SUMMARY OF GEOLOGIC HISTORY
Having finished our geologic ramble through Canyonlands National Park, let us see how this pile of eroded rocks fit into the bigger scheme of thingsthe geologic age and events of the earth as a whole, as depicted in figure 80. As shown in figure 9, the rock strata still preserved in the park range in age from Pennsylvanian to Jurassic, or from about 300 to 175 million years ago, a span of about 125 million years. This seems an in credibly long time, until you note that the earth is some 4.5 billion years old and that our rock pile is but one twenty-fifth, or 4 percent, of the age of the earth as a whole. Thus, in figure 80 the rocks exposed in the park occupy only about the left-hand third of the top whorl of the spiral.
But this is not the whole story. As indicated earlier, about 10,000 feet of younger Mesozoic and Tertiary rocks that once covered the area have been carried away by erosion, and if we include these, the span is increased to about 250 million years, or nearly a full whorl of the spiral.
Deep tests for oil and gas tell us that much older rocks underlie the area, and we have seen that some of these rocks played a part in shaping the park we see todaynote the breaks in the deep-seated Precambrian rocks and the salt in the Paradox Member. In addition to the Precambrian igneous and metamorphic rocks, there are about 2,000 feet of Paleozoic sedimentary-rocks older than the Pennsylvanian Paradox Member. Most of these sedimentary rocks were laid down in ancient seas during Cambrian, Ordovician, Devonian, Mississippian, and Pennsylvanian times (fig. 80). There are some gaps in the rock record caused by temporary emergence of the land above sea level and erosion of the land surface before the land again subsided below sea level so that deposition could resume. Silurian rocks are absent altogether, presumably because here the Silurian Period was dominated by erosion rather than deposition.
While Pennsylvanian and Permian sediments were being deposited in and southwest of the park, a large area to the northeastcalled by geologists the Uncompahgre highland, because it occupied the same general area as the present Uncompahgre Plateaurose slowly above sea level. Whatever Paleozoic rocks there were on this rising land, plus part of the underlying Precambrian rocks, were eroded and carried by streams into deep basins to the northeast and southwest. Thus, while mostly marine or nearshore deposits were being laid down in and near the park, thousands of feet of red beds were being laid down by streams in an area between the park and the Uncompahgre Plateau. During part of Middle Pennsylvanian time a large area including the park and known as the Paradox Basin was alternately connected to or cut off from the sea, so the water evaporated during cutoff periods and was replenished during periods when connection with the sea resumed. In this huge evaporation basin were deposited the layers of salt and gypsum plus some potash salts and shale that now make up the Paradox Member.
The old Uncompahgre highland continued to shed debris into the bordering basins until Triassic time, when it began to acquire a veneer of red sandstone and siltstone of the Chinle Formation (Lohman, 1965). The area remained above sea level during the Triassic Period and most if not all the Jurassic Period, although the Jurassic Carmel Formation was laid down in a sea that lay just to the west.
Late in the Cretaceous Period a large part of central and southeastern United States, including the eastern half of Utah, sank beneath the sea, as shown in figure 81, and received thousands of feet of mud, silt, and some sand that later compacted into the Mancos Shale. This formation and all the younger and some older strata have long since been eroded from the park area but are present in adjacent areas, such as the lower slopes of the Book Cliffs north of Green River, Crescent Junction, and Cisco (fig. 7).
The land rose above the sea at about the close of the Cretaceous and has remained above ever since, although inland basins and lakes received sediment during parts of the Tertiary Period. Compressive forces in the earth's crust produced some gentle folding of the strata at the close of the Cretaceous, but more pronounced folding and some faulting occurred during the Eocene Epoch, when most of the Rocky Mountains took form. During the Miocene Epoch molten igneous rock welled up into the strata to form the cores of the nearby La Sal, Abajo, and Henry Mountains (fig. 7). Additional uplift and some folding occurred in the Pliocene and Pleistocene Epochs.
Much of the course of the Colorado River was established in the Miocene Epoch, with some additional adjustments in the late Pliocene and early Pleistocene Epochs (Hunt, 1969, p. 67). Erosion during much of the Tertiary Period and all of the Quaternary Period, combined with some sagging and breaking of the crust brought on by solution and lateral squeezing of salt beds beneath The Needles, The Grabens, and the Meander anticline, produced the landscape as we now see it.
The Precambrian rocks beneath the area are about 1.5 billion years old, so an enormous span of time is represented by the rocks and events in and beneath Canyonlands National Park.
