A Change of Place

Teton Valley Magazine

By Regan Grandy

Teton Valley has had many names over the course of history. The Native Americans who spent summers in the valley hunting game and gathering berries referred to it as the “Broad Valley.” Mountain men of the fur companies, who held several of their yearly rendezvous in the valley, dubbed it “Pierre’s Hole,” after Pierre Tevanitagon, a French-Iroquois fur trapper. Early settlers, including the Latter-day Saints, called it “Teton Basin,” preferring to use the name Teton, as applied by the romantically deprived French-Canadian trappers in les trois tetons or the three teats. Locals and visitors today respectively call it Teton Valley.

The geology of the valley has changed, too—but over a much longer period of time. Like crime scene investigators who piece together a story based on evidence, geologists have assembled the story of the valley’s geologic history by careful analysis of the evidence set in stone.

About 2,800 million years ago (according to radiometric dating methods) in what is known as the Precambrian Era, ancient volcanic islands in a pre-Pacific ocean west of North America eroded. These islands shed a large quantity of sediment to the east, which then traveled farther east on the moving oceanic plate. Once it reached the continental plate, the sediment was dragged below it, scraped off the oceanic crust onto the continental crust, and deformed by heat and pressure. Over time, these deformed sediments became metamorphic rocks known as gneisses and schists. These rocks, some of the oldest in North America, make up the core of the Teton Range—which are among the continent’s youngest mountains.

About 2,500 million years ago, the oceanic crust that continued to descend below the continental crust melted. Since it was heated and became less dense, the magma began to rise, melting through the continental crust that lay above. Where the plumes of magma reached the surface, they produced Cascade Range-type volcanoes. Mount Rainier, Mount St. Helens, Mount Shasta and Mount Hood are much younger examples of this style of volcanism. As the magma continued to rise, it created narrow dikes (or magma-filled fractures) of light-colored granite among the gneisses and schists. These intrusions can be seen in the jagged rocks of the Teton Range.

A similar intrusive event occurred at the same location about 1,500 million years ago, sending dark-colored magma called gabbro from deep in the mantle through the existing gneisses, schists and dikes. These dark-colored dikes are easily visible in the Middle Teton and Mount Moran.

By 700 million years ago, all of the continental plates had moved together into one landmass, since plates are constantly moving and shifting with the flow of magma below them. Geologists call this the supercontinent Rodinia, and it predates the more commonly known supercontinent Pangaea. During Rodinia’s formation, an unknown continental mass (probably Australia and Antarctica) joined with western North America.

Then, about 650 million years ago, Rodinia began to rift (pull) apart along the western margin of what is now Idaho, again, as a result of crustal plates constantly moving and shifting. (This is known because there are no rocks west of western Idaho that are older than 650 million years.) As the North American continent separated from the unknown continent to the west, magma filled in, spread out in the rifted area, and cooled to form the oceanic crust. Water from the oceans at the termini of the rift began to fill the void, and the Pacific Ocean (as it exists today) was born. The Cascade-type volcanic range, which lay above the metamorphic rocks that now make up the Tetons, eroded. This left the North American continent quite flat and only a few feet above sea level. It also brought the metamorphic rocks closer to the surface.

When the Paleozoic Era began 570 million years ago, life began to flourish on Earth and the sea level began to rise. The water crept inland from western Idaho as far as eastern Montana and Wyoming, leaving Idaho completely submerged.

During this era, about 6,000 feet of sediment accumulated on the ocean floor that covered these states. First, shallow reddish-brown beach sands appeared, followed by gray-colored stone and dolomite. An example of the sandstone can be found at Table Rock, about 1.5 miles west of the Teton peaks. The gray limestone can be observed in the cliffs of Leigh, Teton, Darby and Fox Creek canyons on the east side of the valley. It is also found on the west side of the Snake River and Big Hole ranges, which are south and west of the valley. These limestone deposits contain fossils of exotic tropical sea-dwelling organisms, such as corals, crinoids, brachiopods and bryozoans. The limestone bedrock also dissolves easily, supplying the region with hard water.

The end of the Paleozoic Era (245 million years ago) is marked by the largest extinction on Earth (even larger than the later extinction of the dinosaurs), destroying 90 percent of all species. By the time this mass extinction occurred, the more commonly known supercontinent Pangaea had drifted together, so that once again the Earth held one landmass. In this coming together, eastern North and South America were sutured to western Africa. The Appalachian Range on the east side of the United States is a remnant of this suture zone. There were no continental masses west of North and South America, setting the stage for the Mesozoic Era.

As Pangaea began to rift apart at the beginning of this era, North and South America began drifting westward, and Eurasia and Africa began drifting eastward. This produced the crust of the Atlantic Ocean. These continents continue to drift apart today, with new oceanic crust spreading out from ridges on the ocean floor.

As North and South America moved westward, the continental crust overcame the oceanic crust in front of it, again forcing the oceanic crust below it. This occurred all along the western edge of North and South America, including at the western border of Idaho. As the oceanic crust descended into the mantle and melted, it became less dense and began to rise through the continental crust, producing another Cascade-style range of volcanic mountains.

The ocean water that was inland (in Idaho, Wyoming and Montana) became trapped, forming the Sundance Sea. Shale, limestone and sandstone were deposited on this ocean’s floor to a thickness of about 8,000 feet. These sedimentary layers can be seen in the now-tilted rocks on the east side of the Big Holes and the north side of the Snake River Range.

The Sundance Sea began to recede during the Mesozoic Era, and eastern Idaho became a densely vegetated swamp. Since the Mesozoic Era is known as the age of dinosaurs, these large creatures probably roamed through the swamps, although no evidence of their presence has been found in the vicinity of Teton Valley.

