Part 2

About a million motorists drive south from Yellowstone to Grand Teton National Park each year. As they wind along the crooked highway on the west brink of Lewis River Canyon (fig. 1), the view south is everywhere blocked by dense forest. Then, abruptly the road leaves the canyon, straightens out, and one can look south down a 3-mile sloping avenue cut through the trees. There, 20 to 30 miles away, framed by the roadway, are the snow-capped Tetons, with Jackson Lake, luminous in reflected light, nestled against the east face. This is one of the loveliest and most unusual views of the mountains that is available to the motorist, partly because he is 800 feet above the level of Jackson Lake and partly because this is the only place on a main highway where he can see clearly the third dimension (width) of the Tetons. The high peaks are on the east edge; they rise 7,000 feet above the lake but other peaks and precipitous ridges, progressively diminishing in height, extend on to the west for a dozen miles (fig. 14). Giant, relatively young lava flows, into which the Lewis River Canyon was cut, poured southward all the way to the shore of Jackson Lake and buried the north end of the Teton Range (figs. 13 and 53). South of Yellowstone Park these flows were later tilted and broken by the dropping of Jackson Hole and the rise of the mountains.

A mountaineer’s view

As in many pursuits in life, the greatest rewards of a visit to the Tetons come to those who expend a real effort to earn them. Only by leaving the teeming valley and going up into the mountains to hike the trails and climb the peaks can the visitor come to know the Tetons in all their moods and changes and view close at hand the details of this magnificent mountain edifice.

Even a short hike to Hidden Falls and Inspiration Point affords an opportunity for a more intimate view of the mountains. Along the trail the hiker can examine outcrops of sugary white granite, glittering mica-studded dikes, and dark intricately layered rocks. Nearby are great piles of broken fragments that have fallen from the cliffs above, and the visitor can begin to appreciate how vulnerable are the towering crags to the relentless onslaught of frost and snow. The roar of the foaming stream and the thunder of the falls are constant reminders of the patient work of running water in wearing away the “everlasting hills.” Running his hand across one of the smoothly polished rock faces below Inspiration Point, the hiker gains an unforgettable concept of the power of glacial ice and its importance in shaping this majestic landscape. Looking back across Jenny Lake at the encircling ridge of glacial debris, he can easily comprehend the size of the ancient glacier that once flowed down Cascade Canyon and emerged onto the floor of Jackson Hole.

The more ambitious hiker or mountaineer can seek out the inner recesses of the range and explore other facets of its geology. He can visit the jewel-like mountain lakes—Solitude, Holly, and Amphitheater are just a few—cradled in high remote basins left by the Ice Age glaciers. He can get a closeup view of the Teton Glacier above Amphitheater Lake, or explore the Schoolroom Glacier, the tiny ice body below Hurricane Pass. He may follow the trail into Garnet Canyon to see the crystals from which the canyon takes its name and to examine the soaring ribbonlike black dike near the end of the trail. In Alaska Basin he can study the gently tilted layers of sandstone, limestone, and shale that once blanketed the entire Teton Range and can search for the fossils that help determine their age and decipher their history. From Hurricane Pass he can see how these even layers of sedimentary rock have been broken and displaced and how the older harder rocks that form the highest Teton peaks have been raised far above them along the Buck Mountain fault.

Of all those who explore the high country, it is the mountaineer who has perhaps the greatest opportunity to appreciate its geologic story. Indeed, the success of his climb and his very life may depend on an intuitive grasp of the mountain geology and the processes that shaped the peaks. He observes the most intimate details—the inclination of the joints and fractures, which gullies are swept by falling rocks, which projecting knobs are firm, and which cracks will safely take a piton. To many climbers the ascent of a peak is a challenge to technical competence, endurance, and courage, but to those endowed with curiosity and a sharp eye it can be much more. As he stands shoulder to shoulder with the clouds on some windswept peak, such as the Grand Teton, with the awesome panorama dropping away on all sides, he can hardly avoid asking how this came to be. What does the mountaineer see that inspires this curiosity? From the very first glance, it is apparent that the scenes to the north, south, east, and west are startlingly different.

