Chapter I: Front Matter (1)

View west toward Grand Teton on skyline. Hedrick’s Pond surrounded
by “knob and kettle” topography is in foreground, tree-covered
Burned Ridge moraine is in middle distance, and extending from it to
foot of mountains is gray flat treeless glacial outwash plain.
_National Park Service photo by W. E. Dilley._

_To Fritiof M. Fryxell, geologist, teacher,
writer, mountaineer, and the first ranger-naturalist
in Grand Teton National Park._

_All who love and strive to understand
the Teton landscape follow in his footsteps._

_CREATION OF THE
TETON LANDSCAPE_
The Geologic Story of
Grand Teton National Park

_By
J. D. LOVE AND JOHN C. REED, JR.
U.S. Geological Survey_

_Library of Congress Catalogue Card No._: 68-20628
_ISBN_ O-931895-08-1

1st Edition
1968

1st Revised Edition
1971

Reprinted 1979
Reprinted 1984
Reprinted 1989

Grand Teton Natural History Association
Moose, Wyoming 83012

CONTENTS

Foreword 6
THE STORY BEGINS 8
First questions, brief answers 9
An extraordinary story 10
An astronaut’s view 12
A pilot’s view 14
A motorist’s view 15
View north 15
View west 18
View south 19
A mountaineer’s view 20
CARVING THE RUGGED PEAKS 24
Steep mountain slopes—the perpetual battleground 24
Rock disintegration and gravitational movement 24
Running water cuts and carries 26
Glaciers scour and transport 28
Effects on Jackson Hole 30
MOUNTAIN UPLIFT 36
Kinds of mountains 36
Anatomy of faults 38
Time and rate of uplift 40
Why are mountains here? 41
The restless land 43
ENORMOUS TIME AND DYNAMIC EARTH 45
Framework of time 45
Rocks and relative age 45
Fossils and geologic time 46
Radioactive clocks 47
The yardstick of geologic time 48
PRECAMBRIAN ROCKS—THE CORE OF THE TETONS 51
Ancient gneisses and schists 51
Granite and pegmatite 55
Black dikes 58
Quartzite 63
A backward glance 64
The close of the Precambrian—end of the beginning 64
THE PALEOZOIC ERA—TIME OF LONG-VANISHED SEAS AND THE DEVELOPMENT
OF LIFE 66
The Paleozoic sequence 66
Alaska Basin—site of an outstanding rock and fossil record 66
Advance and retreat of Cambrian seas; an example 69
Younger Paleozoic formations 74
THE MESOZOIC—ERA OF TRANSITION 79
Colorful first Mesozoic strata 79
Drab Cretaceous strata 81
Birth of the Rocky Mountains 82
TERTIARY—TIME OF MAMMALS, MOUNTAINS, LAKES, AND VOLCANOES 86
Rise and burial of mountains 88
The first big lake 92
Development of mammals 95
Volcanoes 98
QUATERNARY—TIME OF ICE, MORE LAKES, AND CONTINUED CRUSTAL
DISTURBANCE 102
Hoback normal fault 103
Volcanic activity 103
Preglacial lakes 104
The Ice Age 105
Modern glaciers 112
THE PRESENT AND THE FUTURE 113
APPENDIX 115
Acknowledgements 115
Selected references—if you wish to read further 116
About the authors 117
Index of selected terms and features 118

_FOREWORD_

Geology is the science of the Earth—the study of the forces, processes, and past life that not only shape our land but influence our daily lives and our Nation’s welfare. This booklet, prepared by two members of the U.S. Geological Survey, discusses how geologic phenomena are responsible for the magnificent scenery of the Teton region.

Recognition of the complex geologic history of our Earth is vital to the enjoyment and appreciation of beautiful landscapes and other natural wonders, to the planning of our cities and highway systems, to the wise use of our water supplies, to the study of earthquake and landslide areas, to the never-ending search for new mineral deposits, and to the conservation and development of our known natural resources. Who can say, in the long run, which of the many uses of this knowledge is the most compelling reason to seek an understanding of the Earth?

