GRAND TETON Creation of the Teton Landscape: The Geologic Story of Grand Teton National Park

The rugged grandeur of the Tetons is a product of four geologic factors: the tough hard rocks in the core, the amount of vertical uplift, the recency of the mountain-making movement, and the dynamic forces of destruction. Many other mountains in Wyoming have just as hard rocks in their cores and an equally great amount of vertical uplift, but they rose 50 to 60 million years ago and have been worn down by erosion from that time on. The Tetons, on the other hand, are the youngest range in Wyoming, less than 10 million years old, and have not had time to be so deeply eroded.

Steep mountain slopes—the perpetual battleground

Any steep slope or cliff is especially vulnerable to nature's methods of destruction. In the Tetons we see the never-ending struggle between two conflicting factors. The first is the extreme toughness of the rocks and their consequent resistance to erosion. The second is the presence of efficient transporting agencies that move out and away from the mountains all rock debris that might otherwise bury the lower slopes.

The rocks making up most of the Teton Range are among the hardest, toughest, and least porous known. Therefore, they resist mechanical disintegration by temperature changes, ice, and water. They consist predominantly of minerals that are subject to very little chemical decay in the cold climate of the Tetons.

Absence of weak layers prevents breaking of the tough rock masses under their own weight. All these conditions, then, are favorable for preservation of steep walls and high rock pinnacles. Nevertheless, they do break down. Great piles of broken rock (talus) that festoon the slopes of all the higher peaks bear witness to the unrelenting assault by the process of erosion upon the mountain citadels (figs. 4 and 31).

Figure 4. Talus at the foot of the jagged frost-riven peaks around Ice Floe Lake in the south fork of Cascade Canyon. Photo by Philip Hyde.

Rock disintegration and gravitational movement

A great variation in both daily and annual temperatures results in minute amounts of contraction and expansion of rock particles. Repeated changes in volume produce stress and strain. Although the rocks in the Tetons are very dense, they eventually yield; a crack forms. Water which seeps in along this surface of weakness freezes, either overnight or during long cold spells, and expands, thereby prying a slab of rock away from the mountain wall. Repeated frost wedging, as the process is called, results eventually in tipping the slab so that it falls.

What happens to the rock slab? It may fall and roll several hundred or thousand feet, depending on the steepness of the mountain surface. Pieces are broken off as it encounters obstacles. All the fragments find their way to a valley floor or slope, where they momentarily come to rest. Thus, rock debris is moved significant and easily observed distances by gravity.

None of this debris is stationary. If it is mixed with snow or saturated with water, the whole mass may slowly flow in the same manner as a glacier. These are called rock glaciers; some can be seen on the south side of Granite Canyon and one, nearly a mile long, is in the valley north of Eagles Rest Peak.

The countless snow avalanches that thunder down the mountain flanks after heavy winter snowfalls play their part, too, in gravitational transport. Loose rocks and debris are incorporated with the moving snow and borne down the mountainsides to the talus piles below. Trees, bushes, and soil are swept from the sites of the slides, leaving conspicuous scars down the slopes and exposing new rock surfaces to the attack of water and frost. Battered, broken, and uprooted trees along many of the canyon trails bear silent witness to the awesome power of snowslides.

These are some of the methods used by Nature in making debris and then, by means of gravity, clearing it from the mountain slopes. There are other ways, too. A weak layer of rock (usually one with a lot of clay in it), parallel to and underlying a mountain slope, may occur between two hard layers. An extended rainy spell may result in saturation of the weak zone so that it is well lubricated; then an earthquake or perhaps merely the weight of the overlying rock sends the now unstable mass cascading down the slope to the valley below. The famous Lower Gros Ventre Slide (fig. 5) was formed in this way on June 23, 1925.

Figure 5. The Lower Gros Ventre slide, air oblique view south. The top of the scar is 2,000 feet above the river; the slide is more than a mile long and one-half mile wide. It dammed the Gros Ventre River in the foreground, impounding a lake about 200 feet deep and 5 miles long. Gros Ventre Mountains are in the distance. Photo by P. E. Millward.

Running water cuts and carries

Running water is another effective agent that transports rock debris and has helped dissect the Teton Range. The damage a broken water main can wreak on a roadbed is well known, as is the havoc of destructive floods. The spring floods of streams in the Tetons, swollen by melting snow and ice (annual precipitation, mostly snow, in the high parts would average a layer of water 5 feet thick), move some rock debris onto the adjoining floor of Jackson Hole.

