Geology

By Wayne Ranney

US Highway 89 cannot entirely be told without saying something about the origin of the land it traverses. And although a small segment of people may continue to insist that it was “the hand of God that made it” they miss the spiritual and intellectual rewards that await those who open their eyes to the details of exactly how and when “he” did it. Geology shaped this land and thus determined the specific route of Highway 89. It is anything but arrow straight. In the first half of the 20th century, before the development of the Interstate Highway System, road builders used far less dynamite in constructing roads and utilized a lot more curvature. In this way, roads were more gently placed upon the land and were made to fit to its shape.  A short introduction to each of these landscapes may be in order:

Basin and Range - Found from the Mexican border to central and northern Arizona, it contains down-faulted valleys (basins) separated by uplifted mountains (ranges). It attained its present shape beginning between 25 and 17 million years ago, when the earths crust was stretched and thinned. Much higher mountains preceded the Basin and Range.

Colorado Plateau - Stretching from northern Arizona to northern Utah, this broad, uplifted series of plateaus has escaped the rigorous deformation and faulting of rocks seen in other nearby provinces. It is characterized by colorful, flat-lying sedimentary rocks that have been deeply incised by rivers and streams and is a unique place on our planet.

Rocky Mountains - An area found along Highway 89 from the Wasatch Front in Utah to the Canadian border, these mountains were formed beginning 70 million years ago when tectonic forces uplifted rocks along a broad front. The Rockies are the stuff of legends in our culture and much of this revolves around its rich mineral heritage, which was emplaced during the creation of these stunning mountains.

Great Plains - Found in the northern half of Montana where the Rockies give way to the Plains. At first it may seem that nothing interesting could be said about “flat land” but the Plains hold many surprises. They originated on the floor of an ancient sea that was last here 70 million years ago. This sea floor has been uplifted and covered by gravel and sand washed out of the Rockies.

These are the provinces that one travels along the route of US Highway 89 from Mexico to Canada. Their history is written in the rocks and beautifully exposed for everyone to enjoy.

THE BASIN AND RANGE

US Highway 89 traverses two, quite different segments within the Basin and Range Province of the United States. The first part begins at its southern terminus at Nogales and snakes north through the cactus-filled Sonoran Desert, up over juniper-studded lava flows and eventually to the colorful red rocks near Sedona. The stretch between Yarnell and Sedona is technically within a province called (in Arizona), the Transition Zone. This interesting landscape incorporates features from both the Basin and Range and the Colorado Plateau and that is why it is considered transitional between the two. However, in this narrative we will consider the Transition Zone as part of the Basin and Range since it looks more like it than the Plateau. US Highway 89 enters its second segment of the Basin and Range as it spills out of Spanish Fork Canyon near Provo, Utah. The highway skirts Utah Lake and the Great Salt Lake on its way to Brigham City, where it turns east towards Logan and enters the Rocky Mountains.

The Basin and Range is a relatively young geologic province which only attained its present form between 25 and 17 million years ago. During this time, the earths crust began to be pulled apart in a process known as extension. The exact cause of this extension is not yet completely understood but suffice it to say that this stretching of the crust is what caused the parallel valleys and mountains to form. Before it formed, this province was the site of a large range of mountains, similar in origin to the Andes found in South America today. These ancient mountains may have been 15,000 to 20,000 feet in elevation. As the crust was extended, these mountains floundered down along faults to form the modern topography.

As a result of this faulting, the Basin and Range tends to be a rather low elevation terrain compared to other nearby provinces and Highway 89 traverses its lowest elevation (at about 1200 feet) where it crosses the Salt River Valley in Phoenix. Between Nogales and Congress, Arizona, the highway takes advantage of easy passage through river valleys and skirts the base of the many ranges. However, beyond Congress road builders constructed one of the most ambitious road alignments in its day - the Yarnell grade. This is where Highway 89 enters mountain country as it ascends the west side of the Bradshaw Mountains. The “White Spar” south of Prescott is a winding, tortuous route on the flanks of a granite outcrop.

North beyond Prescott, the highway divides in two, with the main alignment going north and east to Ash Fork and Williams respectively. The more interesting segment however, is Highway 89A, which rises over the volcanic hulk of Mingus Mountain, then descends precipitously to Jerome and its fabulously rich copper mines. From here there is a further descent across the Verde Fault and into the Verde Valley, where the highway passes the prehistoric ruins of Tuzigoot and courses on to Sedona and the colorful Mogollon Rim. When the red rocks come into view, one has reached the end of Arizonas portion of the Basin and Range.   

THE COLORADO PLATEAU

The Colorado Plateau is something quite special. Its 130,000 square miles expose some of the most breathtaking scenery seen anywhere on our planet. This geologic province is dominated by wide swaths of relatively flat-lying and vividly colored sedimentary rocks. These rocks, although situated on an uplifted plateau, are easily observed because of the regions arid climate. In spite of this aridity, the rocks are deeply dissected by the Colorado River and its tributaries. This odd arrangement, uplifted but undisturbed strata and an arid but deeply eroded landscape, make the Colorado Plateau a unique landscape on planet earth. Forested volcanoes and prehistoric ruins serve as the “icing” on this “layer cake” of strata. 

