J. D. Love

“ 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. ”

“ 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. ”

“ Astronomers deal with distances so great that they are almost beyond understanding; nuclear physicists study objects so small that we can hardly imagine them. Similarly, the geologist is concerned with spans of time so immense that they are scarcely comprehensible. Geology is a science of time as well as rocks, and in our geologic story of the Teton region we must refer frequently to the geologic time scale, the yardstick by which we measure the vast reaches of time in earth history. ”

“ A formation is composed of rock layers which, because of their similar physical characteristics, can be distinguished from overlying and underlying layers. They must be thick enough to be shown on a geologic map. Table 2 lists the various Paleozoic formations present in and adjacent to Grand Teton National Park and gives their thicknesses and characteristics. These sedimentary rocks are of special interest, for they not only record an important chapter of geologic history but elsewhere in the region they contain petroleum and other mineral deposits. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ The last Cretaceous sea still lingered in central Wyoming.
One bit of evidence (there are others) of the first Laramide mountain building west of the Tetons is a tremendous deposit of quartzite boulder debris (several hundred cubic miles in volume) derived from the Targhee uplift (fig. 42) . Nowhere is the uplift now exposed, but from the size, composition, and distribution of rock fragments that came from it, we know that it was north and west of the northern end of the present-day Teton Range. ”

“ 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. ”

“ In all of Precambrian time, which encompasses more than 85 percent of the history of the earth (31 of the 36 inches of our yardstick) , only two events are dated in the Teton Range: the intrusion of granite and pegmatite about 2.5 billion years ago, and the emplacement of the black dikes about 1.3 billion years ago. These dates are indicated by heavy arrows on the time scale (fig. 30) . The ancient gneisses and schists were formed sometime before 2.5 billion years ago, and probably are no older than 3.5 billion years, the age of the oldest rocks dated anywhere in the world. ”

“ 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. ”

“ No other region in the United States contains a thicker or more complete nonmarine Tertiary record; many areas have little or none. The accumulation in Jackson Hole reflects active uplifts of nearby mountains that supplied abundant rock debris, concurrent 89 sinking of nearby basins in which the sediments could be preserved, and proximity to the great Yellowstone-Absaroka volcanic area, one of the most active continental volcanic fields in the United States. The volume and composition of the Tertiary strata are, therefore, clear evidence of crustal and subcrustal instability. ”

“ An intrusive igneous rock must be younger than the rocks that enclosed it at the time it solidified. It may contain pieces of the enclosing rocks that broke off the walls and fell into the liquid. Pebbles of the igneous rock that are incorporated in nearby sedimentary layers indicate that the sediments must be somewhat younger.
All of these criteria tell us only that one rock is older or younger than another. They tell us little about the absolute age of the rocks or about how much older one is than the other. ”

“ 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. ”

“ 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. ”

“ 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 21 get a closeup view of the Teton Glacier above Amphitheater Lake, or explore the Schoolroom Glacier, the tiny ice body below Hurricane Pass. ”

“ Bentonite swells when wet and causes many landslides along access roads into Jackson Hole (fig. 17) .
The Cretaceous rocks in the Teton region are part of an enormous east-thinning wedge that here is nearly 2 miles thick. Most of the debris was derived from slowly rising mountains to the west.
Cretaceous sedimentary rocks are much more than of just scientific interest; they contain mineral deposits important to the economy of Wyoming and of the nation. 82 Wyoming leads the States in production of bentonite, all of it from Cretaceous rocks. ”

“ 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. ”

“ 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 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. ”

“ 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. ”

“ Fossils provide important clues to the ages of the rocks in which they are found. The slow evolution of living things through geologic time can be traced by a systematic study of fossils. The fossils are then used to determine the relative ages of the rocks that contain them and to establish a geologic time scale that can be applied to fossil-bearing rocks throughout the world. Figure 18 shows the major subdivisions of the last 600 million years of geologic time and some forms of life that dominated the scene during each of these intervals. ”

“ 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. ”

“ Small mammals had already existed, though quite inconspicuously, in Wyoming for about 90 million years before Paleocene time. Then about 65 million years ago their proliferation began as a result of the extinction of dinosaurs, obliteration of seaways that were barriers to distribution, and the development of new and varied types of environment. These new environments included savannah plains, low hills and high mountains, freshwater lakes and swamps, and extensive river systems. The mammals increased in size and, for the first time, became abundant in numbers of both species and individuals. ”

“ 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. ”