Utah's National Parks. Ron Adkison
openings, such as the many large arches that are found high on massive sandstone walls in Zion National Park. Arches without openings are not considered in this discussion. Neither are natural bridges because, while arches are common in Utah’s national parks, natural bridges are not. They are formed as a direct result of a meandering stream cutting laterally into a narrow fin of rock, eventually cutting a hole through it. Such features are best seen in appropriately named Natural Bridges Natural Monument, in southeast Utah. In contrast, arches that develop openings below or behind them do so in the absence of streams.
Arches with openings are relatively rare in the world, but are quite common on the Colorado Plateau, and they are found in all of Utah’s national parks. There are two very common types of arches with openings in Utah’s national parks: 1) arches in a thin, vertical wall—or fin—of sandstone; they have a vertical opening entirely below the span; and 2) pothole arches, which have a horizontal opening. Both types of arches owe their existence to chemical and physical weathering by water.
In the first type the process of arch formation begins when groundwater seeps down through a porous sandstone layer, and it encounters an impermeable layer, such as shale. The groundwater then seeps outward along this interface of layers, eventually reaching the face of a cliff. There it dissolves the natural cement that binds sand grains into stone, and then water, wind and gravity remove the sand grains. Thus a small indentation begins to form on the cliff face at the horizontal interface between the two layers. This indentation grows horizontally along the interface and also vertically as rock immediately above the indentation is undercut and destabilized, and breaks away. Over time, more rockfall on a small or a grand scale causes the arch to grow. The indentation becomes larger, forming in the cliff what is called an alcove. If the body of rock behind the cliff is narrow enough, like the fins in Arches and Canyonlands national parks, the alcove will continue to penetrate the rock until it reaches the other side, and an opening forms. If two alcoves form on opposite sides of a fin, the process occurs much more rapidly.
The second type of arch, a pothole arch, is the most common type in Capitol Reef and Canyonlands national parks. A pothole arch begins to form when a pothole close to a cliff face collects water. Although much of the water trapped in a pothole evaporates, some seeps down into the porous sandstone below. When this groundwater reaches an impermeable layer, it seeps outward along the interface to the cliff face, and an alcove eventually forms there, in the same way as described above. The alcove and the pothole above continue to enlarge, and they eventually coalesce, forming a nearly horizontal opening called a pothole arch.
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