Looking west from the rough, narrow, weather-ravaged granite summit of the Grand Teton, one sees far below him the layered gray cliffs of _marine sedimentary rocks_ (solidified sediment originally deposited in a shallow arm of the ocean) overlapping the granite and dipping gently west, finally disappearing under the checkerboard farmland of Teton Basin. Still farther west are the rolling timbered slopes of the Big Hole Range in Idaho. A glance at the foreground, 3,000 feet below, shows some unusual relations of the streams to the mountains. The watershed divide of the Teton Range is not marked by the highest peaks as one would expect. Streams in Cascade Canyon and in other canyons to the north and south begin west of the peaks, bend around them, then flow eastward in deep narrow gorges cut through the highest part of the range, and emerge onto the flat floor of Jackson Hole.

In the view north along the crest of the Teton Range, the asymmetry of the mountains is most apparent. The steep east face culminating in the highest peaks contrasts with the lower more gentle west flank of the uplift. From the Grand Teton it is not possible to see the actual place where the mountains disappear under the lavas of Yellowstone Park, but the heavily timbered broad gentle surface of the lava plain is visible beyond the peaks and extends across the entire north panorama. Still farther north, 75 to 100 miles away, rise the snowcapped peaks (from northwest to northeast) of the Madison, Gallatin, and Beartooth Mountains.

The view east presents the greatest contrasts in the shortest distances—the flat floor of Jackson Hole is 3 miles away and 7,000 feet below the top of the Grand Teton. Along the junction of the mountains and valley floor are blue glacial lakes strung out like irregular beads in a necklace. They are conspicuously rimmed by black-appearing margins of pine trees that grow only on the surrounding glacial moraines. Beyond these are the broad treeless boulder-strewn plains of Jackson Hole. Fifty miles to the east and northeast, on the horizon beyond the rolling hills of the Pinyon Peak Highlands, are the horizontally layered volcanic rocks of the Absaroka Range. Southeast is the colorful red, purple, green, and gray Gros Ventre River Valley, with the fresh giant scar of the Lower Gros Ventre Slide near its mouth. Bounding the south side of this valley are the peaks of the Gros Ventre Mountains, whose tilted slabby gray cliff-forming layers resemble (and are the same as) those on the west flank of the Teton Range. Seventy miles away, in the southeast distance, beyond the Gros Ventre Mountains are the shining snowcapped peaks of the Wind River Range, the highest peak of which (Gannett Peak) is about 20 feet higher than the Grand Teton.

Conspicuous on the eastern and southeastern skyline are high-level (11,000-12,000 feet) flat-topped surfaces on both the Wind River and Absaroka Ranges. These are remnants that mark the upper limit of sedimentary fill of the basins adjacent to the mountains. A plain once connected these surfaces and extended westward at least as far as the conspicuous flat on the mountain south of Lower Gros Ventre Slide. It is difficult to imagine the amount of rock that has been washed away from between these remnants in comparatively recent geologic time, during and after the rise of the Teton Range.

From this vantage point the mountaineer also gets a concept of the magnitude of the first and largest glaciers that scoured the landscape. Ice flowed southwestward in an essentially unbroken stream from the Beartooth Mountains, 100 miles away, westward from the Absaroka Range, and northwestward from the Wind River Range (fig. 57). Ice lapped up to treeline on the Teton Range and extended across Jackson Hole nearly to the top of the Lower Gros Ventre Slide. The Pinyon Peak and Mount Leidy Highlands were almost buried. All these glaciers came together in Jackson Hole and flowed south within the ever-narrowing Snake River Valley.

The view south presents a great variety of contrasts. Conspicuous, as in the view north, is the asymmetry of the range. South of the high peaks of crystalline rocks, gray layered cliffs of limestone extend in places all the way to the steep east face of the Teton Range where they are abruptly cut off by the great Teton fault.

The flat treeless floor of Jackson Hole narrows southward. Rising out of the middle are the previously described steepsided ice-scoured rocky buttes. Beginning near the town of Jackson, part of which is visible, and extending as far south as the eye can see are row upon row of sharp ridges and snowcapped peaks that converge at various angles. These are the Hoback, Wyoming, Salt River, and Snake River Ranges.

CARVING THE RUGGED PEAKS

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).

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.

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 cloudbursts. 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.

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.

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

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).

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 becomes a deep circular or irregular depression called a _kettle_ (fig. 12); many of these now contain small lakes or swamps.

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.