Dakota Sandstone, caps Black Ridge, the highest part of the Monument (7,000 feet) about a mile west of the Coke Ovens. Several airway beacons on this high ridge may be seen for many miles. The Burro Canyon is best seen below the Monument on the west side of Monument Road along the lower part of No Thoroughfare Canyon, where it is about 60 feet thick ([fig. 24]).
BURRO CANYON FORMATION AND DAKOTA SANDSTONE, along west side of No Thoroughfare Canyon, about 2½ miles northeast of the Monument’s East Entrance. Basal sandstone above road and unexposed green shale (brown in photograph) comprise the Burro Canyon, here 58 feet thick. White band two-thirds the way up the slope is 40-foot basal conglomerate of the Dakota Sandstone, above which is 58 feet of carbonaceous shale, a 14-foot bed of sandstone, and 17 feet of sandy shale to the top of the hill. The top of the Dakota has been eroded away. (Fig. 24)
A few fossil plants and shells have come from the Burro Canyon Formation, but the seeming absence of dinosaur bones suggests that possibly these reptiles had to move to areas of greater precipitation, where food was more abundant. Some dinosaurs may have lived in the area at this time, but their bones either were not fossilized or they have not yet been found.
Yet Another Gap in the Rock Record
Deposition of the Burro Canyon Formation was brought to a close by another uplift of the Plateau, and of course the uplift was followed by another period of erosion, which continued through the end of Early Cretaceous time. As noted in the caption for [figure 24], seemingly all but 58 feet of the Burro Canyon was eroded away, but 120 feet remains along East Creek, only about 12 miles to the southeast, which suggests that the old erosion surface was a bit uneven. That this period of erosion was of considerable duration is suggested by the abundance of the white clay mineral, kaolinite, beneath and in the overlying white basal conglomerate of the Dakota Sandstone. This type of clay commonly results from prolonged weathering of many types of rocks and indicates that the period of pre-Dakota erosion was of long duration.
Peat Bogs
By the beginning of Late Cretaceous time the eroded surface of the Monument was part of a low plain near sea level, and the sea was gradually encroaching from the east or northeast. Gravel and sand carried in by streams combined with the white kaolinite on the surface to form the 40-foot basal conglomerate of the Dakota Sandstone ([fig. 24]).
As the land gradually subsided nearer to sea level, swamps which were formed along the coast supported considerable vegetation. As the trees and plants died and were covered by silt and mud, they gradually changed to peat which finally became compacted into coal and brown or black coaly shale containing plant remains. You can dig out some of this coal and perhaps find some plant remains near the top of the west canyon wall just below the highest sandstone bed in [figure 24], which is outside the Monument.
For awhile the coast alternately sank slightly below and rose slightly above sea level. Beach sand covered the swamp deposits, then more swamp deposits covered the sand. Some of the sand contains seashells, such as oysters and clams.
Except on Black Ridge, the Dakota has been entirely eroded from the Monument, but it crops out with the underlying Burro Canyon in a series of low hills south of the Colorado River. The Dakota Sandstone is about 200 feet thick.
The Sea Covers the Plateau
Still later in the Cretaceous Period the whole region sank beneath the sea and stayed there a long time. Silt and limy mud were piled layer upon layer on the sea floor and hardened into the gray and black Mancos Shale. Thin layers of sand were cemented into sandstone, and layers of calcium-carbonate mud became chalk or limestone. Seashells and bones of sharks and seagoing reptiles have been found in the Mancos in many places.
The Mancos and all younger rocks have been stripped off the Monument, but they may be seen one after the other as you travel northeastward. Thin remnants of the Mancos cap low hills just south of the Colorado River, and the entire 3,800 feet of the Mancos underlies the Grand Valley and Book Cliffs. The upper part is clearly exposed in the towering, barren Book Cliffs, where the soft shale is protected by a caprock of hard sandstone—the lowermost unit of the overlying Late Cretaceous Mesaverde Group ([fig. 25]).
The Sea’s Final Retreat
Slow uplift of the Plateau, including the Monument region, caused the gradual retreat of the Mancos sea. Deposition of mud on the sea bottom gave way to deposition of beach sand, coal swamps, and then more beach sand and coal swamps. Finally, in Late Cretaceous time, the sea withdrew entirely, never again to return to the Colorado Plateau region.
Streams deposited sand, silt, and mud on the newly uplifted coastal areas. All these deposits, including some high-grade bituminous coal that was formed in the swamps, we now know as the Mesaverde Group. The thick cliff-forming sandstones of this unit are beautifully displayed in DeBeque Canyon of the Colorado River between Palisade and DeBeque, just upstream from the Grand Valley. There are several active coal mines in the Mesaverde between Palisade and Cameo, and outcrops of coal may be seen on the east side of the road just south of Cameo. The electric generating station of the Public Service Company of Colorado at Cameo is conveniently situated over a coal mine and next to the Colorado River, which supplies cooling water.
