The Rocky Mountains at Glacier National Park, Photo by NPS, Public Domain
Audio – trip to Seattle, Part I (174)
The Rocky Mountains are the backbone of North America. They dominate the western half of the continent and seem to form (on maps, anyway) a 3000 mile long wall separating the Great Plains to their east from the Great Basin and more to their west. As I learned back in 1975, though, when I tried to follow the Rockies from New Mexico to northern Canada on a long, car trip, the Rockies are not just one thing, and, sometimes, it is hard to be sure exactly where they are!
More than one hundred separate mountain ranges make up the Rockies, and these ranges differ from each other in terms of their specific ages, geological histories and physiographic features. These diverse mountain systems are often divided up into four, large, but still quite heterogeneous, geographical groups: the Canadian Rockies (which includes the mountains of Montana and northern Idaho), the Middle Rockies (the mountains of Wyoming, Utah and southern Idaho), the Southern Rockies (the mountains of Colorado and New Mexico), and the Colorado Plateau (the great uplift centered around the “Four Corners” connection point of New Mexico, Arizona, Utah and Colorado).
The modern Rocky Mountains formed during a tectonic event called the Laramide orogeny. This mountain building sequence began as two tectonic plates just off the western coast of North America (the Kula and Farallon plates) began a shallow dive under the North American plate. These plates pushed well to the east before they triggered the rise of hot magma from the mantle that then powered the rise of the mountain chains that make up the Rockies.
Colorado Rockies, Public Domain
The Southern Rockies in Colorado and New Mexico formed in this Laramide orogeny between 80 and 55 million years ago. The Canadian Rockies formed later (between 65 and 35 million years ago), and most of the remaining ranges formed sometime in between these south and north events. There are ranges in the Rockies, though, that formed much more recently (The Tetons of the Middle Rockies in Wyoming, for example, are just 10 million years old!). The Rocky Mountains have a very complex, and extremely drawn out origin!
Once the mountains had risen, erosive forces of wind and water and also glacier sculpting, wore them down and carved them into their present day conformations. In places where these sculpting forces were most extreme or where the risen rocks were less resistant, few great peaks remain. Conversely, where erosion proceeded more slowly or where the uplifted rocks were more resistant, complex, jagged peaks and steeply walled canyons predominate.
There is a tendency to refer to the Southern Rockies as “the” Rocky Mountains. For example, as we were planning our trip from Greeley to Seattle we were told that we would “drive up into Wyoming and go across the north edge of the Rockies” on our way west. All of those gorgeous peaks of the Colorado Rockies do seem to define the Rocky Mountains! Their descriptive numbers are compelling: 78 of the 100 tallest peaks of the Rockies are in Colorado and these include the 30 tallest peaks of the entire range! The Colorado Rockies, though, this Front Range that I have written about before and which looms just to the west of my home, are just one part of this vast system of mountains.
Photo by D. Sillman
The first part of our trip to Seattle involved getting over the Rocky Mountains. We started out on a cold, wet, May morning and drove due west from Greeley, directly toward the Front Range. It started snowing as soon as we got into the foot hills just west of Fort Collins. The snow and sleet intensified when we turned north toward Wyoming. We have always heard that the weather in southern Wyoming was more severe and more changeable than the weather in Greeley. Altitude explains part of this: southern Wyoming is 1500 to 2000 feet higher in elevation than Greeley (essentially Wyoming is a vast, high plateau punctuated by mountain ranges). A section of the Wyoming Plateau separates the Southern Rockies from the Middle Rockies in the southwest section of the state. This “gap” or “low spot” in the mountains lets western winds and weather fronts pass over the barrier of the Rockies with less of an altitude challenge and allows weather fronts to strike the southern Wyoming plains at full force.
This broad, relatively low region in the mountain range (if you can call 7000’ elevation “low!”) was the preferred path for wagon trains trundling west back in the mid-Nineteenth Century on trails like the Oregon Trail. This flattened mountain section was also later the path of the transcontinental railroad and is the current location of Interstate 80. Our trip to the west coast was following some very rich, historical lines! We drove from Laramie (7165’ elevation) to Rawlins (6834’) and then “over the hump” through Rock Springs (6388’) Lyman (6706’) and Evanston (6749’) and then out onto the flat, western sagebrush plains.
Photo by NPS, Public Domain
In the broad, open rangeland on either side of the road, small herds of dark cattle grazed in the ice and snow. Just south of connecting with I-80 in a snow-covered field west of the highway we saw a herd of very funny looking “cattle.” The grazing animals were brown and white and had small, cylindrical bodies, skinny legs and black masks and horns! It was a small herd of pronghorns!
Pronghorns (Antilocarpa americana) are sometimes referred to as “antelopes” although they are more closely related to giraffes than to Old World antelopes. The early European explorers of the American west noted the pronghorn’s similarity of appearance and habits to the African antelope and made the leap of logic to assume they were similar animals. They are, though, an example of convergent evolution from very different starting species. The song “Home on the Range” immortalized the pronghorns (although it did mis-name them), and marked them as the “playmates” of deer out on the western plains of North America!
Although they are much smaller than cattle, pronghorns are quite substantial animals. The are four or five feet long from nose to tail, stand about three and half feet tall and can weigh 90 to 150 pounds. They are very distinctively colored with a reddish-brown coat and extensive white highlights on their rumps, sides, chests, bellies and across their throats. Their horns are black with a solid bone core covered with a yearly shed coating of keratin. A forward facing process on their horns is the feature that generated their common name of “pronghorn.”
Photo by NPS, Public Domain
Pronghorns are built for speed. They are the fastest land mammal in North America and can run at 35 mph for four miles! Authorities differ on their maximum sprinting speeds but most estimates fall in the 53 to 65 mph range! Only the African cheetah can run faster than a pronghorn, but the cheetahs’ endurance and distances over which their top speeds can be sustained do not come close to matching those of the pronghorn. The evolution of the pronghorn’s ability to run so fast was undoubtedly a mechanism to evade predators, although its present day predators do not come close to matching its running ability so it seems a very extravagant adaptation. Past predators, though, now extinct in North America, include the cheetah, and it widely agreed that there was an ancient, co-evolutionary “arms race” between cheetahs and pronghorns that cycled up their relative running speeds to their present day maximums!
I wonder if a pronghorn feels sorry for the poor, slow-footed coyotes that try to chase them down!
Interestingly, although pronghorns are able to run quite rapidly, they are not able to jump all that well. Often they actually duck under wire, cattle fences while running at top speeds! A number of conservation programs have encouraged ranchers to either remove the bottom strand of fence wire or, at least, replace it with non-barbed wire, to reduce the chance of injuries to the dashing, ducking pronghorns.
Next week: a little more about pronghorns and much more about Utah and Idaho!