Utah's National Parks. Ron Adkison
nearly white to reddish-brown, and usually form ledges, low cliffs, and slopes. In Zion, the Kayenta is soft, and consequently it erodes into slopes rather than the ledgy cliffs it forms in Arches, Capitol Reef, and Canyonlands.
The resistant Navajo Sandstone was also deposited around 200 million years ago, when once again the region was buried under drifting sands. Perhaps no other rock formation on the Colorado Plateau is as famous as the Navajo. These rocks erode into gigantic cliffs both sheer and rounded, and great domes, for which Capitol Reef was named. The Navajo also composes the great cliffs of Zion Canyon, the Petrified Dunes of Arches, and the domes that cap the Island in the Sky mesa in Canyonlands. The Navajo achieves its greatest thickness, approximately 2200 feet, in Zion.
Cross-bedding, the sweeping diagonal lines most obvious in the hummocky structures of the Navajo, reflects the advance of the ancient dunes across the landscape as northerly winds swept the region.
Overlying the Navajo in Zion is a thin, discontinuous layer of shale and sandstone, the Temple Cap Formation. Most of this reddish-brown layer has been removed from the Markagunt Plateau by erosion.
Bulging Navajo Sandstone walls embrace the narrow wash of lower Courthouse Wash
Less than 200 million years ago, the limestones, shales, sandstones, and gypsum of the Carmel Formation were deposited. The soft rocks of the Carmel cap much of the Markagunt Plateau in Zion, and outcrop on the eastern flanks of the Waterpocket Fold in Capitol Reef.
The Entrada Sandstone is the most widespread rock unit in Arches, and its rocks form most of the scenic highlights of the Park. This formation has been divided into three subunits, each with its own special characteristics. The Dewey Bridge member is the lowermost and softest layer of the Entrada, and erosion attacks it vigorously. Dark red in color, the beds of this silty sandstone are often contorted and irregular. Its relatively soft nature allows it to erode more readily than the overlying Slick Rock member. The result has been the formation of many fascinating hoodoos, such as those found along The Great Wall, and these rocks form the pedestals for many balanced rocks in Arches.
The Slick Rock member of the Entrada Sandstone is the dominant rock in Arches, and its presence is responsible for the unusual scenic beauty of the Park. This rock is orange- or salmon-hued, occasionally nearly white. The Slick Rock member forms rounded-to-vertical cliffs, domes, and fins—the narrow sandstone walls that dominate the scene in Devils Garden and Fiery Furnace in Arches. This is the rock that Arches visitors encounter more than any other, for the arches they have come to see are formed almost exclusively in it.
The uppermost member of the Entrada, the Moab Tongue, could easily be mistaken for the Navajo Sandstone, since it has many of the same characteristics and is similar in color. The Moab Tongue is a white, fairly thin bed of wind-deposited dune sand, and this rock displays the cross-bedding common in wind-blown sand deposits.
Anyone familiar with the Entrada in Arches will hardly recognize that formation in Capitol Reef. Here much of the soft red beds of the Entrada has been eroded away, and only a few low domes crop out in and near the valleys east of the Waterpocket Fold. In Capitol Reef, the Entrada is a soft, thinly bedded formation composed of sandstones and siltstones.
Tower Arch is formed entirely within the Entrada’s Slick Rock Member
The Curtis Formation lies above the Entrada only in Capitol Reef’s Cathedral District. This gray, limey marine sandstone is prominent on the rim of The Hartnet, and it forms a protective caprock on the monoliths of Cathedral Valley and South Desert.
Another formation unique to Capitol Reef is the Summerville, composed of thinly bedded red mudstones and siltstones. Three members of the Morrison Formation are found in Arches, and two of these also occur in Capitol Reef. Sand, mud, silt, and volcanic ash make up the various layers of the Morrison.
The red silty shales of the Tidwell member of the Morrison outcrop in a few locations only in Arches National Park. This is a relatively thin layer, and it is most easily identified where the abundant white concretions, composed of silica, have weathered from the formation and now litter surrounding slopes.
The Salt Wash Sandstone, though not noted for forming spectacular scenery, nevertheless adds its own distinct character to the landscape wherever it occurs in Capitol Reef and Arches. Light yellow sandstones and conglomerates compose this member of the Morrison, along with some mudstones, siltstones, and limestones. Most of the fossil dinosaur bones found on the Colorado Plateau are located in the Salt Wash Sandstone. A great deal of organic debris is also present. This fossil plant debris is a major source of uranium ore, and the Salt Wash Sandstone contains the world’s most significant reserves of this ore. The areas east and south of Arches were focal points for uranium mining during the uranium boom of the 1950s.
Geologic Time Scale
The Brushy Basin Shale is famous for its badlands and colorful “painted desert” scenery. One of the most striking rock formations on the Colorado Plateau, it is composed ofclayand mud deposits. The formation is nutrient-poor, and the clays on its surface swell rapidly when wet and shrink upon drying. The surface absorbs little moisture and is usually devoid ofvegetation.
Similar to the Brushy Basin Shale, the Cedar Mountain Formation, exposed only in Arches, consists primarily of soft mudstones that form slopes and beds of conglomerates that form ledges. The mudstones are colorful—mostly light green, gray, and lavender—and in Arches these rocks are difficult to distinguish from the underlying Brushy Basin Shale.
About 135 million years ago were deposited the sediments that compose the thin layer of the Dakota Formation—one of the most widespread sedimentary formations in the western U.S. Attesting to its marine origin are the vast amounts of fossil shells in the Dakota’s upper layer. One of the best places to see these fossil shells is in the Oyster Shell Reef in Capitol Reef’s South District. The Dakota is most common in Capitol Reef, but there are also a few minor outcrops in Arches and atop Zion’s highest summit, Horse Ranch Mountain.
Sediments forming sandstone and shale were subsequently deposited atop the Dakota, and they are collectively called the Mancos Formation. These rocks are among the youngest and the most widespread rocks on the Colorado Plateau. The great buttes and mesas east of Capitol Reef near Hanksville are composed of both shales and sandstones of the Mancos. Shales of the Mancos also dominate the floor of lower Salt Valley and Cache Valley near Wolfe Ranch in Arches. Tropic Shale, a local name for the Mancos, forms the nearly barren hills and slopes surrounding the Paria Valley east of Bryce.
Around 100 million years ago, the Straight Cliffs and Wahweap formations were deposited in a layer 1000–2000 feet thick. Since the Straight Cliffs sandstone when eroded typically forms a cliff, and since it is interlayered with the less resistant Wahweap, these formations resemble a series of giant stairs on the eastern escarpment of the Paunsaugunt Plateau in Bryce.
The final episode of the Mesozoic era, around 65 million years ago, was the deposition of the Kaiparowits Formation. Clays, sands, and gravels that were ultimately cemented into stone form this discontinuous layer, which outcrops just below the Pink Cliffs in Bryce.
About 60 million years ago, lakebed deposits composed of sand, gravel, silt, and limey clay made up the varied layers of the Claron Formation—the rugged Pink Cliffs for which Bryce is so famous. The varied layers of this formation contain beds both hard and soft. Each bed erodes at a different rate, and this differential erosion has resulted in a fantastic array of pinnacles, towers, and finlike ridges collectively called hoodoos. Altogether, the hoodoos and badlands of Bryce’s Pink Cliffs are referred to as breaks.
Volcanic activity that began about 37 million years ago enveloped much of southwest Utah in a blanket of lava, mostly basalt. The subsequent uplift of Utah’s High Plateaus and erosion have since made the volcanic layer discontinuous in this part of the Colorado Plateau. However, these rocks