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)
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.
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.
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)
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
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.
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.
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.
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.
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 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.
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.
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.
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).