Now and then the range is deluged by summer cloud bursts. Water funnels down the maze of gullies on the mountainsides, quickly gathering volume and power, and plunges on to the talus slopes below, as if from gigantic hoses. The sudden onslaught of these torrents of water on the saturated unstable talus may trigger enormous rock and mudflows that carry vast quantities of material down into the canyons. During the summer of 1941 more than 100 of these flows occurred in the park.

Wherever water moves, it carries rock fragments varying in size from boulders to sand grains and on down to minute clay particles. Erosion (wearing away) by streams is conspicuous wherever the water is muddy, as it always is each spring in the Snake, Buffalo Fork, and Gros Ventre Rivers. Clear mountain streams likewise can erode. Although the volume of material moved and the amount of downcutting of the stream bottom may not seem great in a single stream, the cumulative effect of many streams in an area, year after year and century after century, is enormous. Streams not only transport rocks brought to them by gravitational movement but also continually widen and deepen their valleys, thereby increasing the volume of transported debris.

The effectiveness of streams as transporting agents in the Tetons is enhanced by steep gradients (slopes); these increase water velocity which in turn expands the capability of the streams to carry larger and larger rock fragments.

Glaciers scour and transport

Mountain landscapes shaped by frost action, gravitational transport, and stream erosion alone generally have rounded summits, smooth slopes, and V-shaped valleys. The jagged ridges, sharply pointed peaks, and deep U-shaped valleys of the Tetons show that glaciers have played an important role in their sculpture. The small present-day glaciers still cradled in shaded recesses among the higher peaks (fig. 6) are but miniature replicas of great ice streams that occupied the region during the Ice Age. Evidence both here and in other parts of the world confirms that glaciers were once far more extensive than they are today.

Figure 6. The Teton Glacier on the north side of Grand Teton, air oblique view west. Photo by A. S. Post, August 19, 1963.

Glaciers form wherever more snow accumulates during the winter than is melted during the summer. Gradually the piles of snow solidify to form ice, which begins to flow under its own weight. Rocks that have fallen from the surrounding ridges or have been picked up from the underlying bedrock are incorporated in the moving ice mass and carried along. The ability of ice to transport huge volumes of rock is easily observed even in the small present-day glaciers in the Tetons, all of which carry abundant rock fragments both on and within the ice.

Recent measurements show that the ice in the present Teton Glacier (fig. 6) moves nearly 30 feet per year. The ancient glaciers, which were much wider and deeper, may have moved as much as several hundred feet a year, like some of the large glaciers in Alaska.

As the glacier moves down a valley, it scours the valley bottom and walls. The efficiency of ice in this process is greatly increased by the presence of rock fragments which act as abrasives. The valley bottom is plowed, quarried, and swept clean of soil and loose rocks. Fragments of many sizes and shapes are dragged along the bottom of the moving ice and the hard ones scratch long parallel grooves in the underlying tough bedrock (fig. 7). Such grooves (glacial striae) record the direction of ice movement.

Figure 7. Rock surface polished and grooved by ice on the floor of Glacier Gulch.

The effectiveness of glaciers in cutting a U-shaped valley is particularly striking in Glacier Gulch and Cascade Canyon (figs. 2 and 8).

Figure 8. East face of the Teton Range showing some of the glacial features, air oblique view. Cascade Canyon, the U-shaped valley, was cut by ice. The glacier flowed toward the flats, occupied the area of Jenny Lake (foreground), and left an encircling ring of morainal debris, now covered with trees. The flat bare outwash plain in foreground was deposited by meltwater from the glacier The lake occupies a depression that was left when the ice melted away. National Park Service photo by Bryan Harry.

The rock-walled amphitheater at the head of a glaciated valley is called a cirque (a good example is at the upper edge of the Teton Glacier, fig. 6). The steep cirque walls develop by frost action and by quarrying and abrasive action of the glacier ice where it is near its maximum thickness. Commonly the glacier scoops out a shallow basin in the floor of the cirque. Amphitheater Lake, Lake Solitude, Holly Lake, and many of the other small lakes high in the Teton Range are located in such basins.

The sharp peaks and the jagged knife-edge ridges so characteristic of the Tetons are divides left between cirques and valleys carved by the ancient glaciers.