MOUNT GARFIELD, a prominent point on the Book Cliffs bordering the northeastern side of the Grand Valley. Slopes are Mancos Shale; ledge about halfway upslope is the toe of an ancient landslide deposit of Mesaverde sandstone blocks marking the level of an ancestral Grand Valley; capping beds of sandstone at crest are basal beds of Mesaverde Group. (Fig. 25)
PHOTO INDEX MAP, showing localities where most of the photographs were taken. Arrows point to distant views. Numbers refer to figure numbers. (Fig. 26)
Photographs for four figures are not shown because figures 5, 25 and 36 were taken outside the map borders and figure 1 was taken at an undisclosed locality in the monument
[High-resolution Map]
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The remains of dinosaurs have been discovered in rocks of this age elsewhere, but near the Monument only their tracks have been found. Some of these, in coal mines along the Book Cliffs and near Cedaredge, are 38 inches across and their placement indicates an incredible stride of 16¼ feet! Had there been highways in Mesaverde time, this bipedal giant could have crossed them in two strides.
Both the Mancos and Mesaverde once covered the Monument area but were removed long ago by erosion.
End of the Dinosaurs
The end of the Cretaceous Period was also the end of the dinosaurs. Exactly why the “terrible lizards” died out after dominating the world for more than 150 million years is not known for sure, but many guesses have been made.
One likely idea is that widespread uplift and mountain building that began late in Cretaceous time, accompanied by changes in climate, may have greatly reduced the supply of soft edible plants. If so, it is easy to imagine how huge dinosaurs accustomed to a ton or more of lush plant food each day would soon starve to death.
Many dinosaurs were vegetarians. As they died out, the flesheaters, such as Tyrannosaurus, soon ran short of food also, and probably began to eat each other. Tyrannosaurus closely resembled the Jurassic Allosaurus shown in [figure 23], except that Tyrannosaurus was much larger and more formidable—in fact it probably was the most terrible predator that ever roamed the surface of the Earth. The dinosaurs had become too highly specialized to their environment to adapt themselves to changes of this kind.
Another fascinating notion is that the growing population of small primitive mammals devoured dinosaur eggs (which were left unattended like those of turtles and alligators) nearly as fast as mamma dinosaur could lay them. But whatever the reason, it is clear that some worldwide condition caused the gradual extinction of the ponderous over-specialized dinosaurs and allowed the rise to power of the next types of animals destined to rule the Earth—the brainier and more adaptable mammals.
At this time the rocks were gently bent into upfolds, called anticlines or arches, and downfolds, called synclines or basins ([fig. 27]). One upfold that began to take form was the Uncompahgre arch, the crest of which shapes Piñon Mesa just south of the Monument. But this gentle upfold was to grow larger and to have its flanks wrinkled and broken in the next geologic era—the Cenozoic.
The Age of Mammals
The beginning of the Cenozoic Era 65 million years ago—give or take a few million years—marked the beginning of a long span of geologic time during which mammals became the ruling land animals. Remains of some small primitive mammals have been found in Mesozoic rocks ([p. 50]), but these tiny newcomers did not have a chance to flourish until the formidable dinosaurs died out.
The Cenozoic Era is divided into the long Tertiary Period—The Age of Mammals—and the short (about 2 million years) Quaternary Period—The Age of Man. The Tertiary in turn is divided into five epochs—the Paleocene, Eocene, Oligocene, Miocene, and Pliocene ([fig. 61]). Events during parts of the Tertiary Period had an important bearing upon the Monument even though no rocks of this period now occur in the area.
COMMON TYPES OF ROCK FOLDS. Top, anticline, or upfold; closed anticlines are called domes. Middle, syncline or downfold; closed synclines are called structural basins. Bottom, monocline, a common type on the Plateau in which the dip of the beds changes in amount but not in direction; axes may be mapped along trends of upper fold, middle flexure, or lower fold. Top and middle diagrams from Hansen (1969, p. 31, 108). (Fig. 27)
Early Deposits and Events
The broad inland basins that were formed late in the Cretaceous Period received sand, silt, and mud brought in by streams from the uplifted or folded areas. These materials became compacted into the Wasatch Formation—the red or pink rock from which Bryce Canyon National Park was sculptured. One such basin lay just northeast of the Monument. The Monument probably was covered by some of these stream deposits after the main basin was partly filled.
