Volcanoes

Figure 52.Layers of volcanic conglomerate separated by thin white tuff beds in Wiggins Formation. These cliffs, on the north side of Togwotee Pass, are about 1,100 feet high and represent a cross section of part of the enormous blanket of waterlaid debris that spread south and east from the Yellowstone-Absaroka volcanic area. These and younger deposits from the same general source filled the basins and almost completely buried the mountains in this part of Wyoming.

Volcanoes are one of the most interesting parts of the geologic story of the Teton region. Although ash from distant volcanoes had settled in northwestern Wyoming at least as far back in time as Jurassic, the first nearby active volcanoes (since the Precambrian) erupted in the Yellowstone-Absaroka region during the early Eocene, about 50 million years ago. From then on, the volcanic area grew in size and the violence of eruptions and volume of debris increased until Pliocene time. This debris had a profound influence on the color and composition of the sediments and on the environment and types of plants and animals.

The color of the volcanic rocks and the sediments derived from them varies significantly from one epoch toanother. For example, the middle Eocene rocks are white to light-green, red, and purple, upper Eocene are dark-green, Oligocene are light-gray, white, and brown, Miocene are dark-green, brown, and gray, and Pliocene are white to red-brown.

Figure 53.Air oblique view south, showing the north end of the Teton Range disappearing beneath Pleistocene lava flows. Light-colored bare area at lower left is vertical Paleozoic limestone surrounded on three sides by nearly horizontal rhyolite lava flows. Bare slope at lower right is west-dipping Pinyon Conglomerate, also overlapped by lava. Grand Teton is on right skyline and Mt. Moran is rounded summit on middle skyline.

As mentioned earlier, it is probable that the vast outpouring of volcanic rocks during late Tertiary time in the Teton region and to the north and northeast is directly related to the subsidence of Jackson Hole and the rise of the Tetons.

The spectacular banded cliffs of the Wiggins Formation on both sides of Togwotee Pass (fig. 52) and farther north in the Absaroka Range are remnants of Oligocene volcanic conglomerate and tuff that once spread as a blanketseveral thousand feet thick across eastern Jackson Hole and partially or completely buried the nearby older folded mountain ranges.

Figure 54.Obsidian, a volcanic glass less than 10 million years old, especially prized by Indians who used it for spear and arrow points and for tools.

About 25 million years ago, with the start of the Miocene Epoch, volcanic vents opened up within, and along the borders of, Grand Teton National Park. Major centers of eruption were at the north end of the Teton Range, east of Jackson Lake, and south of Spread Creek. They emitted a prodigious amount of volcanic ash and fragments of congealed lava. For example, adjacent to one vent a mile in diameter, about 4 miles north-northeast of Jackson Lake Lodge, is a continuous section, 7,000 feet thick, of waterlaid strata derived in large part from this volcanic source. These sedimentary rocks comprise the Colter Formation which is darker colored and contains more iron and magnesium than the Wiggins Formation. The site of deposition at this locality was a north-trending trough that represented an early stage in the downwarping of Jackson Hole.

Pliocene volcanoes erupted in southern and central Yellowstone Park. The volcanoes emitted viscous, frothy, pinkish-gray and brown lava calledrhyolite. This is an extrusive igneous rock that has the same compositionas granite, but is much finer grained. In several places, lava apparently flowed into the north end of Teewinot Lake, chilled suddenly, and solidified into a black volcanic glass calledobsidian. Because it chips easily into thin flakes having a smooth surface, obsidian was prized by the Indians, who used it for spear and arrow points (fig. 54). Some of this obsidian has a potassium-argon date of 9 million years.

Figure 55.East face of Signal Mountain showing Bivouac Formation (upper Pliocene or Pleistocene). Tilted ledge is rhyolitic welded tuff 2.5 million years old, and slopes above and below it are conglomerate. National Park Service photo by W. E. Dilley.

After Teewinot Lake was filled with sediment, the floor of Jackson Hole became a flat boulder-covered surface. Nearby vents erupted heavy fiery clouds of gaseous molten rock that rolled across this plain and then congealed into hard layers with the general appearance of lava flows. Under a microscope, however, the rock is seen to be made up of compressed fragments of glass that matted down and solidified when the clouds stopped moving. This kind of rock is called awelded tuff. One of these forms the conspicuous ledge in the Bivouac Formation on the north and east sides of Signal Mountain (fig. 55), and is especially important because it has a potassium-argon date of 2.5 million years. More of thiswelded tuffflowed southward from Yellowstone National Park, engulfed the north end of the Teton Range (fig. 53), and continued southward along the west side of the mountains for 35 miles and along the east side for 25 miles.

Figure 56.The final 3 million years on our yardstick of time, enlarged to show approximate dates of major events.