Over time, the swamps began to accumulate peat (organic material) to a thickness of approximately 4,000 feet. Eventually this peat was buried by about 3,000 feet of sandstone and converted to coal by time, pressure and heat. Settlers in the mining town of Sam, in the Horseshoe Creek area of the Big Hole Mountains, mined this coal in the early 20th century. The Mesozoic Era ended 65 million years ago with another mass extinction, claiming the lives of the dinosaurs.

As the Cenozoic Era began, erosion was eating away at the volcanic range west of the Sundance Sea. Small island masses (or microcontinents) in the Pacific Ocean began docking against the suturing to North America. This added more land to the western edge of the continent and shifted the sinking of the oceanic plate westward to the current west coast of North America. The Sundance Sea also dried up at the beginning of the Cenozoic Era, and for the first time since the Paleozoic Era began (about 500 million years previous), the land now part of Idaho was entirely above sea level.

Due to the increasing compressional forces from the microcontinents docking and suturing to the west coast, the sedimentary rocks deposited in Idaho during the Paleozoic and Mesozoic eras were forced eastward, detaching from the older igneous and metamorphic rocks below them.

As they slid, these originally horizontal rock layers folded and faulted. In some cases, the older (Paleozoic) rocks were thrust over the younger Mesozoic rocks. This produced what geologists call the Idaho-Wyoming Fold and Overthrust Belt, of which the Big Hole and Snake River ranges are a part. Between Swan Valley and Victor, Highway 32 passes through approximately eight inactive thrust faults of the Big Hole Range, indicated, for example, by Canal, Mike Spencer, Fleming, Poison and Tie canyons.

The compressional forces pushing on the west coast of North America continued until about 13 million years ago. For not-well-understood reasons, these forces reversed and the continent began extending, or stretching. These extensional forces pull the crust, rupturing rocks and creating fault lines. This movement is known as normal faulting and produces sequences of basins and ranges by moving one side of the fault up relative to the other side, which moves down. This process continues today and is primarily responsible for the seismic (earthquake) activity in the Intermountain Seismic Belt that runs through western Montana, western Wyoming and eastern Utah. The Swan Valley and Grand Valley faults, on the west side of the Big Hole and Snake River ranges, and the notorious Teton Fault, on the east side of the Teton Range, are all normal faults.

Since its formation, the Teton normal fault has hoisted the metamorphic rocks and igneous dikes of the Precambrian Era about 23,000 feet from within the crust, exposing them in the Teton Range. As the normal fault lifts these rocks higher, the overlaying sedimentary rocks (sandstone and limestone) tilt westward.

The movement of the fault usually results in an event of Richter magnitude 7.0 every 2,000 years, according to an average produced by Robert Smith, a professor of geology and geophysics at the University of Utah. This continuing movement makes the Teton Range one of the youngest in North America. However, the Teton Fault has not moved in about 7,000 years and therefore could be 5,000 years overdue. On the other hand, Smith acknowledges, the fault may be in a period of slumber and may not move again for another several thousand years.

Just before the compressional forces reversed, about 15 million years ago, a large hot spot of magma (about 40 miles in diameter) began welling upward from the mantle of the Earth through the crust of the North American continent, near the Idaho-Oregon-Nevada borders. The resulting volcanism produced a large crater known as a caldera. While the hot spot has remained stationary in the mantle, the North American continent (crust/plate) has moved toward the southwest, causing mountains to melt and a series of calderas to form as the land passed over the hot spot. These processes make the hot spot itself appear to be migrating towards the northwest.

This melting activity produced the broad, relatively flat volcanic region of the Snake River plain, immediately northwest of Teton Valley, and caused the valley to open to the north. Green Canyon Hot Springs (on the north end of the Big Holes) and Heise Hot Springs (on the east side of the Big Holes), lie near the margin of one of the inactive calderas and owe their existence to the hot waters moving up along its edges.

About 2 million years ago, the hot spot was at Island Park, about 50 miles northwest of the valley. (On a clear day, the outside slopes of the topographically high caldera can still be seen to the north.) As the crust collapsed into the hot spot, avalanches of hot gas and ash swept down the flanks of the volcano. The valley was deluged with about 400 feet of tuff (ashy rock), which obliterated life in the vicinity. Most of the tuff rock is still exposed on the north end of the valley; the rest has subsequently been buried below sediments from the Teton Range. Because the tuff rock is still exposed on the north end of the valley; the rest has subsequently been buried below sediments from the Teton Range. Because the tuff was easy to quarry, early settlers built many structures in Teton Valley with it, including the Corner Drug building in Driggs, and the Dewey house (now home to the restaurant) in Victor. Today the hot spot is located beneath Yellowstone National Park (northwest of Teton Valley) and is responsible for the intense seismic and geyser activity in that region.

During the Pleistocene Ice Age, which began about 1.6 million years ago, the planet cooled and glaciers extended from Canada as far south as Yellowstone and the Teton Range. The ice that flowed out of the high country chiseled out Leigh, Teton, Darby and Fox Creek canyons. As the glaciers scoured the canyon walls, large quantities of sediment were deposited in the basin. The prevailing winds later picked up and redistributed the fine silty material, known as loess, which has made Teton Valley’s seed potato industry famous.

This once-oceanic seascape, teaming with sea life and swampy vegetation, has been transformed into a high mountain region through the uplift of rock from below sea level. The work of enormous volcanoes, earthquakes and glaciers, performed over millions of years, has made this region one of the most geologically diverse on the continent. Today, mountain climbers, skiers, campers and farmers alike owe their pursuits to these geological processes, which have built this beautiful and productive land.
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