W. T. Pecora, _Director_
U.S. Geological Survey

_This booklet is based on geologic investigations by the
U. S. Geological Survey in cooperation with the National Park Service,
U. S. Department of Interior._

“_Something hidden. Go and find it.
Go and look behind the Ranges—
Something lost behind the Ranges.
Lost and waiting for you. Go._”
KIPLING—_THE EXPLORER_

THE STORY BEGINS

The Teton Range is one of the most magnificent mountain ranges on the North American Continent. Others are longer, wider, and higher, but few can rival the breath-taking alpine grandeur of the eastern front of the Tetons. Ridge after jagged ridge of naked rock soar upward into the western sky, culminating in the towering cluster of peaks to which the early French voyageurs gave the name “_les Trois Tetons_” (the three breasts). The range hangs like a great stone wave poised to break across the valley at its base. To the south and east are lesser mountains, interesting and scenic but lacking the magic appeal of the Tetons.

This is a range of many moods and colors: stark and austere in morning sun, but gold and purple and black in the softly lengthening shadows of afternoon; somber and foreboding when the peaks wrap themselves in the tattered clouds of an approaching storm, but tranquil and ethereal blue and silver beneath a full moon.

These great peaks and much of the floor of the valley to the east, _Jackson Hole_ (a _hole_ was the term used by pioneer explorers and mountain men to describe any open valley encircled by mountains), lie within Grand Teton National Park, protected and preserved for the enjoyment of present and future generations. Each year more than 3 million visitors come to the park. Many pause briefly and pass on. Others stay to explore its trails, fish its streams, study the plants and wildlife that abound within its borders, or to savor the colorful human history of this area.

Most visitors, whatever their interests and activities, are probably first attracted to the park by its unsurpassed mountain scenery. The jagged panorama of the Tetons is the backdrop to which they may turn again and again, asking questions, seeking answers. How did the mountains form? How long have they towered into the clouds, washed by rain, riven by frost, swept by wind and snow? What enormous forces brought them forth and raised them skyward? What stories are chronicled in their rocks, what epics chiseled in the craggy visage of this mountain landscape? Why are the Tetons different from other mountains?

First questions, brief answers

_How did the Tetons and Jackson Hole form?_ They are both tilted blocks of the earth’s crust that behaved like two adjoining giant trapdoors hinged so that they would swing in opposite directions. The block on the west, which forms the Teton Range, was hinged along the Idaho-Wyoming State line; the eastern edge was uplifted along a fault (a fracture along which displacement has occurred). This is why the highest peaks and steepest faces are near the east margin of the range. The hinge line of the eastern block, which forms Jackson Hole, was in the highlands to the east. The western edge of the block is downdropped along the fault at the base of the Teton Range. As a consequence, the floor of Jackson Hole tilts westward toward the Tetons (see cross section inside back cover).

_When did the Tetons and Jackson Hole develop the spectacular scenery we see today?_ The Tetons are the youngest of all the mountain ranges in the Rocky Mountain chain. Most other mountains in the region are at least 50 million years old but the Tetons are less than 10 million and are still rising. Jackson Hole is of the same age and is still sinking. The Teton landscape is the product of many earth processes, the most recent of which is cutting by water and ice. Within the last 15,000 years, ice sculpturing of peaks and canyons and impounding of glacial lakes have added finishing touches to the scenic beauty.

_Why did the Tetons rise and Jackson Hole sink?_ For thousands of years men have wondered about the origin of mountains and their speculations have filled many books. Two of the more popular theories are: (1) continental drift (such as South America moving away from Africa), with the upper lighter layer of the earth’s crust moving over the lower denser layer and wrinkling along belts of weakness; and (2) convection currents within the earth, caused by heat transfer, resulting in linear zones of wrinkling, uplift, and collapse.

These concepts were developed to explain the origin of mountainous areas hundreds or thousands of miles long but they do not answer directly the question of why the Tetons rose and Jackson Hole sank. As is discussed in the chapter on mountains, it is probable that semifluid rock far below the surface of Jackson Hole flowed north into the Yellowstone Volcanic Plateau-Absaroka Range volcanic area, perhaps taking the place of the enormous amount of ash and lava blown out of volcanoes during the last 50 million years. The origin of the line of weakness that marks the Teton fault along the east face of the Teton Range may go back to some unknown inequality in the earth’s composition several billion years ago. Why it suddenly became active late in the earth’s history is an unanswered question.

The ultimate source of heat and energy that caused the mountains and basins to form probably is disintegration of radioactive materials deep within the earth. The Tetons are a spectacular demonstration that the enormous energy necessary to create mountains is not declining, even though our planet is several billion years old.

