Chapter II: Front Matter (2)

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.

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.

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.

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?

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.

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).

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.

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.”

_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).

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.

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]

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.

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.

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).

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.

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.

Piracy on the High Plateaus

Rivers, like people, do not always choose their courses wisely. After a few million years of downcutting through the soft sedimentary rocks, mainly what is now called the Mancos Shale, the ancestral Colorado and Gunnison Rivers found themselves cutting through the hard Proterozoic rocks in a deep gorge athwart the slowly rising Uncompahgre arch, which greatly slowed the downcutting power of the combined streams. Note in figure 34_A_ that while the mighty ancestral Colorado and Gunnison Rivers were in this frustrating predicament, a young upstart tributary began cutting northward from what is now the mouth of the Dolores River (fig. 34_D_). Although the combined main rivers could lower their channel only very slowly because of the hard rock in Unaweep Canyon, the tributary was able to cut downward and headward quite rapidly through the soft Mancos Shale. It eventually cut around the northwestward dipping Uncompahgre arch and headed southeastward toward the ancestral Colorado River near the present site of Palisade.

Then occurred an act of piracy that put to shame the mightiest exploits of Blackbeard and Captain Kidd. In latest Pliocene or earliest Pleistocene time additional uplift of the Uncompahgre arch, an unusually large flood, or both, caused the ancestral Colorado River to overflow its banks and spill across a low shale divide into the headwaters of the tributary. Some ponding may have preceded the spillover. With this enormously increased supply of water, the tributary cut down rapidly through the soft shale and captured the entire flow of the Colorado River, but the ancestral Gunnison River still flowed through Unaweep Canyon, as shown in figure 34_B_. Stream capture of this type is appropriately called “piracy.”

But the piracy had not ended. Note in figure 34_B_ that the “new” river sent out several tributaries, one of which headed for and, with the aid of yet additional and greater uplift, soon captured the ancestral Gunnison River, as shown in figure 34_C_. This second act of piracy left Unaweep Canyon really “high and dry” except for small streams that carried off what little water the canyon received from local rain and snow. While these piracies were taking place, the Book Cliffs and the edge of Grand Mesa gradually retreated away from the valley because of erosion, and more of the Uncompahgre arch was uncovered.

The rising Uncompahgre arch, whose renewed uplifts in latest Pliocene or earliest Pleistocene times played such an important role in the ultimate abandonment of ancestral Unaweep Canyon, was asymmetric in that the crest was not in the middle but was near the southwest side. Although sharp, locally faulted monoclines are found on both sides of the arch, including the part within the Monument, in general, the northeastern flank has a rather gentle northeastward dip; whereas, the southwestern flank of the arch also is bordered by normal faults of considerable vertical displacement.[31] Thus, after abandonment, the minor drainage in Unaweep Canyon continued to flow northeastward from a new divide near the southwestern border, and ancestral West Creek began cutting northeastward toward the new divide. The drainage pattern depicted in figure 34_C_ differs slightly from my earlier interpretation and results from additional fieldwork.[32]

At the time of abandonment, ancestral Unaweep Canyon was a V-shaped canyon resembling Glenwood Canyon just upstream from the city of Glenwood Springs, Colorado. The reasons for its change in shape and appearance to the beautiful U-shaped canyon we find today and the profound effect the abandonment of Unaweep Canyon had on the deepening of the Grand Valley and the canyons of the Monument will be brought out in the section “Canyon Cutting.”

The Age of Man

Like the dinosaurs before them, a few of the Tertiary mammals were so long on brawn and short on brains that they evolved into grotesque monsters and overspecialized themselves into early extinction. Fortunately, however, most of the mammals evolved more slowly and moderately into the forms we find today.

One group—the anthropoid primates—began to think, so they developed their brains rather than their brawn, particularly the Tertiary ancestors of man. Few remains of these ancestors have been found in Tertiary rocks, but many more have been discovered in rocks of the next geologic period—the Quaternary. Thus, this period may properly be regarded as the age of man, for man then began to dominate the Earth for better or for worse.

The Quaternary—latest and shortest of the geologic periods—is divided into the Pleistocene and Holocene (recent) Epochs (fig. 61).

The Ice Age

During the Pleistocene Epoch, all continents of the Northern Hemisphere and some of the Southern Hemisphere were partly covered at least four times by huge glaciers. Each glacial advance in Europe and North America was ended by a warmer interval during which the glaciers melted and retreated northward; then, vegetation and soil had time to become re-established. Thus, the Pleistocene has properly been called the ice age.

None of the continental glaciers reached the Monument or the Uncompahgre Plateau, or arch, but small alpine glaciers grew in the high Rocky Mountains to the east, sculpturing sharp-crested peaks and ridges and forming beautiful valleys and lakes. Many of the beautiful lakes on Grand Mesa were formed by glaciation, but some near the edges were formed by landslides.

