Introduction to the Rocky Mountain Landscape

by Wayne Ranney

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.

Yellowstone National Park

US Route 89 Road Trip Map Book - 2022-2023 Edition

US Route 89 Road Trip Map Book—2022-2023 Edition Cover

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.

Glacier National Park

US Highway 89 officially enters the Rocky Mountain Province as it descends the Colorado Plateau to Provo, Utah. It reascends the Rockies near Logan, Utah and then begins a winding course to the foot of the Grand Tetons and across the volcanically active Yellowstone Plateau. It enters the Great Plains Province just north of White Sulfur Springs, Montana. The Great Plains are essentially the uplifted sea bed of the Great Interior Seaway.

Waterton Lakes National Park

Logan Canyon National Scenic Byway

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 valley’s 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. On our tour of the Rockies, we’ll learn more about their uplift, the active volcanoes near Yellowstone, how the Ice Age affected their modern shape and how humans interact with these very active processes – that are still very much ongoing.

Salt River Pass

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 professional geologist in the American Southwest, he is an adjunct faculty member at Coconino Community College in Flagstaff and frequently travels to exotic landscapes worldwide. His 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.

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