An extraordinary story

Visitors whose curiosity is whetted by this unusual and varied panorama are not satisfied with only a few questions and answers. They sense that here for the asking is an extraordinary _geologic_ (_geo_—earth; _logic_—science) story. With a little direction, many subtle features become evident—features that otherwise might escape notice. Here, for example, is a valley with an odd U-shape. There is a sheer face crisscrossed with light- and dark-colored rocks. On the valley floor is a tuft of pine trees that seem to be confined to one particular kind of rock. On the rolling hills is a layer of peculiar white soil—the only soil in which coyote dens are common. All these are geologically controlled phenomena. In short, with a bit of initial guidance, the viewer gains an ability to observe and to understand so much that the panorama takes on new depth, vividness, and excitement. It changes from a flat, two-dimensional picture to a colorful multi-dimensional exhibit of the earth’s history.

BLOCK DIAGRAM VIEW SOUTHWEST SHOWING THE TETON RANGE AND JACKSON
HOLE]

An astronaut’s view

The Tetons are a short, narrow, and high mountain range, distinctive in the midst of the great chain of the Rocky Mountains, the backbone of western North America. Figure 1 shows how the Tetons and their surroundings might appear if you viewed them from a satellite at an altitude of perhaps a hundred miles. The U. S. Geological Survey topographic map of Grand Teton National Park shows the names of many features not indicated on figure 1 or on the geologic map inside the back cover. The Teton Range is a rectangular mountain block about 40 miles long and 10-15 miles wide. It is flanked on the east and west by flat-floored valleys. Jackson Hole is the eastern one and Teton Basin (called Pierre’s Hole by the early trappers) is the western.

The Teton Range is not symmetrical. The highest peaks lie near the eastern edge of the mountain block, rather than along its center, as is true in conventional mountains, and the western slopes are broad and gentle in contrast to the precipitous eastern slopes. The northern end of the range disappears under enormous lava flows that form the Yellowstone Volcanic Plateau. Even from this altitude the outlines of some of these flows can be seen.

On the south the Teton Range abuts almost at right angles against a northwest-trending area of lower and less rugged mountains (the Snake River, Wyoming, and Hoback Ranges). These mountains appear altogether different from the Tetons. They consist of a series of long parallel ridges cut or separated by valleys and canyons. This pattern is characteristic of mountains composed of crumpled, steeply tilted rock layers—erosion wears away the softer layers, leaving the harder ones standing as ridges.

On the east and northeast, Jackson Hole is bounded by the Gros Ventre and Washakie Ranges, which are composed chiefly of folded hard and soft sedimentary rocks. In contrast, between these mountains and the deepest part of Jackson Hole to the west, thick layers of soft nearly flat-lying sedimentary rocks have been sculptured by streams and ice into randomly oriented knife-edge ridges and rolling hills separated by broad valleys. The hills east of the park are called the Mount Leidy Highlands and those northeast are the Pinyon Peak Highlands.

A pilot’s view

If you descend from 100 miles to about 5 miles above the Teton region, the asymmetry of the range, the extraordinary variety of landscapes, and the vivid colors of rocks become more pronounced.

Figure 2 shows a panorama of the Teton Range and Jackson Hole from a vantage point over the Pinyon Peak Highlands. The rough steep slopes and jagged ridges along the east front of the range contrast with smoother slopes and more rounded ridges on the western side. Nestled at the foot of the mountains and extending out onto the floor of Jackson Hole are tree-rimmed sparkling lakes of many sizes and shapes. Still others lie in steep-sided rocky amphitheaters near the mountain crests.

One of the most colorful flight routes into Jackson Hole is from the east, along the north flank of the Gros Ventre Mountains. For 40 miles this mountain range is bounded by broad parallel stripes of bright-red, pink, purple, gray, and brown rocks. Some crop out as cliffs or ridges, and others are _badlands_ (bare unvegetated hills and valleys with steep slopes and abundant dry stream channels). In places the soft beds have broken loose and flowed down slopes like giant varicolored masses of taffy. These are mudflows and landslides. The colorful rocks are bounded on the south by gray and yellow tilted layers forming snowcapped peaks of the Gros Ventre Mountains.

These landscapes are the product of many natural forces acting on a variety of rock types during long or short intervals of geologic time. Each group of rocks records a chapter in the geologic story of the region. Other chapters can be read from the tilting, folding, and breaking of the rocks. The latest episodes are written on the face of the land itself.

A motorist’s view

Most park visitors first see the Teton peaks from the highway. Whether they drive in from the south, east, or north, there is one point on the route at which a spectacular panorama of the Tetons and Jackson Hole suddenly appears. Part of the thrill of these three views is that they are so unexpected and so different. The geologic history is responsible for these differences.