The increased streamflow from the greater precipitation and from melting alpine glaciers in the Rockies, particularly during times of glacial retreat, helped the Colorado River cut through the rocks faster, thus assisting in the formation of Colorado National Monument as we see it today. The river carried thousands of cubic miles of sediment to the Gulf of California, including a lot of rock that once covered the Monument, and the river is still actively at work on this immense earthmoving project.

If the ancestral Colorado River carried sediment at about the same rate as the present river since the building of Hoover Dam, it may have carried about 3 cubic miles of sediment each century. Now most of the rock debris is being dumped into Lake Powell—the new reservoir behind Glen Canyon Dam. When this, Lake Mead, and other reservoirs ultimately become filled with sediment, the Gulf of California will again be the burial ground.

But other events during the Pleistocene also played a role in shaping the area. The Uncompahgre arch was again uplifted and deformed in the Pleistocene soon after the abandonment of Unaweep Canyon. This caused added tilting of the strata and more bending and breaking along some of the folds and faults in the Monument.

Capture of East Creek

East Creek, which drains the northeastern half of Unaweep Canyon, was forced to change its course during the Pleistocene Epoch by another act of piracy. After capture of the Gunnison River by the newly routed Colorado River, East Creek joined the Gunnison by way of Cactus Park. Then, a tributary of North East Creek headed southward and captured East Creek, as shown in figure 34_D_.

Canyon Cutting

When the Colorado River was diverted into its new course through the Grand Valley past the Monument, the stream channel seems to have been only about 600 to 800 feet higher than it is today, but the present divide in Unaweep Canyon is now about 2,500 feet higher than the channel. The difference of 1,700 to 1,900 feet was caused by the additional uplift of the Uncompahgre arch during the Pleistocene.

Thus, the Grand Valley and its tributary canyons, such as those of Colorado National Monument, were cut since the abandonment of Unaweep Canyon, probably mainly during the Quaternary Period. This suggests that the cutting of the Monument’s canyons began only about 2 million years ago, but that much of the canyon cutting occurred only a few hundred thousand years ago. Indeed, the canyons are still slowly being deepened, lengthened, and widened.

As you stand on any of the lookout points and gaze down into the canyons of the Monument, you may well wonder how such immense chasms could have been cut by such puny streams that are dry most of the time. The streams flow only for short periods after heavy thundershowers or after rapid melting of snow. If you are lucky enough to see them flow, you will notice that the water is red or brown because of the suspended mud, silt, and sand. If the flow is large, you may see or hear pebbles and cobbles rolling along the bed. Accordingly, the streams and their cutting tools are slowly deepening the channels. But, you may ask, how does this account for such wide, broad-bottomed, cliff-walled canyons? Such streams act mainly as storm sewers to carry off the rock debris formed by other types of erosion.

When cutting first began, the Monument’s canyons were narrow, steep, and V-shaped. When the top of the hard, dark Proterozoic rocks was reached, however, downcutting slowed just as it had earlier in Unaweep Canyon. While the streams were thus hung up, other erosional processes caused the cliff walls to recede away from the streams, forming broad, flat-bottomed, U-shaped canyons.

Recession of the cliffs away from the middle of the canyons probably was caused partly by undercutting of the soft Chinle Formation by wind and in places by streams. This allowed slabs of the overlying Wingate Sandstone and younger rocks to break off and fall into the canyons—eventually to break up and to be carted off as sand and mud by streams.

But other processes are probably the ones chiefly responsible for the present shape and width of the canyons. The summer sun heats the cliff faces until they are hot to the touch, but in the present desert climate of the Monument the rocks cool rapidly after sundown. Oftentimes the hot cliff faces are chilled rapidly by summer thundershowers. Repeated heating, cooling, wetting, and drying causes expansion and contraction of the rocks so that thin layers break off and fall. This process goes on slowly even in winter on sun-facing cliffs, but it does not occur on the cliffs that face away from the winter sun.

Even more important, perhaps, is the alternate freezing at night and thawing by day on sun-facing cliff faces during the winter. Water in cracks near the cliff faces alternately freezes and melts, gradually prying off slabs of rock. Canyon walls that are shaded from the sun most of the winter, however, stay cold or frozen much of the winter; hence, they are not subject to repeated heating and cooling or freezing and thawing. Thus, you will notice that because of talus accumulation many such canyon walls are sloping rather than vertical.

To illustrate the above conjectures concerning the cutting and shaping of the canyons, let us consider several canyons that trend in different directions. We have seen in figure 12 that the left side of northeastward trending Red Canyon is a nearly vertical cliff that faces the sun most of the winter; whereas, the right side, which is shaded most of the winter, slopes gently enough to be climbed at many places. The sides of Ute Canyon, which trends more nearly northward (fig. 52), slope about equally, as would be expected. However, the west arm of the Canyon, which trends slightly southeastward, has sides whose slopes differ markedly (fig. 35).

This brings us to the remarkable transformation of the original V-shape of Unaweep Canyon to the beautiful U-shape of the present canyon, which is shown in figure 36. The abandonment of Unaweep Canyon discussed earlier removed the gigantic storm sewer that for millions of years carried off the products of vigorous erosion of the canyon walls by the processes just described. Rock materials that now fall from the cliffs of the inner gorge in hard Proterozoic rocks and that fall from the overlying softer sedimentary rocks simply pile up at the foot of the cliffs to form a canyon equally as U-shaped as those cut by glaciers in the high mountains. Indeed, Unaweep Canyon has been mistaken for a glacial canyon by many, including some geologists.

