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
Desert Flora
The national parks of Utah, despite their arid-to-semiarid climates, high temperatures in summer and bitter cold in winter, lack of dependable precipitation, and poorly developed soil cover, are home to hundreds of species of plants.
Animal forms, though diverse in numbers and species, are infrequently seen due to their mobility and their mostly nocturnal habits. On the other hand, each plant occupies its own niche in the desert, growing only in a habitat that meets its specific environmental requirements.
It is beyond the scope of this guide to catalog all or even many of the hundreds of plant species visitors are likely to encounter in Utah’s national parks, but the trail descriptions do identify certain plants at various locations. Anyone interested in learning more about desert flora can obtain one or more of the books listed in the bibliography of this book.
Plants of riparian woodlands and hanging gardens (assemblages of water-loving plants found on canyon walls) are hardy enough to withstand extreme heat, but they could be called drought-escaping plants, since they grow only where a continuous supply of moisture is available, either on the surface or underground.
Most other desert plants have developed various methods for coping with extremes of heat and drought. Phreatophytes are plants that have deep, extensive root systems that enable them to tap underground sources of moisture. Once their roots reach the water table, their growth is not dependent on rainfall.
Buffaloberry
Most desert plants, however, are xerophytes. Xerophytes include succulent plants, which have fleshy leaves or stems that