The portal to Highway 89s plateau section of scenery opens at Sedona, Arizona, tagged by USA Today as our country’s most beautiful location for a small city. The road then climbs through spectacular switchbacks in Oak Creek Canyon and once the Mogollon Rim is attained, travelers are treated to the cool scent of North America’s largest stand of ponderosa pines. Soon, the graceful outline of the San Francisco Peaks, the highest mountain in Arizona at 12,633 feet, and just one of over 600 volcanoes located in the area. The Peaks were formed in the last two million years and most likely were an additional 3,000 feet higher before they catastrophically lost their top, either in a big blast or a violent collapse. Sunset Crater Volcano National Monument, just north of the Peaks off of Highway 89, is the youngest volcano in this field and was actively erupting about 950 to 750 years ago when Sinagua Indian farmers tilled their fields of corn.

As Highway 89 descends this volcanic plateau, it enters the heart of the Colorado Plateau. Travelers are treated to stunning views of the Painted Desert, an eerily stark landscape littered with petrified logs. The Echo Cliffs loom large to the east and act as a giant rampart that guides wayfarers north. Another option awaits as 89 climbs the Echo Cliffs to Page, Lake Powell and the southern sections of Grand Staircase-Escalante National Monument. 89A continues north at the base of the Echo Cliffs which leads to the only natural crossing of the Colorado River for hundreds of miles upstream or downstream, located near Lees Ferry and the modern crossing at Navajo Bridge. On this route travelers get a short reprieve from red rock scenery by climbing the forested Kaibab Plateau, the feature that makes the Grand Canyon so deep and so grand!

The two routes converge in Kanab, Utah and trend north along historic Long Valley. Zion awaits revelers to the west and Bryce to the east in this area. After crossing a drainage divide at the junction with Utah State Highway 14, the road follows the headwaters of the Sevier River all the way down to Gunnison, Utah. Here, the route traverses through the Marysvale Volcanic Field, where the core of 20 to 30 million year old volcanoes have been exposed by erosion to provide stunning scenery, as seen at the Big Rock Candy Mountain. Leaving the Sevier River Valley, Highway 89 climbs towards the Wasatch Plateau and its eventual descent to the Utah portion of the Basin and Range in Spanish Fork Canyon.  

THE ROCKY MOUNTAINS 

Perhaps no landscape is etched upon the American psyche as firmly as the Rocky Mountains. This national pride arose from the journals of Lewis and Clark and continues to thrive in modern Americans. But the Rockies are also of great interest to the geologist since they are among the most studied, yet ironically, one of the most perplexing mountain ranges on planet earth. US Highway 89 traverses one of the more spectacular sections of this immense range and travelers can learn much about the formative years of the western landscape.

The Rockies began to rise about 70 million years ago – a long time for sure but rather recent to geologists. Prior to their uplift, the interior of North America was flooded by an immense ocean that connected the present day Gulf of Mexico with the Arctic Ocean. All of the coal deposits found today within the Rocky Mountain region were laid down in the swampy backwaters of this Great Interior Seaway. As North America drifted farther west from its prior attachment to Europe and Africa, it encountered a stubborn oceanic plate on its western edge that caused the crust beneath this seaway to buckle up. This was the start of the Rocky Mountains as we know them today.

The enigma of the Rockies is that they are found so far away from the continent’s edge – Yellowstone and the Tetons for example, are almost 700 miles from the Pacific Ocean. Most mountain ranges that form from a collision with an oceanic plate are located within a few hundred miles of the coast. Why are the Rocky Mountains so different? Some geologists believe that when the ocean crust was shoved under the edge of North America 70 million years ago, it did so at a very shallow angle – such that it affected the crust of the overlying continent a very far distance inland. Other theories are possible but this is perhaps the best explanation given.

The enigma of the Rockies is that they are found so far away from the continent’s edge – Yellowstone and the Tetons for example, are almost 700 miles from the Pacific Ocean. Most mountain ranges that form from a collision with an oceanic plate are located within a few hundred miles of the coast. Why are the Rocky Mountains so different? Some geologists believe that when the ocean crust was shoved under the edge of North America 70 million years ago, it did so at a very shallow angle – such that it affected the crust of the overlying continent a very far distance inland. Other theories are possible but this is perhaps the best explanation given.

Travelers on US Highway 89 can expect stupendous scenery and variable weather conditions due to the surrounding high mountains but the geology is such that the road generally follows mountain valleys rather than cross high alpine passes. Snow is possible any time of year and many portions of the road can be closed in winter. Earthquakes are infrequent but have happened historically, especially around Salt Lake City and Yellowstone. The Rockies are still an active growing range and as such are a great place to study geology.

(Insert brief Great Plains info here.)

Wayne Ranney is a guide, educator and author who specializes in making the diverse landscapes and geologic history of our planet come alive for curious and interested travelers. Trained as a geologist in the American Southwest, he is an adjunct faculty member at Yavapai College in Sedona and frequently travels and lectures to exotic landscapes worlwide. He has led trips for the Smithsonian Institution, Lindblad Expeditions, Abercrombie and Kent, and TCS Expeditions. He has visited over 65 countries, from Antarctica to Zimbabwe, taking travelers on journeys of discovery to earthly and human landscapes.

Wayne is the author of numerous books including Sedona Through Time; Ancient Landscapes of the Colorado Plateau; and Carving Grand Canyon. Wayne’s primary interests are in understanding landscape development through time and how human cultures intersect with these landscapes. This confluence of landscapes and cultures has, in many instances, helped to determine the course of human history. His enthusiasm and excitement is infectious as his students and fellow travelers discover the immense beauty and fascinating story of our home planet.