_View north._—

Throughout the first 4 miles north of the town of Jackson, the view of the Tetons from U. S. 26-89 is blocked by East Gros Ventre Butte. At the north end of the butte, the highway climbs onto a flat upland at the south boundary of Grand Teton National Park. Without any advance warning, the motorist sees the whole east front of the Teton Range rising steeply from the amazingly flat floor of Jackson Hole.

From the park boundary turnout no lakes or rivers are visible to the north but the nearest line of trees in the direction of the highest Teton peaks marks the approximate position of the Gros Ventre River. The elevation of this river is surprising, for the route has just come up a 150-foot hill, out of the flat valley of a much smaller stream, yet here at eye-level is a major river perched on an upland plain. The reason for these strange relations is that the hill is a fault scarp (see fig. 16A for a diagram) and the valley in which the town of Jackson is located was dropped 150 feet or more in the last 15,000 years.

On the skyline directly west of the turnout are horizontal and inclined layers of rocks. These once extended over the tops of the highest peaks but were worn away from some parts as the mountains rose. All along the range, trees grow only up to _treeline_ (also called _timberline_—a general elevation above which trees do not grow) which here is about 10,500 feet above the sea. To the southeast and east, beyond the sage-covered floor of Jackson Hole, are rolling partly forested slopes marking the west end of the Gros Ventre Mountains. They do not look at all like the Tetons because they were formed in a very different manner. The Gros Ventres are folded mountains that have foothills; the Tetons are faulted mountains that do not.

Figure 3. _The Teton landscape as seen from Signal Mountain._

Three steepsided hills called _buttes_ rise out of the flat floor of Jackson Hole. They are tilted and faulted masses of hard, layered rock that have been shaped by southward-moving glaciers. Six miles north of the boundary turnout is Blacktail Butte, on the flanks of which are west-dipping white beds. Southwest of the turnout is East Gros Ventre Butte, composed largely of layered rocks that are exposed along the road from Jackson almost to the turnout. These are capped by very young lava that forms the brown cliff overlooking the highway at the north end of the butte. To the southwest is West Gros Ventre Butte, composed of similar rocks.

_View west._—

The motorist traveling west along U. S. 26-287 is treated to two magnificent views of the Teton Range. The first is 8 miles and the second 13 miles west of Togwotee Pass. At these vantage points, between 20 and 30 miles from the mountains, the great peaks seem half suspended between earth and sky—too close, almost, to believe, but too distant to comprehend.

Only from closer range can the motorist begin to appreciate the size and steepness of the mountains and to discern the details of their architecture. The many roads on the floor of Jackson Hole furnish ever-changing vistas, and signs provided by the National Park Service at numerous turnouts and scenic overlooks help the visitor to identify quickly the major peaks and canyons and the principal features of the valley floor. Of all these roadside vantage points, the top of Signal Mountain, an isolated hill rising nearly 1,000 feet out of the east margin of Jackson Lake, probably offers the best overall perspective (fig. 3). To the west, across the shimmering blue waters of Jackson Lake, the whole long parade of rugged peaks stretches from the north horizon to the south, many of the higher ones wearing the tattered remnants of winter snow. From here, only 8 miles away, the towering pinnacles, saw-toothed ridges, and deep U-shaped canyons are clearly visible.

Unlike most other great mountain ranges, the Tetons rise steeply from the flat valley floor in a straight unbroken line. The high central peaks tower more than a mile above the valley, but northward and southward the peaks diminish in height and lose their jagged character, gradually giving way to lower ridges and rounded hills. Some of the details of the mountain rock can be seen—gnarled gray rocks of the high peaks threaded by a fine white lacework of dikes, the dark band that cleaves through Mount Moran from base to summit, and the light brown and gray layers on the northern and southern parts of the range.

At first glance the floor of Jackson Hole south of Signal Mountain seems flat, smooth, and featureless, except for the Snake River that cuts diagonally across it. Nevertheless, even the flats show a variety of land forms. The broad sage-covered areas, low isolated hills, and hummocky tree-studded ridges that form the foreground are all parts of the Teton landscape, and give us clues to the natural processes that shaped it. A critical look to the south discloses more strange things. We take for granted the fact that the sides of normal valleys slope inward toward a central major stream. South of Signal Mountain, however, the visitor can see that the Snake River Valley does not fit this description. The broad flat west of the river should slope east but it does not. Instead, it has been tilted westward by downward movement along the Teton fault at the base of the mountains.

_View south._—

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.

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.

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.

