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The Rock Formations of Utah’s National Parks

Utah’s national parks are as diverse as they are beautiful. But what they all have in common are their colorful, magnificently sculpted rocks, which make these parks some of the finest scenic attractions in the world. The effects of erosion on more than two dozen different sedimentary rock formations have made the landscapes of the parks exciting and dramatic.

Following is a very brief description of the sedimentary rocks that compose the core of Utah’s national parks.

Beginning about 345 million years ago, salts accumulated to a thickness of more than 5000 feet in a vast inland marine depression in eastern Utah known as the Paradox Basin. These salts are now the rocks of the Paradox Formation, and their presence beneath thousands of feet of younger rocks has created an unstable foundation and has had a profound effect in the shaping of the landscapes we see today in Arches and Canyonlands national parks.

Later, the marine sediments that compose the gray and red limestones, shales, and sandstones of the Honaker Trail Formation were deposited. These rocks are exposed only in Canyonlands, where they form the broken cliffs in the inner gorges of both the Green and Colorado rivers, from Cataract Canyon upriver for several miles above the rivers’ confluence.

Around 300 million years ago, the limestones, shales, and sandstones of the Elephant Canyon Formation were deposited just offshore in a retreating sea near the present-day confluence of the Green and Colorado rivers in Canyonlands National Park. These rocks form the broken cliffs just below the canyon rims in the lower reaches of the Green and Colorado rivers.

The next stage of deposition was responsible for the red- and white-banded Cedar Mesa Sandstone, the principal scenery producer in the Maze and Needles districts of Canyonlands. The striking Cedar Mesa Sandstone is composed of offshore sand-bar and coastal sand-dune deposits. However, the Cedar Mesa also contains red beds, composed of red, stream-deposited sands that interfinger with the white sandstones.

Overlying the Cedar Mesa in Canyonlands are thick deposits of reddish-brown siltstones and sandy shales of varying hardnesses collectively known as the Organ Rock Shale. These dark red rocks are contorted and irregular, much like the Entrada Sandstone’s Dewey Bridge member in Arches National Park. The Organ Rock forms the monoliths of Land of Standing Rocks in the Maze District and the pillars of Monument Basin in the Island District.

Overlying the Organ Rock Shale is another rock formation that is widespread in Canyonlands but crops out only in the deepest canyons in Capitol Reef, the White Rim Sandstone. Despite the thinness of the White Rim, it is a distinct scenery producer in Canyonlands. These rocks form a prominent bench above the inner gorges of the Green and Colorado rivers beneath the Island in the Sky mesa. This sandstone also forms the western rim of The Maze, and resistant caprocks of the White Rim protect the softer beds of the Organ Rock Shale in places, such as on the Chocolate Drops and Nuts and Bolts in the Maze District and the spires of Monument Basin in the Island District.

Following deposition of the White Rim, a dolomitic stratum of marine origin was deposited in south-central Utah and northern Arizona, the Kaibab Limestone. This mostly white rock forms the broken cliffs in the Goosenecks of Sulphur Creek and in the Fremont River canyon in Capitol Reef National Park.

Deposition of the red Moenkopi Formation beds occurred about 225 million years ago. These rocks are exposed in all Utah national parks except Bryce Canyon. The Moenkopi is composed of deep red, thinly layered sandy shale and silty sandstone, forming slopes, low cliffs, fluted columns, and ledges. Tracks of small reptiles and amphibians are found in the Moenkopi, but most abundant are ripple marks, formed after shallow, wind-rippled water created a corrugated surface on the sediments just below the water surface.

Around 200 million years ago the wide variety of sediments that compose the Chinle Formation were deposited. Dinosaur tracks, fossil freshwater fish, fossil plants and leaves, and petrified wood from a coniferous genus found in the Chinle show the evolution of life on the planet.

Like the Moenkopi, Chinle rocks are found in all Utah national parks except Bryce Canyon, where younger rocks predominate. The Chinle consists of sandstones, conglomerates, bentonitic mudstones, limestones, and siltstones. It is a varicolored unit, ranging from red and purple to yellow, green and gray. The gray layer is composed primarily of volcanic ash.

The Shinarump Conglomerate, the lowermost layer of the Chinle, forms a prominent white or tan cliff band that discontinuously caps the Moenkopi in Zion and Capitol Reef. This rock consists of lithified coarse sand and stream-polished pebbles. In Canyonlands, however, the Shinarump is absent. Instead, its place in the stratigraphic sequence is taken by the Mossback member of the Chinle. The Mossback is similar in composition to the Shinarump, but its dark gray-green color sets it apart.

Around 200 million years ago, vast deserts of drifting sand dominated the Colorado Plateau landscape. One of these windblown sand deposits is the Wingate Sandstone, and whenever it is exposed in Capitol Reef, Arches, or Canyonlands, it forms cliffs that have been major barriers to travel. The Wingate most often forms sheer, but sometimes broken and fluted, cliffs as high as 400 feet. These rocks are typically orange-red in color, but are often stained with patches and streaks of desert varnish—a dark brown, black, or bluish coating of iron and manganese oxides formed as mineral-laden water evaporates on the rock’s surface.

The vast Wingate desert did not extend as far southwest as Zion National Park. Instead, its corollary in Zion is the thin but prominent Moenave Formation, composed of red siltstones and yellow sandstones of floodplain origin.

The thin beds of the Kayenta Formation that overlie the Wingate and Moenave range in color from 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 outcrop close to Bryce, forming the steep southern face of the Black Mountains north of the Park, which are visible from most overlooks there. Volcanic rocks also cap the plateaus of Boulder and Thousand Lake mountains to the southwest and northwest of Capitol Reef. Stream-rounded boulders of basalt litter much of that Park, transported there by debris flows from the aforementioned mountains.

Following that volcanic activity, a now-discontinuous sedimentary layer was deposited atop the Claron beds, which escaped entombment in the lava. The Brian Head Conglomerate is most obvious where it forms the caprock of Boat Mesa, a high prominence rising above the Paunsaugunt Plateau between Fairyland and Sunrise points in Bryce. This formation contains stream-polished pebbles, many of which are volcanic in origin.

Hamburger Rocks of Capitol Reef

Volcanic rocks of more recent origin are exposed in parts of Zion and Capitol Reef. In the Cathedral District of Capitol Reef, particularly in the Middle Desert-Cathedral Valley area, there are many curious volcanic intrusions that were emplaced within the sedimentary layers about 4 million years ago. A dike is where molten magma is injected into a more or less vertical crustal fracture such as a joint or a fault. Erosion of the softer sedimentary rocks that surround a dike leaves it standing as a thin wall of rock. A sill is formed where molten magma is injected into a zone of weakness parallel to the bedding planes of the sedimentary strata.

Volcanic rocks also occur sporadically throughout the southwest and central parts of Zion National Park. These young (Pleistocene and Holocene) basaltic lava flows issued from vents and cinder cones, flooding canyons and forming terraces atop the Lower Kolob Plateau.

Since that time, erosion has been the dominant force in shaping the landscapes of Utah’s national parks: deposition, such as the accumulation of alluvium or of drifting sand, has played only a minor role.

Double Arch, Arches National Park