The Ice Age
With the uplift of the Teton Range and the formation of Jackson Hole late in Cenozoic time the landscape gradually began to assume the general outlines that we see today. Rain, wind, snow, and frost shaped the first crude approximations of the present ridges and peaks. Streams cut into the rising Teton fault block, eroding the ancestral canyons deeper and deeper as the uplift continued. The most recent great chapter in the story of the Teton landscape, however, remained to be written by the glaciers of the Ice Age.
The reasons for the climatic changes that caused the Ice Age are still a matter of much scientific debate. Various theories have been advanced that attribute them to changes in solar radiation, changes in the earth’s orbit and inclination to the sun, variations in the amount of carbon dioxide in the atmosphere, shifts in the positions of the continents or the poles, and to many other factors, but none has met with universal acceptance. No doubt the explanation lies in some unusual combination of circumstances, for widespread glaciation occurred only twice before in the earth’s history—once in the late Precambrian and once during the Permian. It is quite clear, however, that the glaciers did not form in response to any local cause such as the uplift of the Teton Range, for concurrent climatic changes and ice advance took place throughout many parts of the world.
At least three times in the last 250,000 years glaciers from the surrounding highlands invaded Jackson Hole. The oldest and most widespread glaciation probably took place about 200,000 years ago; it was called the Buffalo Glaciation by Prof. Eliot Blackwelder in 1915 (see selected references). The age estimate is based on measurements of the thickness of the decomposed layer on the surface of obsidian pebbles in the glacial debris. Major sources of ice were the Beartooth Mountains ([fig. 1]), the Absaroka Range, and the Wind River Range. The Gros Ventre Mountains and Teton Range furnished lesser amounts of ice.
The ice from the Beartooth and Absaroka centers of ice accumulation converged in the northeastern part of Grand Teton National Park and flowed south along the face of the Teton Range in a giant stream that in many places was 2,000 feet thick ([fig. 57]). All but the highest parts of the Pinyon Peak and Mount Leidy Highlands were buried and scoured. Signal Mountain, Blacktail Butte, and the Gros Ventres Buttes were overridden and shaped by ice at this time. Another glacier, this one from the Wind River Range, flowed northwest along the Continental Divide, then down the Gros Ventre River Valley, and merged with the southward-moving main ice stream west of Lower Slide Lake. Where Jackson Hole narrows southward, the glacier became more and more confined, but nevertheless flowed all the way through the Snake River Canyon and on into Idaho.
Figure 58. Glacial deposits, outwash, and loess exposed along Boyle Ditch in Jackson Hole National Elk Refuge. Indicated are middle Pliocene Teewinot formation (A), oldest till (B), Bull Lake outwash gravel (C), and post-Bull Lake loess (D), which here contains snail shells dated by Carbon-14 as 15,000 years old. Height of cliff is about 30 feet.
The volume of this great ice mass was probably considerably more than 1,000 cubic miles. When it melted, nearly all the previously accumulated soil in Jackson Hole was washed away and a pavement of quartzite boulders mantled much of the glaciated surface. In areas not subsequently glaciated, the lack of soil and abundance of quartzite boulders drastically influenced the topography, later drainage, distribution of all types of vegetation, especially conifers and grass, and the pattern of human settlement and industry.
Figure 59. View west from the Snake River overlook showing at upper right the Burned Ridge moraine (with trees) merging southward with the highest (oldest) Pinedale outwash plain. The next lower surface is composed of outwash from the Jackson Lake moraine which lies to the right, out of the picture. At the bottom is Deadman’s Bar, a gravel deposit at the present river level. Photo by H. D. Pownall.
The second glaciation, named Bull Lake, was less than half as extensive as the first. A large tongue of ice from the Absaroka center of accumulation flowed down the Buffalo River Valley and joined ice from the Tetons on the floor of Jackson Hole. An enormous outwash fan of quartzite boulders extended from near Blacktail Butte southward throughout most of southern Jackson Hole. Glaciers in the Gros Ventre Mountains did not advance beyond the east margin of the valley floor. Carbon-14 ages and data from weathered obsidian pebbles suggest that this glaciation took place between 35,000 and 80,000 years ago.
Bull Lake moraines and outwash deposits are overlain directly in the southern part of Jackson Hole by fine silt, rather than by deposits of the third glaciation ([fig. 58]). This silt, of windblown origin, is called loess and contains fossil shells dated by Carbon-14 as between 13,000 and 19,000 years old. Wherever the loess occurs, it is marked by abundant modern coyote dens and badger burrows.