The Quaternary Period is represented by less than 15-thousandths of the last inch on our yardstick of time (fig. 56) and the entire Ice Age takes up less than 2-thousandths of an inch (less than the thickness of this page). Nevertheless, the spectacular effects of various forces of nature on the Teton landscape during this short interval of time are of such significance that they warrant a separate discussion.The role of glaciers in carving the rugged Teton peaks and shaping the adjacent valleys was mentioned in the first part of this booklet, but is discussed in more detail here. The magnitude and complexity of crustal movements increased during the final 2 million years of time—so much so that the beginning of Quaternary time has not yet been identified with any single event.Figure 56shows the major events described below.

TheHoback normal fault, 30 miles long, with a mile or more displacement, developed in the southernmost part of Jackson Hole about 2 million years ago. This fault is on the east side of the valley. Thus, the valley block was downdropped between this fault and the Teton fault that borders the west side.

During or shortly after major movement on the Hoback fault, and perhaps related to it, there was a complex series of volcanic eruptions west and north of the town of Jackson, along the south boundary of the park. In rapid succession, lavas of many types, with a combined thickness of more than 1,000 feet, were extruded and volcanic plugs intruded into the near-surface sedimentary rocks. These volcanic rocks can be seen on the East and West Gros Ventre Buttes.

There are no active volcanoes in the Teton region today and no postglacial lava flows or cinder cones. Five miles north of Grand Teton National Park are boiling springs (Flagg Ranch hot springs) that are associated with the youngest (late Quaternary) lavas in southern Yellowstone Park. Elsewhere in Jackson Hole are a number of lukewarm springs but their relation to volcanic rocks has not been determined.

What happened to the vast thicknesses of volcanic debris? We know they existed because sections of them have been measured on the eroded edges of uptilted folds and fault blocks. Many cubic miles of these rocks are now buried beneath the floor of Jackson Hole, but a much greater volume was carried completely out of the region by water,ice, and wind during the final chapter of geologic history.

Remnants of two sets of lake deposits in Jackson Hole record preglacial events in Quaternary time. Downdropping of southern Jackson Hole along the Hoback and Teton faults blocked the southwestward drainage of the Snake River, and a new lake formed overlapping and extending south of the site of the long-vanished Teewinot Lake. Incorporated in the lake sediments are fragments of lava like that in nearby Quaternary flows. From this we know that the lake formed after at least some of the lava was emplaced. Apparently subsidence was more rapid than filling, for a time, at least, because this new lake was deep. Fossil snails preserved in olive-drab to gray fine-grained claystone overlying lava flows at the north end of East Gros Ventre Butte are the kind now living at depths of 120 to 300 feet in Lake Tahoe, California-Nevada. Near the margins of the lake, pink and green claystone and soft sandstone were deposited. The duration of this lake is not known but it lasted long enough for 200 feet of beds to accumulate. Subsequent faulting and warping destroyed the lake, left tilted remnants of the beds perched 1,000 feet up on the east side of Jackson Hole, and permitted the Snake River to reestablish its course across the mountains to the southwest.

Later downdropping of Jackson Hole impounded a second preglacial lake. Little is known about its extent because nearly everywhere the soft brown and gray shale, claystone, and sandstone deposited in it were scooped out and washed away during subsequent glaciations. A few remnants of the lake deposits are preserved in protected places, however; two are within the Gros Ventre River Valley—one downstream from Lower Slide Lake about a mile east of the park and the other 4 miles farther east. The latter remnant is nearly 500 feet thick and the upper half is largely very fine grained shale and claystone. This fine texture suggests that the lake existed for a good many thousand years, for such deposits commonly accumulate more slowly than coarser grained debris.

Figure 57.Map showing extent and direction of movement of first and largest ice sheet. Seefigure 41for State lines and location map.

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 theIce 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 inclinationto 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 theBuffalo Glaciationby 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, namedBull 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 calledloessand containsfossil 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 thePinedale, 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 ornunatak.

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 theBurned 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 61is 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 asrecently 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 theclimatic optimumabout 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.

About a dozen small rapidly dwindling glaciers exist today in shaded reentrants high in the Teton Range. They are probably vestiges of ice masses built up since the climatic optimum, during the so-called “Little Ice Age.” These glaciers, while insignificant compared to those still present in many other mountain ranges, are fascinating working models of the great ice streams that shaped the Tetons during Pleistocene time.

The Teton Glacier (fig. 6) is one of the best known. It is an ice body about 3,500 feet long and 1,100 feet wide that lies at the head of Glacier Gulch, shaded by the encircling ridges of the Grand Teton, Mount Owen, and Mount Teewinot. Ice in the central part is moving at a rate of more than 30 feet a year.

The geologic story of the Teton country from the time the earth was new to the present day has been summarized. What can we learn from it? We become aware that events recorded in the rocks are not a chaotic jumble of random accidents but came in an orderly, logical succession. We see the majestic parade of life evolving from simple to complex types, overcoming all natural disasters, and adapting to ever-changing environments. We can only speculate as to the motivating force that launched this fascinating geologic and biologic venture and what the ultimate goal may be. New facts and new ideas are added to the story each year, but many unknown chapters remain to be studied; these offer an irresistible, continuing challenge to inquisitive minds, strong bodies, and restless, adventurous spirits.

