For many travelers, Utah's striking landforms and natural history provide the impetus for a visit to the state.
Utah's 84,990 square miles place it 11th in size among the 50 states. The varied landscape is divided into three major physiographic provinces: the Basin and Range Province to the west; the Middle Rocky Mountains Province of the soaring Uinta, Wasatch, and Bear River Ranges to the north and northeast; and the Colorado Plateau Province of canyons, mountains, and plateaus in the south.
Basin and Range Province
Rows of fault-block mountain ranges follow a north-south alignment in this province in the Great Basin west of the Wasatch Range and the High Plateaus. Most of the land lies at elevations between 4,000 and 5,000 feet. Peaks in the Stansbury and Deep Creek Mountains rise more than 11,000 feet above sea level, creating "biological islands" inhabited by cool-climate plants and animals.
Erosion has worn down many of the ranges, forming large alluvial fans in adjacent basins. Many of these broad valleys lack effective drainage, and none have outlets to the ocean. Terraces mark the hills along the shore of prehistoric Lake Bonneville, which once covered most of this province. Few perennial streams originate in these rocky mountains, but rivers from eastern ranges end their voyages in Great Salt Lake, Sevier Lake, or barren silt-filled valleys.
Middle Rocky Mountains Province
The Wasatch Range and the Uinta Mountains, which form this province, provide some of the most dramatic alpine scenery in the state. In both mountainous areas, you'll find cirques, arêtes, horns, and glacial troughs carved by massive rivers of ice during periods of glaciation. Structurally, however, the ranges have little in common. The narrow Wasatch, one of the most rugged ranges in the country, runs north-south for about 200 miles between the Idaho border and central Utah. Slippage along the still-active Wasatch Fault has resulted in a towering western face with few foothills. Most of Utah's ski resorts lie in this area. The Uinta Mountains in the northeast corner of the state present a broad rise about 150 miles west-east and 30 miles across. Twenty-four peaks exceed 13,000 feet, with Kings Peak (elev. 13,528 feet) the highest mountain in Utah. An estimated 1,400 tiny lakes dot the glacial moraines of the Uintas.
Colorado Plateau Province
World-famous for its scenery and geology, the Colorado Plateau covers nearly half of Utah. Elevations lie mostly between 3,000 and 6,000 feet, but some mountain peaks reach nearly 13,000 feet. The Uinta Basin forms the northern part of this vast complex of plateaus; it's bordered on the north by the Uinta Mountains and on the south by the Roan Cliffs. Although most of the basin terrain is gently rolling, the Green River and its tributaries have carved some spectacular canyons into the Roan and Book Cliffs. Farther south, the Green and Colorado Rivers have sculpted remarkable canyons, buttes, mesas, arches, and badlands.
Uplifts and foldings have formed such features as the San Rafael Swell, Waterpocket Fold, and Circle Cliffs. The rounded Abajo, Henry, La Sal, and Navajo Mountains are examples of intrusive rock—an igneous layer that is formed below the earth's surface and later exposed by erosion. The High Plateaus in the Escalante region drop in a series of steps known as the Grand Staircase. Exposed layers range from the relatively young rocks of the Black Cliffs (lava flows) in the north to the increasingly older Pink Cliffs (Wasatch Formation), Gray Cliffs (Mancos Shale), White Cliffs (Navajo Sandstone), and Vermilion Cliffs (Chinle and Wingate Formations) toward the south.
The land now contained in Utah began as undersea deposits when the North American continental plate sat near the equator, about 500 million years ago. The spectacular canyon country, now known as the Colorado Plateau, began as a basin of silt and sand deposits at the verge of a shallow sea. This basin sat on a continental plate that rose and fell; it was sometimes below the waters of ancient seas—at which time fossils of early marinelife were encased in the deposits—and sometimes, during more arid periods, above sea level, with vast sand dunes covering the landscape.
Beginning about 200 million years ago, in the Mesozoic era, the North American continental plate broke away from Europe and Africa and began its westward movement over the top of the Pacific Ocean seafloor. This massive collision of tectonic plates resulted in the buckling of rock formations—which formed mountains, including the Rockies and the Uintas—and in thrust faulting, where older formations were pushed up and onto younger rocks; one such range is the Oquirrh Mountains.
All of this activity happened at the verges of the Colorado Plateau, which by the Cretaceous period—the age of the dinosaurs, about 65 million years ago—had again sunk back to sea level, resulting in thick formations of sand, mud, and ancient vegetation. These formations would later be revealed in the region's mighty canyons and in the coal fields of northeastern Utah. In some places, fossilized mud footprints of dinosaurs provide unmistakable evidence of the era's far damper climate.
Basin and Range
In the Tertiary era, the new formations west of the old Colorado Plateau were shot through with volcanoes. Then, as the North American continental plate pivoted to the southwest, the earth's crust under this region—which would become the Basin and Range Province—stretched thinner and thinner. In fact, the Great Basin of Utah and Nevada is about twice as wide as it was about 18 million years ago. This stretching has resulted in a much thinner layer of underlying basement rock here than in other parts of the continent, and the entire area is riven by faults where parts of the crust have pulled apart. Given the differential forces at work in the earth's mantle, sometimes half of a fault would be pushed up to mountain heights while the other half would sink, causing a basin. The spectacular fault-block mountains of the Great Basin result from such parallel rising and falling along fault lines. The most famous instance of this type of formation is the rugged Wasatch Mountains, which rise directly above the basin of Great Salt Lake.
To the east of this momentous fault-block mountain building, the old Colorado Plateau remained relatively undisturbed. However, within the last 10 million years the entire intermountain region bowed up in a broad arch, elevating the old sandstones of the Colorado Plateau; this corner of Utah has risen 5,000 feet during this time. The rivers that once wound across the surface of eastern Utah were forced to cut ever deeper canyons as the formations rose. The erosive power of the Green, Colorado, San Juan, and other rivers and streams have cut down through hundreds of millions of years of rock.
Ice Age Utah
During the geologically recent Pleistocene era, ice-age mountain glaciers and climatic changes brought an abundance of moisture to Utah. The runoff and meltwater flooded the basins of fault-block mountain ranges, forming enormous lakes. The largest of these was Lake Bonneville, the name given to the ice-age predecessor of Great Salt Lake. At its greatest extent, Lake Bonneville covered nearly all of northern and west-central Utah and was nearly 900 feet deeper than the current Great Salt Lake. Even at that depth, finding an outlet to the sea was not simple. It was only after the lake waters breached Red Rock Pass in Idaho that the lake found an outlet into the Snake and Columbia River systems, about 16,000 years ago.
After the ice ages ended, about 10,000 years ago, Lake Bonneville diminished in size and dropped below the level necessary to cut through Red Rock Pass, resulting in the saline Great Salt Lake. You can easily see the old lake shorelines along the Wasatch Front, and cities like Logan, Provo, and Salt Lake City sprawl along these stairstep-like ledges. Much of the old lake bottom west of Salt Lake City is salt desert and extremely flat. In the Bonneville Salt Flats, the valley is so flat and unbroken that the curvature of the earth can be seen.
© W.C. McRae and Judy Jewell from Moon Utah, 9th Edition