A Look into the Future

This ends the brief geologic story of Colorado National Monument, except for a peek into the future, a description of trips through and around the Monument, and a comparison with other Parks and Monuments on the Plateau. The temporary nature of lakes, rivers, and even mountains has been discussed—the Monument of today and the new course of the Colorado River are no exceptions.

The Colorado River did not solve its problems by abandoning its hard-rock course in Unaweep Canyon in favor of a soft-rock course through Ruby and Westwater Canyons—it just postponed them. The river has again cut down into its old nemesis—the hard Proterozoic rock—in Ruby Canyon just within the Colorado border, in Westwater Canyon in Utah, and the Gunnison River has reached the hard rock at its confluence with Dominguez Creek, not far above Whitewater, as shown in figure 34_D_. Thus, once again hard rock is slowing down old man river, and will slow him down for a long time to come. Someday, Westwater and Ruby Canyons will be deep gorges like Unaweep Canyon. Then it is quite possible that another young tributary may sneak around the Uncompahgre arch some miles northwest of these canyons and pirate the river into a new soft-rock course.

By this time, the Monument will have changed appearance considerably. Some of the canyons will have come together by eating away the ramparts that separated them—just as the two entrances of Monument Canyon have already done. But as the lower canyons thus eliminate themselves, the headwaters will bite deeper into Piñon Mesa, so perhaps the Monument will simply creep slowly southwestward. However, renewed uplift, more volcanos, changes in climate, or other events could alter the picture.

Still, if the geologic clock ran as fast as the ones we use, the picture of the Monument we see today would be on the screen only a small fraction of a split second. But the geologic clock ticks on, slowly but surely, and, someday, the Holocene Epoch in which we live will become just another brief chapter in the long geologic history of the Earth.

How to See the Monument

How to see the monument depends in large part on how long you can stay, but it depends also upon the direction you are travelling to or through Grand Junction and Fruita, and on the mode of transportation. Moreover, the Monument has four entrances—two main entrances from Fruita (West Entrance) and Grand Junction (East Entrance), and two subordinate entrances from the Glade Park area to the southwest.

Though by no means as well known as our large National Parks, Colorado National Monument is more readily accessible than many. It is on two transcontinental highways (U.S. 6 and 50), is the western terminus of U.S. 24, and is on nearly completed Interstate 70, one of the most scenic transcontinental Interstate Highways. Highway I-70 supersedes many stretches of U.S. 6, 24, and 50, but the latter are still used in parts of the Grand Valley and elsewhere. The Monument also is on the main line of the Denver and Rio Grande Western Railroad, which still maintains limited passenger service between Denver and Salt Lake City, but this service may eventually be terminated. Grand Junction’s Walker Field is served by several airlines, and both Grand Junction and Fruita are served by busses.

Many people driving through the Grand Valley for the first time are unaware of the Monument’s existence unless they happen to see it on a roadmap or see road signs pointing toward it, but unfortunately the signs are unevenly and poorly distributed. People entering Grand Junction from the east on U.S. 6 and 24 or from the southeast on U.S. 50 are apt to see one or more of the signs—particularly when crossing Grand Avenue on First or Fifth Streets. If they are heading westward and can devote at least half a day, they may drive to the East Entrance, follow Rim Rock Drive for 22 miles northwestward through the Monument, stop at some or most of the scenic overlooks and the Visitor Center, leave via the West Entrance, and proceed northwestward on U.S. 6 and 50 or better yet on I-70. However, as will be described below, longer stops are much more rewarding. Those driving eastward on I-70 may see the sign at the Fruita interchange pointing southward toward the Monument—and may take the above described quickie trip in reverse. Those heading northwestward on I-70, however, may not be aware of the Monument until they see the sign at the Fruita interchange; then, they may not have or take time to double back southeastward through the Monument. If they do drive southeastward through the Monument, however, they can return to Fruita following a very scenic northwesterly route through The Redlands on Broadway (Colo. Highway 340) and South Broadway, or they may take a paved shortcut from near the East Entrance to South Broadway via South Camp Road (p. 118). Drinking water and sanitary restrooms are available in the headquarters area in the campgrounds and picnic grounds and Visitor Center, and in the Devils Kitchen Picnic Area. Food is not available in the Monument, so those planning to remain all day should bring lunches.

I have conducted many groups through the Monument, always choosing to travel northwestward from Grand Junction through The Redlands, just northeast of the Monument, to the West Entrance, then returning southeastward through the Monument.[33] On most days, taking the trip in these directions affords good lighting for photographing most of the scenic features. This and other routes are described in the next section and are plainly labeled so that the visitor may start with any trip he or she chooses. Regardless of how long you stay or which routes you follow, it is advisable to be well supplied with color film.