If we consider the geologic formations that make up the Colorado Plateauincluding national parks (N.P.), national monuments (N.M.) (excluding small historical or archeological ones), Monument Valley, San Rafael Swell, and Glen Canyon National Recreation Areacertain formations or groups of formations play starring roles in some parks or monuments, some play supporting roles, and in a few places the entire cast of rocks gets about equal billing. Let us compare them and see how and where they fit into the geologic time spiral (fig. 80).
Dinosaur N. M., with exposed rocks ranging in age from Precambrian to Cretaceous, represents the greatest time span (nearly 2 billion years) but has one unitthe Jurassic Morrison Formationin the starring role, for this unit contains the many dinosaur fossils that give the monument its name and fame; several older units have supporting roles. Grand Canyon N. P. and N. M. are next, with rocks from Precambrian through Permian (excluding the Quaternary lava flows in the N. M.), but here there is truly a team effort, for the entire cast gets about equal billing. Canyonlands N. P. stands third in size of cast, with rocks ranging from Pennsylvanian to Jurassic, but we would have to give top billing to the Permian Cedar Mesa Sandstone Member of the Cutler Formation, from which The Needles, The Grabens, and most of the arches were sculptured; the Triassic Wingate Sandstone and Kayenta Formation get second billing for their roles in forming and preserving Island in the Sky and other high mesas.
Now let us consider those with only one or few players in the cast, beginning at the bottom of the time spiral. Black Canyon of the Gunnison N. M., cut entirely in rocks of early Precambrian age (except for only a veneer of much younger rocks), obviously has but one star in its cast. Colorado N. M. contains rocks ranging from Precambrian to Cretaceousequal to Dinosaur in this respectbut it is unique in that all the rocks of the long Paleozoic Era and some others are missing from the cast; of those that remain, the Triassic Wingate and Kayenta are the stars, with strong support from the Jurassic Entrada Sandstone.
All the bridges in Natural Bridges N: M. were carved from the Permian Cedar Mesa Sandstone, also the star in Canyonlands N. P. In Canyon de Chelly (pronounced "dee shay") N. M. and Monument Valley (neither N. P. nor N. M., as it is owned and administered by the Navajo Tribe), the de Chelly Sandstone Member of the Cutler Formationa Permian member younger than the Cedar Mesaplays the starring role.
Wupatki N. M., near Flagstaff, Ariz., stars the Triassic Moenkopi Formation. Petrified Forest N. P. (which now includes part of the Painted Desert) also has but one starthe Triassic Chinle Formation, with its many petrified logs and stumps of ancient trees. The Triassic-Jurassic Glen Canyon Group (fig. 9), which includes the Triassic Wingate Sandstone and Kayenta Formation and the Triassic-Jurassic Navajo Sandstone, receives top billing in recently enlarged Capitol Reef N. P., but the Triassic Moenkopi and Chinle Formations enjoy supporting roles.
The Triassic-Jurassic Navajo Sandstone, erosional remnants of which are found on the high mesas of Canyonlands N. P., is the undisputed star of Zion N. P., Rainbow Bridge N. M., and Glen Canyon National Recreation Area, despite the fact that the latter is the type locality of the entire Glen Canyon Group (fig. 9). The Navajo also forms the impressive reef at the eastern edge of the beautiful San Rafael Swell (a dome, or closed anticline, fig. 7), now crossed by Highway I-70 between Green River and Fremont Junction, Utah.
As we journey upward in the time spiral (fig. 80), we come to the Jurassic Entrada Sandstone, which stars in Arches N. P., with help from the underlying Navajo Sandstone and a supporting cast of both older and younger rocks. The Entrada also forms the grotesque erosional forms called "hoodoos and goblins" in Goblin Valley State Park, north of Hanksville, Utah.
Moving ever upward in the spiral, we come to the Cretaceousthe age of the starring Mesaverde Group, whose caves in Mesa Verde N. P. now house beautifully preserved ruins once occupied by the Anasazi, the same ancient people who once dwelt in Canyonlands N. P.
This brings us up to the Tertiary Period, during the early part of which the pink limestones and shales of the Paleocene and Eocene Wasatch Formation were laid down in inland basins. Beautifully sculptured cliffs, pinnacles, and caves of the Wasatch star in Bryce Canyon N. P. and nearby Cedar Breaks N. M. This concludes our climb up the time spiral, except for Quaternary volcanoes and some older volcanic features at Sunset Crater N. M., near Flagstaff, Ariz.
Thus, one way or another, many geologic units that formed during the last couple of billion years have performed on the stage of the Colorado Plateau and, hamlike, still lurk in the wings eagerly awaiting your applause to recall them to the footlights. Don't let them downvisit and enjoy the national parks and monuments of the Plateau, for they probably are the greatest collection of scenic wonderlands in the world.
Last Updated: 28-Dec-2006