Utah's Powder Keg

Famous for deep powder that attracts skiers from across the globe, the Wasatch Front of Utah is about to unleash its bounty on those who live along its base. Thanks to a persistent atmospheric trough that produced a chilly, wet spring, the snowpack is near record levels and has barely begun to melt, a process that generally peaks in May. When summer heat suddenly arrives, a torrent of meltwater will sweep through the steep canyons of that majestic range, inundating the towns and cities that lie in its path.

Unlike in other areas of the country, the rivers that drop into the Great Basin soon spread across the flat terrain, forming broad, shallow lakes that evaporate in the intense sun of the high desert. Of course, Salt Lake City and other urban centers now occupy that basin and, when reservoirs are unable to contain the flow, flooding results. In light of the deep snowpack and delayed melting cycle, such a scenario appears to be unavoidable.

The bounty of mountain snow is vital to the ecosystems of the Western U.S., sustaining forests and producing corridors of vegetation across the semiarid landscape that lies below; without it, humans could not have colonized the region. But nature is fickle, bringing drought one year and floods the next; we must understand the risks that come with her varied landscapes and learn to live with her changing moods.

The Virgin River Staircase

The North and East Forks of the Virgin River rise on the south face of the Markagunt Plateau, in southwest Utah, amidst colorful outcrops of the Claron Formation. From these early Tertiary sediments, the tributaries begin to flow back in time, entering Zion National Park through Cretaceous Dakota Sandstone before carving magnificent canyons of Jurassic Navajo Sandstone; the North Fork has produced the more famous Zion Canyon while the East Fork sculpted Parunaweep Canyon.

The Forks join west of Zion, where the Virgin River begins a journey across Triassic redbeds, interrupted by Quaternary basalt flows along the Hurricane Fault. Beyond St. George, the river dips into the northwest corner of Arizona, where it has carved the spectacular Virgin River Gorge through Permian limestone and older, metamorphosed Paleozoic strata. West of the Grand Wash Fault, which marks the west edge of the Colorado Plateau, the canyon widens and the river enters the stark landscape of the Mojave Desert. After flowing into Nevada, the Virgin River curves to the south and joins the Colorado River within Lake Mead.

Western rivers are appealing to the naturalist in many ways. Producing ribbons of vegetation through arid landscapes, they also reveal the complex geology that underlies and explains the rugged, surface topography. Though relatively short, the Virgin River of Utah-Arizona-Nevada has bestowed its gifts in spectacular fashion.

The Uinta Mountains

Looking at a map of North America, one notices that the major mountain ranges trend north to south; this reflects the compressive, subduction and, in the case of fault-block ranges, stretching forces that produced the uplifts, which have generally come from an east-west direction. Two exceptions are the Transverse Range of Southern California, created by northward compression along the San Andreas Fault, and the Uinta Mountains of northeast Utah.

The Uintas, the highest range in the State, formed during the Laramide Orogeny (70-60 million years ago), which lifted the Rocky Mountain chain. Pressure within the North American craton, presumably secondary to the Farallon Plate subduction along the west coast, crumpled ancient Precambrian bedrock up through the overlying Paleozoic and Mesozoic sediments. Parallel faults in northeastern Utah, running west to east, dictated the unusual orientation of the Uinta uplift; furthermore, subsequent shifts in the crust forced the initial anticline northward atop the edge of the Green River basin, tilting the block. This latter movement, along a thrust fault, explains why the crest of the Uinta Range is much closer to its steep northern slope than to its more gradual southern slope. The uplift of this massive range also produced deep basins to its north, south and east, which held the Green River Lakes of the Eocene, among the largest freshwater lake systems in the history of our planet.

During the Pleistocene, glaciers scoured the summits of the range and produced broad, U-shaped valleys on its flanks, especially along the south side of the Uintas; this glaciation also created massive domes and flat expanses of tundra, with numerous glacial lakes, atop the range. Drainage along the south side of the Uintas is via streams that flow southeastward across the Uinta Basin to join the Green River, a major tributary of the Colorado; most streams on the north side also drain to this River though the western third of the north flank feeds the Bear River, which flows to the Great Salt Lake.

Jurassic Parks

The Colorado Plateau of the Western U.S. harbors one of the most extensive exposures of Jurassic strata on our planet. Stretching from 200 to 135 million years ago, the Jurassic Period covered the heart of the Mesozoic Era, the Age of Dinosaurs.

A number of our National Parks and Monuments are famous for their Jurassic deposits and, by extension, their cargo of dinosaur fossils. Dinosaur National Monument, in northwest Colorado and northeast Utah, is a showcase for the Morrison Formation; deposited in a long, shallow basin, from Canada to New Mexico, this layer cake of mudstones, siltstones, coal and sandstone is famous for its late Jurassic fossils. A bit older, the Entrada Sandstone, deposited in the mid Jurassic, is highlighted at Arches National Park, in eastern Utah, where it has been sculpted into a spectacular array of fins, natural bridges and arch formations.

Perhaps most famous of the Jurassic sedimentary rocks is the Navajo Sandstone, deposited about 190 million years ago when a vast desert covered the region. Relatively soft and heavily jointed, this sandstone forms the upper cliffs of Canyonlands National Park, the scenic domes of Capitol Reef National Park and the towering walls of Glen Canyon, now drowned by Lake Powell. Separated from the Navajo Sandstone by the Kayenta Formation, the Wingate Sandstone yields the middle cliffs of Canyonlands and the majestic, sheer walls of Colorado National Monument; though officially dated from the onset of the Jurassic, some geologists argue that, based on its fossil contents, the Wingate was deposited near the end of the Triassic.

Claron Lake

When the Rocky Mountain chain began to rise, some 70 million years ago, the crust to its west was stretched, producing a mosaic of ridges and broad basins. Drainage into the basins created large inland lakes, including the Green River Lakes of the Utah-Colorado-Wyoming Tristate and Claron Lake of southwest Utah. All of these lakes gradually filled with sediments of the Paleocene and Eocene Periods (60-50 million years ago); the deposits in the Green River Lakes would eventually yield the famous oil-shales of the Roan Plateau while deposits in Claron Lake would become prized more for their natural beauty.

Compacting into layers of limestone, dolomite, siltstone and conglomerates and capped by volcanic tuff from the Oligocene Period (30 million years ago), the Claron Formation was lifted with the rest of the Colorado Plateau during the Miocene-Pliocene Uplift, some 25-10 million years ago. Erosion and faulting would eventually expose the Claron beds along the edge of regional plateaus; Cedar Breaks National Monument, a spectacular natural amphitheater on the west edge of the Markagunt Plateau and Bryce Canyon National Park, on the east edge of the Paunsaugunt Plateau, protect the most scenic exposures.

Rich in iron oxides and manganese oxides, the colorful limestones and dolomites have eroded into striking rock formations, known as hoodoos. These pinnacles, capped by more resistant rock layers have been split apart by stream erosion and freeze-thaw fracturing. Once lying beneath an ancient lake, they now gleam in the bright Utah sun and adorn calendars across the globe.