JACKSON, Wyo. — While the Teton Range and the Gros Ventre Mountains are distinct features separated by today’s Jackson Hole, the Gros Ventres once extended all the way past the Tetons to the west.
The Gros Ventre Mountains were formed as part of the Laramide Orogeny mountain-building event, the result of a compression that squeezed the mountains up 65 to 55 million years ago. John Hebberger Jr., retired geologist and former vice president of Geologists of Jackson Hole, says when the range was uplifted it originally extended all the way over into eastern Idaho, right through what is now the Teton Range. Today, the Gros Ventres end on the east side of the National Elk Refuge.
The Gros Ventre Mountains remained extended until about 30 million years ago, when Hebberger confirms the U.S. ceased undergoing compression and began undergoing extension that stretched and pulled it apart. According to Hebberger, this extensional event reached this area roughly four and a half million years ago, initiating the Teton Fault’s creation of the down-dropped basin that became Jackson Hole.
The Teton Range rose a bit, formed from what Hebberger calls “the bones” of the older Gros Ventre Range, and the combination of down-dropping and uplifting led to the truncation of the Gros Ventres so that they no longer extend so far west.
But the evidence of that extension is still visible in the Teton Range, where the 2.7 billion-year-old crystalline rocks that form the center of the mountains are seen above 500 million-year-old limestone. This is because uplift of the Gros Ventre Mountains raised a segment of PreCambrian rocks several thousand feet up. In Creation of the Teton Landscape by David Love et al., this is referred to as the Buck Mountain thrust fault. According to the book, later movement activated on the recent Teton Fault, a “normal fault” occurring as a response to extension instead of compression, uplifted the hard, basement rocks even more to form the Cathedral Group, a key element of the new (geologically speaking) Teton Range.
Looking up Avalanche Canyon, Hebberger says to note how the Cambrian carbonate rocks, mainly limestone, can be seen lower than the much older igneous, crystalline rock. Those crystalline rocks were once overlain by the limestones, as Sheep Mountain in the westernmost extent of today’s Gros Ventres still is.
The Gros Ventre Mountains also offer expansive evidence of the dynamics of their geology. Hebberger says the uplift of rocks dipping gently to the north from the crest of the range is a result of the thrust fault on the southern edge. The Gros Ventre River cut the deep valley on the north side of the range, and this undercutting of the northward dipping rocks caused the famous 1925 Lower Gros Ventre Slide. Hebberger says the slide was possibly enabled by a wet spring and an earthquake; its scar is still prominent today, and is one of many mapped slides along the Gros Ventre River.
“By uplifting those mountains and by having a river cut into them, you set up a situation where landslides occur,” Hebberger tells Buckrail. “There’s a lot of them in that valley. Most are completely tree-covered today and people wouldn’t readily recognize them, but there are a lot more that have occurred. It is quite likely landslides will happen again in this area.”
Love et al. writes that Jackson Hole contains more landslides than almost any other part of the Rocky Mountain region, emphasizing how the land surface is still restless even millions of years after the initial uplifts and readjustments. Despite the differences in formation, where the Gros Ventres were formed by compression and the Tetons were formed by later extension, the history they reveal in today’s landscape is tied to geological activity that still continues today.