Effects on Jackson Hole

Rock debris is carried toward the end of the glacier or along the margins where it is released as the ice melts. The semicircular ridge of rock fragments that marks the downhill margin of the glacier is called a terminal moraine; that along the sides is a lateral moraine (figs. 9 and 10). These are formed by the slow accumulation of material in the same manner as that at the end of a conveyor belt. They are not built by material pushed up ahead of the ice as if by a bulldozer. Large boulders carried by ice are called erratics; many of these are scattered on the floor of Jackson Hole and on the flanks of the surrounding mountains (fig. 11).

Figure 9. Cutaway view of a typical valley glacier.

Figure 10. Recently-built terminal moraine of the Schoolroom Glacier, a small ice mass near the head of the south fork of Cascade Canyon. The present glacier lies to the right just out of the field of view. The moraine marks a former position of the ice terminus; the lake (frozen over in this picture) occupies the depression left when the glacier wasted back from the moraine to its present position. Crest of the moraine stands about 50 feet above the lake level. Many of the lakes along the foot of the Teton Range occupy similar depressions behind older moraines. Photo by Philip Hyde.

Figure 11. Large ice-transported boulder of coarse-grained pegmatite and granite resting on Cretaceous shale near Mosquito Creek, on the southwest margin of Jackson Hole. Many boulders at this locality are composed of pegmatite rock characteristic of the middle part of the Teton Range. This occurrence demonstrates that boulders 40 feet in diameter were carried southward 25 miles and left along the west edge of the ice stream, 1,500 feet above the base of the glacier on the floor of Jackson Hole.

Great volumes of water pour from melting ice near the lower end of a glacier. These streams, heavily laden with rock flour produced by the grinding action of the glacier and with debris liberated from the melting ice, cut channels through the terminal moraine and spread a broad apron of gravel, sand, and silt down-valley from the glacier terminus (end). Material deposited by streams issuing from a glacier is called outwash; the sheet of outwash in front of the glacier is called an outwash plain. If the terminus is retreating, masses of old stagnant ice commonly are buried beneath the outwash; when these melt, the space they once occupied became a deep circular or irregular depression called a kettle (fig. 12); many of these now contain small lakes or swamps.

Figure 12. "The Potholes," knob and kettle topography caused by melting of stagnant ice partly buried by outwash gravel. Air oblique view north from over Burned Ridge moraine (see fig. 61 for orientation). Photo by W. B. Hall and J. M. Hill.

As a glacier retreats, it may build a series of terminal moraines, marking pauses in the recession of the ice front. Streams issuing from the ice behind each new terminal moraine are incised more and more deeply into the older moraines and their outwash plains. Thus, new and younger layers of bouldery debris are spread at successively lower and lower levels. These surfaces are called outwash terraces.

Just as the jagged ridges, U-shaped valleys, and ice-polished rocks of the Teton Range attest the importance of glaciers in carving the mountain landscape, the flat gravel outwash plains and hummocky moraines on the floor of Jackson Hole demonstrate their efficiency in transporting debris from the mountains and shaping the scenery of the valley.

Glaciers sculptured all sides of Jackson Hole and filled it with ice to an elevation between 1,000 and 2,000 feet above the present valley floor. The visitor who looks eastward from the south entrance to the park can see clearly glacial scour lines that superficially resemble a series of terraces on the bare lower slopes of the Gros Ventre Mountains. Southward-moving ice cut these features in hard rocks. Elsewhere around the margins of Jackson Hole, especially where the rocks are soft, evidence that the landscape was shaped by ice has been partly or completely obliterated by later events. Rising 1,000 feet above the floor of Jackson Hole are several steepsided buttes (figs. 13 and 55) described previously, that represent "islands" of hard rock overridden and abraded by the ice. After the ice melted, these buttes were surrounded and partly buried by outwash debris.

Figure 13. Radar image of part of Tetons and Jackson Hole. Distance shown between left and right margins is 35 miles. Lakes from left to right: Phelps, Taggart, Bradley, Jenny, Leigh, Jackson. Blacktail Butte is at lower left. Channel of Snake River and outwash terraces are at lower left. Burned Ridge and Jackson Lake moraines are in center. Lava flows at upper right engulf north end of Tetons. Striated surfaces at lower right are glacial scour lines made by ice moving south from Yellowstone National Park. Image courtesy of National Aeronautics and Space Administration. (click on image for an enlargement in a new window)

The story of the glaciers and their place in the geologic history of the Teton region is discussed in more detail later in this booklet.