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]

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