The mammals that roamed the area during the Paleocene Epoch were primitive, but more advanced forms appeared later, in Eocene time. Some of their fossil remains have been found in the Wasatch Formation in Plateau Creek Valley north of Grand Mesa and near Rifle, about 60 miles northeast of Grand Junction. The entire 5,000 feet of the Wasatch may be seen along U.S. Highway I-70 between the towns of DeBeque and Grand Valley, and much of it helps support towering Grand and Battlement Mesas.
Lake Uinta
In Eocene time the northern part of the Colorado Plateau sagged downward and gradually filled with water until it became a huge lake, now known as Lake Uinta. The waters in it teemed with plants and animals, particularly micro-organisms such as algae, whose remains, coated with calcium carbonate, settled to the bottom along with the sand, silt, and mud washed into the lake by streams. These sediments compacted into the remarkable Green River Formation which contains, among many rock types, large deposits of rich oil shale.
The light-colored Green River Formation, which is about 3,800 feet thick, may be seen from U.S. Highway I-70 in the upper part of the towering Roan Cliffs on the northwest side of the Colorado River between DeBeque and Rifle. It also underlies the volcanic caprock of Grand and Battlement Mesas. John R. Donnell, of the U.S. Geological Survey, estimated that the oil shale in the Piceance Creek Basin, northwest of the Colorado River alone, contains more than one trillion barrels of oil. The Monument was at or near the south shore of this lake, and may have been covered with a few hundred feet of the Green River Formation.
The Mountains Rise Again
Lakes, like mountains, are temporary things. Even as lakes are forming, sediment begins to fill them until ultimately they are obliterated. So it was with Lake Uinta. Sometime after this lake dried up, the Earth’s crust again became restless. The gentle folds that were formed late in the Cretaceous were lifted higher and bent more sharply, and the flanks of some folds were wrinkled and broken (figs. [27], [28]). The sharply bent or broken rocks along the northeastern border of the Monument are thought to have been deformed mainly at this time, but in part both earlier and later. That pronounced folding of the rocks followed the deposition of the Eocene Green River Formation is clearly shown along the Grand Hogback monocline between the towns of Rifle and Meeker, Colorado, where the once flat lying beds of the Green River and Wasatch Formations now stand vertical.
The folds and faults along the northeastern border of the Monument, which are shown on the geologic map ([fig. 8]), are discussed briefly here—more details are given later in “Trips through and around the Monument.” The folded and faulted northeastern border of the Monument, which is shown in [figure 29] and in several ensuing photographs, is believed to have resulted from renewed uplift of the area southwest of the folds and faults, including the Monument. The Redlands fault (figs. [8], [29], [37], [38], [40], [41]) generally is a normal fault but locally is a reverse fault, as discussed on [page 92] and as shown in [figure 40] and in the cross section of [figure 8]. This fault has a maximum vertical displacement of 700 or 800 feet, but dies out in scissors fashion at each end. Beyond the end of the Redlands fault in the upper right of [figure 29] may be seen another unbroken monocline. A close-up view of the northwestern end of this fold in shown in [figure 30].
COMMON TYPES OF FAULTS. Top, normal, or gravity, fault which generally results from tension in and lengthening of a segment of the Earth’s crust, which allows the lower block to subside. However, some normal faults, particularly some that are vertical or nearly so, may result from uplift of the upper block. Low-angle reverse faults generally are called overthrust faults or simply overthrusts. In both the normal and reverse faults note amount of displacement and repetition of strata. Displacement of such faults may range from a few inches to many thousands of feet, and in overthrusts may reach many miles. From Hansen (1969, p. 116). (Fig. 28)
If we proceed about a quarter of a mile northeast of the point from which [figure 30] was taken, walk about 50 feet north, and look to the northwest, we see quite a different structure, for here the gentle lower fold of the Lizard Canyon monocline has become the east end of the Kodels Canyon fault ([fig. 31]).