MOUNTAIN UPLIFT

Mountains appear ageless, but as with people, they pass through the stages of birth, youth, maturity, and old age, and eventually disappear. The Tetons are youthful and steep and are, therefore, extremely vulnerable to destructive processes that are constantly sculpturing the rugged features and carrying away the debris. The mountains are being destroyed. Although the processes of destruction may seem slow to us, we know they have been operating for millions of years—so why have the mountains not been leveled? How did they form in the first place?

Kinds of mountains

There are many kinds of mountains. Some are piles of lava and debris erupted from a volcano. Others are formed by the bowing up of the earth’s crust in the shape of a giant dome or elongated arch. Still others are remnants of accumulated sedimentary rocks that once filled a basin between preexisting mountains and which are now partially worn away. An example of this type is the Absaroka Range 40 miles northeast of the Tetons (figs. 1 and 52).

The Tetons are a still different kind—a _fault block mountain range_ carved from a segment of the earth’s crust that has been uplifted along a fault. The _Teton fault_ is approximately at the break in slope where the eastern foot of the range joins the flats at the west edge of Jackson Hole (see map inside back cover), but in most places is concealed beneath glacial deposits and debris shed from the adjacent steep slopes. The shape of the range and its relation to Jackson Hole have already been described. Clues as to the presence of the fault are: (1) the straight and deep east face of the Teton Range, (2) absence of foothills, (3) asymmetry of the range (fig. 14), and (4) small _fault scarps_ (cliffs or steep slopes formed by faulting) along the mountain front (fig. 15).

Recent geophysical surveys of Jackson Hole combined with data from deep wells drilled in search of oil and gas east of the park also yield valuable clues. By measuring variations in the earth’s magnetic field and in the pull of gravity and by studying the speed of shock waves generated by small dynamite explosions, Dr. John C. Behrendt of the U. S. Geological Survey has determined the depth and tilt of rock layers buried beneath the veneer of glacial debris and stream-laid sand and gravel on the valley floor. This information was used in constructing the geologic cross section in the back of the booklet. The same rock layers that cap the summit of Mount Moran (fig. 27) are buried at depths of nearly 24,000 feet beneath the nearby floor of Jackson Hole but are cut off by the Teton fault at the west edge of the valley. Thus the approximate amount of movement along the fault here would be about 30,000 feet.

Anatomy of faults

The preceding discussion shows that the Tetons are an upfaulted mountain block. Why is this significant? The extreme youth of the Teton fault, its large amount of displacement, and the fact that the newly upfaulted angular mountain block was subjected to intense glaciation are among the prime factors responsible for the development of the magnificent alpine scenery of the Teton Range. An understanding of the anatomy of faults is, therefore, pertinent.

A fault is a plane or zone in the earth’s crust along which the rocks on one side have moved in relation to the rocks on the other. There are various kinds of faults just as there are various types of mountains. Three principal types of faults are present in the Teton region: normal faults, reverse faults, and thrust faults. A _normal fault_ (fig. 16A) is a steeply _dipping_ (steeply inclined) fault along which rocks above the fault have moved _down_ relative to those beneath it. A _reverse fault_ (fig. 16B) is a steeply inclined fault along which the rocks above the fault have moved _up_ relative to those below it. A _thrust fault_ (fig. 16C) is a gently inclined fault along which the principal movement has been more nearly horizontal than vertical.

Normal faults may be the result of tension or pulling apart of the earth’s crust or they may be caused by adjustment of the rigid crust to the flow of semi-fluid material below. The crust sags or collapses in areas from which the subcrustal material has flowed and is bowed up and stretched in areas where excess subcrustal material has accumulated. In both areas the adjustments may result in normal faults.

Reverse faults are generally caused by compression of a rigid block of the crust, but some may also be due to lateral flow of subcrustal material.

Thrust faults are commonly associated with tightly bent or folded rocks. Many of them are apparently caused by severe compression of part of the crust, but some are thought to have formed at the base of slides of large rock masses that moved from high areas into adjacent low areas under the influence of gravity.

The Teton fault (see cross section inside back cover) is a normal fault; the Buck Mountain fault, which lies west of the main peaks of the Teton Range, is a reverse fault. No thrust faults have been recognized in the Teton Range, but the mountains south and southwest of the Tetons (fig. 1) display several enormous thrust faults along which masses of rocks many miles in extent have moved tens of miles eastward and northeastward.

Time and rate of uplift

When did the Tetons rise?

