|Medicine Bow Mountains and the Snowy Range rise above the Laramie Plains.
Near Laramie, as they looked west at distant mountains, McPhee was struck by the illusion: "... the Medicine Bow Mountains and the Snowy Range stood high, sharp, and clear, each so unlike the other that they gave the impression of actually being two ranges ... the flat-crested Medicine Bows, dark with balsam [subalpine fir], spruce, and pine; and, in the far high background, the white and treeless Snowy Range."
In fact, the latter sits atop the former, and geologically they are one.
In 1868, just a few months after Laramie was established, Arnold Hague of Clarence King's Fortieth Parallel Survey became the first geologist to explore the Medicine Bows. In his 1877 report, Hague described a flat-crested mountain range with "elevated plateau country, nearly 10,000 feet above sea-level ... dotted over with numerous alpine lakes." From this high surface rose a sharp-edged ridge that "culminates in Medicine Peak, a grand, broad central mass." [The plateau country is now Libby Flats, the ridge the Snowy Range, and the high point Medicine Bow Peak.]
|The Snowy Range on the horizon, above Libby Flats.
|Pebbles in quartzite, cited by Hague as evidence of sedimentary origins.
Two decades later, a geology graduate student from the University of Wisconsin, Charles Van Hise, crossed the Medicine Bow Mountains on horseback in just three days. During his brief visit, he examined the rocks at the crest, taking notes for his PhD dissertation about North America's Precambrian rocks (equivalent to "Archean series" used by Hague). Like Hague, he described the quartzite as ancient and sedimentary.
|Cross-section through the Medicine Bow Mountains in the area of the Snowy Range. Note the thickness of the Medicine Peak quartzite! (labeled "D"). Blackwelder 1926.
|Medicine Peak quartzite above Mirror Lake. Added arrow points to a huge dike—magma injected into the quartzite. Blackwelder 1926.
|Cross-bedding preserved in Medicine Peak quartzite; block is about 1 m long.
|Earth's plates (source)—very different from 2 billion years ago.
|Modified from Mitchell and others, 2021.
Collision and a continental suture
|Inferred location of the Cheyenne Belt, a continental suture (original source unknown).
|Deformed rock of the Cheyenne Belt, Medicine Bow Mountains (field trip stop 1); ruler is 15 cm.
That change was uplift of the Medicine Bow Mountains. It happened during a great mountain building event called the Laramide Orogeny, which started 80 million years ago, lasted almost 40 million years, and created mountain ranges from Mexico to Canada—the Rockies. In contrast with the previous collision, the plate jostling this time was remote. Almost a thousand miles to the west, the oceanic Farallon plate was diving under the North American plate, compressing the continent and pushing up mountains far inland.
|Rocky Mountains due to subduction far to the west (source).
Field trip—you too can read the rocks!
|The Snowy Range at the crest of the Medicine Bow Mountains, just 50 miles west of Laramie.
|Cross-bedded quartzite en route to Medicine Bow Peak from west Lake Marie trailhead.
|Summit of Medicine Bow Peak—a giant pile of quartzite boulders.