Figure 60. Air oblique view west toward the Teton Range, showing effects of Pinedale Glaciation on the landscape. Mt. Moran is at top left; the mountain front is broken by U-shaped valleys from which ice emerged into the area now occupied by Jackson Lake. The timbered area bordering Jackson Lake is the Jackson Lake moraine. One of the braided outlet channels breaching the Jackson Lake moraine can be seen crossing the outwash plain at the left center. Lakes at lower right occupy “potholes” near where the 9,000-year-old snail shells occur. Snake River is in foreground. Photo by R. L. Casebeer.
The third and last glaciation, named the Pinedale, was even less extensive than the others. Nevertheless it was of great importance for it added the final touches to the present landscape. The jagged intricately ice-carved peaks ([fig. 4]) and the glittering lakes and broad gravelly plains are vivid reminders of this recent chapter in geologic history.
Pinedale glaciers advanced down Cascade, Garnet, Avalanche, and Death Canyons and spilled out onto the floor of Jackson Hole, where they built the outermost loops of the conspicuous terminal moraines that now encircle Jenny, Bradley, Taggart, and Phelps Lakes ([fig. 13]). Ice streams from Glacier Gulch and Open Canyon also left prominent moraines on the valley floor, but these do not contain lakes. Ice from Leigh Canyon and all of the eastward-draining valleys to the north combined to form a large glacier in roughly the present position of Jackson Lake. This ice entirely surrounded Signal Mountain, leaving only the upper few hundred feet projecting as an island or nunatak.
Figure 61. The Pinedale Glaciers in the central part of Jackson Hole as they might have appeared at the time the Jackson Lake moraine was built. Solid color areas are lakes; dark irregular pattern shows areas of moraine deposited during the maximum advance of the Pinedale Glaciers. Pattern of open circles shows older Pinedale outwash plains; pattern of fine dots shows outwash plains built at the time the glaciers were in the positions shown in the drawing. Coarser dots near the margins of the glaciers represent concentrations of rock debris in the ice.
The southernmost major advance of Pinedale ice from Jackson Lake is marked by a series of densely timbered moraines that cross the Snake River Valley. This series is collectively named the Burned Ridge moraine ([fig. 61]). Extending southward for 10 miles from this moraine is a remarkably flat surfaced gravelly outwash deposit. It was spread by streams that poured from the glacier at the time the moraine was being built ([fig. 59]). East of the Snake River, the main highway from a point just north of Blacktail Butte to the Snake River overlook is built on this flat untimbered surface. We assume that the outwash is younger than 15,000 years because it apparently overlies loess of that age.
The glacier withdrew rapidly northward from the Burned Ridge moraine, leaving behind many large irregular masses of stagnant, debris-covered ice. The sites of these became kettles, locally known as “The Potholes” ([fig. 12]). The main glacier retreated to a position marked by the loop of moraines just south of Jackson Lake ([fig. 60]). [Figure 61] is a sketch map showing how the glaciers in this part of Jackson Hole might have appeared at the time the Jackson Lake moraine was built.
Abundant snail shells have been found in lake sediments in the bottoms of the kettles north of the Burned Ridge moraine ([fig. 60]) as well as on low ridges between them. Carbon-14 age determinations indicate that the snails lived about 9,000 years ago, either in a lake already present before the Pinedale ice advanced and formed the Burned Ridge moraine or in ponds that filled kettles left as the ice melted behind this moraine.
In either case, the shells indicate that the Pinedale glaciers probably existed on the floor of Jackson Hole as recently as 9,000 years ago, at a time when Indians were already living in the area. We can easily imagine the fascination with which these primitive peoples may have watched as year after year the glaciers wasted away, slowly retreating back into the canyons, then withdrawing into the sheltered recesses of the high mountains, eventually to dwindle and disappear.
Many bits of evidence, both from North America and Europe, indicate that there was a period called the climatic optimum about 6,000 years ago when the climate was significantly warmer and drier than at present. We suspect, though there is as yet no direct proof, that the Pinedale glaciers wasted away entirely during this interval.
The modern pattern of vegetation in Jackson Hole is strongly influenced by the distribution of Pinedale glacial moraines and outwash deposits. Almost without exception the moraines are heavily forested, whereas the nearby outwash deposits are covered only by a sparse growth of sagebrush. This is probably because the moraines contain large amounts of clay and silt produced by the grinding action of the glaciers. Material of this type retains water much better and, because of the greater variety of chemical elements, is more fertile than the porous quartzite gravel and sand on the outwash plains.