Most geologic processes that developed the Teton landscape have been beneficial to man; a few have interfered with his activities, cost him money, time, effort, and on occasion, his life. Postglacial faulting and tilting along the southern margin of Grand Teton National Park diverted drainage systems (such as Flat Creek, southwest of the Flat Creek fault on the south edge of thegeologic map), raised hills, dropped valleys, and made steep slopes unstable. Flood-control engineers wage a never-ending struggle to keep the Snake River from shifting to the west side of Jackson Hole as the valley tilts westward in response to movement along the Teton fault. Each highway into Jackson Hole has been blocked by a landslide at one time or another and maintenance of roads across slide areas requires much ingenuity. We see one slide (the Gros Ventre) that blocked a river; larger slides have occurred in the past, and more can be expected. Abundant fresh fault scarps are a constant reminder that public buildings, campgrounds, dams, and roads need to be designed to withstand the effects of earthquakes. Some of these problems have geologic solutions; others can be avoided or minimized as further study increases our understanding of this region.

Man appeared during the last one-fiftieth of an inch on our yardstick of time gone by. In this short span he has had more impact on the earth and its inhabitants than any other form of life. Will he use wisely the lessons of the past as a guide while he writes his record on the yardstick of the future?

This booklet could not have been prepared without the cooperation and assistance of many individuals and organizations. We are indebted to the National Park Service for the use of facilities, equipment, and photographs, and for the enthusiasm and interest of all of the park staff. We especially appreciate the cooperation, advice, and assistance rendered by the late Fred C. Fagergren, former superintendent of Grand Teton National Park; Willard E. Dilley, former chief park naturalist; and R. Alan Mebane, former assistant chief park naturalist.

Profs. Charles C. Bradley and John Montagne of Montana State University and Bruno J. Giletti of Brown University generously provided us with unpublished data. Cooperators during the years of background research were the late Dr. H. D. Thomas, State Geologist of Wyoming, and Dr. D. L. Blackstone, Jr., Chairman, Department of Geology, University of Wyoming.

Helpful suggestions were made by many of our colleagues with the U. S. Geological Survey; S. S. Oriel, in particular, gave unstintingly of his time and talents in the review and revision of an early version of the manuscript. A later version had the further benefit of critical review by three other people, all experienced in presenting various types of scientific data to public groups: John M. Good, former chief park naturalist of Yellowstone National Park; Bryan Harry, former assistant chief park naturalist of Grand Teton National Park; and Richard Klinck, “1965 National Teacher of the Year.”

We are indebted to Ann C. Christiansen, Geologic Map Editor, for advice and guidance on the illustrations and to R. C. Fuhrmann and his staff for preparation of many of the line drawings. Block diagrams and photo artwork were prepared by J. R. Stacy and R. A. Reilly. All photographs without specific credit lines are by the authors. From the beginning of the Teton field study to editing and proofing of the final manuscript, our wives, Jane M. Love and Linda H. Reed, have been enthusiastic and indispensable participants.

J. D. Love, a native of Wyoming, received his bachelor and master of arts degrees from the University of Wyoming and his doctor of philosophy degree from Yale University. His first field season in the Teton country, in 1933, was financed by the Geological Survey of Wyoming. After 12 years of geologic work ranging from New England to Utah and Michigan to Mississippi, he returned to the Teton region. Beginning in 1945, he spent parts or all of 20 field seasons in and near the Tetons. He compiled the first geologic map of Teton County. He is the senior author of the geologic map of Wyoming, and author or co-author of more than 70 other published maps and papers on the geology of Wyoming. In 1961, the University of Wyoming awarded him an honorary doctor of laws degree for his work on uranium deposits that “led to the development of the uranium industry in Wyoming.” The Wyoming Geological Association made him an honorary life member and gave him a special award for his geologic studies of the Teton area. He is a Fellow of the Geological Society of America and is active in various other geological organizations, as well as having been president of the Wyoming Chapters of Sigma Xi (scientific honorary) and Phi Beta Kappa (scholastic honorary) societies.

John C. Reed, Jr., joined the U.S. Geological Survey in 1953 after receiving his doctor of philosophy degree from the John Hopkins University. His principal geologic work before coming to the Teton region was in Alaska and in the southern Appalachians. Beginning in 1961, he spent five field seasons studying and mapping the Precambrian rocks in Grand Teton National Park, including all the high peaks in the Teton Range. He is a noted mountaineer, a Fellow of the Geological Society of America, a member of the Arctic Institute of North America, and the American Alpine Club. His numerous publications, in addition to those on the Tetons, describe the geology of mountainous areas in Alaska, the Appalachians, and Utah.

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