LADDER CREEK MONOCLINE AND REDLANDS FAULT, telephoto view looking northwest from point near Little Park Road east of the Monument. No Thoroughfare Canyon in foreground, which is bordered on the left by northeastward-dipping beds of Wingate Sandstone at northwest end of Ladder Creek monocline. The old Serpents Trail, the lower part of which is barely visible, ascends this dipping block of rock. The dark Proterozoic rocks form the flat-topped bluff to the right and are exposed by the Redlands fault which lies just above the sharply upturned remnants of the Wingate Sandstone. (Fig. 29)
LIZARD CANYON MONOCLINE, looking southeastward across mouth of Lizard Canyon from southeasternmost loop of Rim Rock Drive just before it ascends Fruita Canyon. Note gentle lower bend at lower left and sharper upper one at upper right. Lower bend changes to Kodels Canyon fault in Fruita Canyon behind camera station. Grand Mesa forms left skyline. (Fig. 30)
KODELS CANYON FAULT, looking northwest across mouth of Fruita Canyon from point on Rim Rock Drive just described in text. Here, along a normal fault dipping steeply northeastward, the 350-foot cliff of Wingate Sandstone at upper left has been sheared and squeezed into only a few feet of broken rock overlain by a steep slope of the Kayenta Formation covered by piñon and juniper. The thinner cliff at right is the Entrada Sandstone which belongs high atop the cliffs at left. Book Cliffs form distant skyline at right. (Fig. 31)
If you doubt that [figure 31] shows a fault, a glance at [figure 32] in the next major canyon eight-tenths of a mile to the northwest should convince you. Here, on the northwest side of Kodels Canyon, the Wingate was not thinned but was rent completely asunder by the vertical Kodels Canyon fault ([fig. 32]). Kodels Canyon is not readily accessible to visitors.
The Lizard Canyon monocline, Kodels Canyon fault, and other structures are clearly shown in the stereoscopic pair of aerial photographs in [figure 33].
Another structural feature within the Monument is the Glade Park fault ([fig. 8]), which lies mainly south of the Monument but just cuts across the south end of No Thoroughfare Canyon in the latest addition to the Monument. It is well shown both from the air and the ground in figures [58] and [59]. It is unique among all the major faults in the area in that the rocks south of the fault subsided with respect to those on the north side.
KODELS CANYON FAULT, looking northwestward across canyon of same name. Base of Wingate cliff on left is just about opposite the top of the Wingate on right. Here, nature was kind to the geologist, for the vertical displacement (rise of left side with respect to right side) is virtually the thickness of the Wingate Sandstone—about 350 feet. The Wingate on the right is lighter colored than that on the left seemingly because rockfalls removed desert-varnish-coated rocks and exposed the true color of the sandstone. (Fig. 32)
GEOLOGIC STRUCTURES AT FRUITA ENTRANCE TO COLORADO NATIONAL MONUMENT. The stereoscopic pair of aerial photographs may be viewed without optical aids by those accustomed to this procedure, or by use of a simple double-lens stereoscope, such as the folding ones used by the armed forces during and after World War II. Geologic details may be identified by comparing photographs with the geologic map, [figure 8]. If viewer is unable to see stereoscopic pairs in three dimensions, looking at either photograph alone will convey a good idea of the geologic structure. The monocline near top of the photographs may be seen on the right-hand side of the highway in [figure 43]. Photographs taken in 1937 by U.S. Soil Conservation Service, hence, alinement of then unpaved Colorado Highway 340 differs from the paved present highway. (Fig. 33)
At this point in our story it might be well to point out that the folding and faulting of the rocks just described occurred when thousands of feet of younger rocks covered the area. Additional folding and faulting, drainage changes, and gradual removal of the overlying rocks occurred during the remainder of the Tertiary and Quaternary Periods, as will be discussed further.
Nearby Lava Flows[30]
Grand and Battlement Mesas, respectively east and northeast of the Monument, are capped by several resistant thick flows of dark basaltic lava. The molten rock welled up through fissures at the east end of Grand Mesa and flowed westward and northwestward over the eroded surface of Eocene rocks. Radiometric dating of a sample of the basalt indicated an age of 9½ million years plus or minus half a million years, placing the event in the Miocene Epoch of the Tertiary Period ([fig. 61]).
A small remnant of the lava on the crest of the Roan Cliffs just southwest of the present town of Grand Valley indicates that the flows crossed this part of the ancestral Colorado River Valley and may have pushed the young stream westward.
The lava flows are about 800 feet thick on the eastern part of Grand Mesa but are only about 200 feet thick above the western rim of the mesa. As the ancestral Gunnison River is believed to be pre-Miocene in age, it is not known whether or not the lava flows crossed the old river valley and reached as far west as the Monument.
Ancestral Colorado River
During most of the Pliocene Epoch the ancestral Colorado River did not flow past what is now Grand Junction; instead, it joined with the ancestral Gunnison River about 10 miles southeast of the present city, and the combined streams flowed southwestward across the slowly rising Uncompahgre arch through what was later to be called Unaweep Canyon ([fig. 36]). Southwest of the canyon, near the site of the present town of Gateway, the ancestral Colorado River was joined by the combined flows of the ancestral San Miguel River and the previously diverted ancestral Dolores River, then it flowed northwestward to what is now the mainstem of the Colorado River.
I have attempted to show my ideas of this ancient drainage system as it may have existed in middle to late Pliocene time in [figure 34]. But the stage was set for more spectacular drainage changes to follow.