A study of the youngest sedimentary rocks on the floor of Jackson Hole shows that the Teton Range began to rise rapidly and take its present shape less than 9 million years ago. The towering peaks themselves are direct evidence that the rate of uplift far exceeded the rate at which the rising block was worn away by erosion. The mountains are still rising, and comparatively rapidly, as is indicated by small faults cutting the youngest deposits (fig. 15).

How rapidly? Can the rate be measured?

We know that in less than 9 million years (and probably in less than 7 million years) there has been 25,000 to 30,000 feet of displacement on the Teton fault. This is an average of about 1 foot in 300-400 years. The movement probably was not continuous but came as a series of jerks accompanied by violent earthquakes. One fault on the floor of Jackson Hole near the southern boundary of the park moved 150 feet in the last 15,000 years, an average of 1 foot per 100 years.

In view of this evidence of recent crustal unrest, it is not surprising that small earthquakes are frequent in the Teton region. More violent ones can probably be expected from time to time.

Figure 16. _Types of faults._

Why are mountains here?

Why did the Tetons form where they are?

At the beginning of this booklet we discussed briefly the two most common theories of origin of mountains: continental drift and convection currents. The question of why mountains are where they are and more specifically why the Tetons are here remains a continuing scientific challenge regardless of the wealth of data already accumulated in our storehouse of knowledge.

The mobility of the earth’s crust is an established fact. Despite its apparent rigidity, laboratory experiments demonstrate that rocks flow when subjected to extremely high pressures and temperatures. If the stress exceeds the strength at a given pressure and temperature, the rock breaks. Flowing and fracturing are two of the ways by which rocks adjust to the changing environments at various levels in the earth’s crust. These acquired characteristics, some of which can be duplicated in the laboratory, are guides by which we interpret the geologic history of rocks that once were deep within the earth.

The site of the Teton block no doubt reflects hidden inequalities at depth. We cannot see these, nor in this area can we drill below the outer layer of the earth; nevertheless, measurements of gravity and of the earth’s magnetic field clearly show that they exist.

We know that the Tetons rose at the time Jackson Hole collapsed but the volume of the uplifted block is considerably less than that of the downdropped block. This, then, was not just a simple case in which all the subcrustal material displaced by the sinking block was squeezed under the rising block (the way a hydraulic jack works). What happened to the rest of the material that once was under Jackson Hole? It could not be compressed so it had to go somewhere.

As you look northward from the top of the Grand Teton or Mount Moran, or from the main highway at the north edge of Grand Teton National Park, you see the great smooth sweep of the volcanic plateau in Yellowstone National Park. Farther off to the northeast are the strikingly layered volcanic rocks of the Absaroka Range (fig. 52). For these two areas, an estimate of the volume of volcanic rock that reached the surface and flowed out, or was blown out and spread far and wide by wind and water, is considerably in excess of 10,000 cubic miles. On the other hand, this volume is many times more than that displaced by the sagging and downfaulting of Jackson Hole.

Where did the rest of the volcanic material come from? Is it pertinent to our story? Teton Basin, on the west side of the Teton Range, and the broad Snake River downwarp farther to the northwest (fig. 1) are sufficiently large to have furnished the remainder of the volcanic debris. As it was blown out of vents in the Yellowstone-Absaroka area, its place could have been taken deep underground by material that moved laterally from below all three downdropped areas. The movement may have been caused by slow convection currents within the earth, or perhaps by some other, as yet unknown, force. The sagging of the earth’s crust on both sides of the Teton Range as well as the long-continued volcanism are certainly directly related to the geologic history of the park.

In summary, we theorize as to how the Tetons rose and Jackson Hole sank but are not sure why the range is located at this particular place, why it trends north, why it rose so high, or why this one, of all the mountain ranges surrounding the Yellowstone-Absaroka volcanic area, had such a unique history of uplift. These are problems to challenge the minds of generations of earth scientists yet to come.

The restless land

Among the greatest of the park’s many attractions is the solitude one can savor in the midst of magnificent scenery. Only a short walk separates us from the highway, torrents of cars, noise, and tension. Away from these, everything seems restful.

Quiescent it may seem, yet the landscape is not static but dynamic. This is one of the many exciting ideas that geology has contributed to society. The concept of the “everlasting hills” is a myth. All the features around us are actually rather short-lived in terms of geologic time. The discerning eye detects again and again the restlessness of the land. We have discussed many bits of evidence that show how the landscape and the earth’s crust beneath it are constantly being carved, pushed up, dropped down, folded, tilted, and faulted.