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Introduction
Grand Teton National Park is the destination of more than 3 million vistors a year who are searching for a vacation spot that provides scenic splendor, a varied ecosystem with abundant wildlife, and excellent recreational opportunities. The Teton Range is the focal point of the park, rising more than 7,000 feet above the floor of Jackson's Hole which borders the mountains on the east (Fig. 1). The steep eastern front of the range is unique in the Rocky Mountains and is the result of erosion of the Precambrian crystalline rocks along the steeply dipping Teton normal fault. The summit of the Grand Teton stands 13,770 feet above sea level and is the second highest summit in the state of Wyoming.
Figure 1. Grand Teton Range and adjacent Jackson's Hole, Wyoming, U.S.A. - aerial view from the north.
Rocks of the Teton Range
The bedrock of the Teton Range is primarily ancient, hard crystalline rock including darker-colored metamorphic rocks (such as the Mt. Moran Gneiss) and lighter-colored igneous rocks (such as the Mt. Owen Granite) (Fig. 2). These rocks were formed during the Archean portion of Precambrian time, prior to 2.5 billion years ago, and were subsequenty intruded by later Precambrian (Proterozoic) dark-colored diabase dikes around 1.1-1.3 billion years ago. The best exposure of these dikes is on the eastern face of Mt. Moran where the "Black Dike" extends from the summit of the peak downward to Leigh Lake on the floor of the valley. The geologic map and cross section in Figure 2 portray these Precambrian crystalline rocks in purple.
Figure 2. Geologic Map and Cross-Section, Grand Teton National Park and Vicinity, Northwestern Wyoming, U.S.A.
The Precambrian crystalline rocks are overlain by a blanket of Paleozoic sedimentary rocks (Fig. 2). These rocks are exposed on the western slope of the Teton Range and form "The Wall", a great cliff of stratified rocks which may be seen from the floor of Jackson's Hole as one looks westward up the great U-shaped valleys which transect the range. Most of the sedimentary rocks were deposited as layers of sand, mud and lime in great inland seas whose shorelines fluctuated in position across this region from the Cambrian period at the beginning of the Paleozoic Era (around 550 million years ago) through the Mississippian Period (around 320 million years ago). The Paleozoic sedimentary rocks are colored light blue on the map and cross section in Figure 2.
Younger Paleozoic and Mesozoic rocks were deposited in the region but were removed by erosion from the western slope of the Tetons following uplift along the Teton Fault. These younger rocks can be seen in the Gros Ventre Mountains which bound Jackson's Hole on the east. A drive along the Gros Ventre River to the Gros Ventre Slide takes visitors past exposures of these younger and brightly-colored sedimentary rocks, some of which were deposited in terrestrial river environments during the Mesozoic Era, as little as 150 to 100 million years ago.
The boundary between the Precambrian crystalline rocks and the overlying sedimentary rocks is a major unconformity (or gap) in the rock record. This unconformity can be seen by hikers who traverse the Skyline Trail on the west flank of the range, but it is also visible from the floor of Jackson's Hole on the summit of Mt. Moran (Fig. 3). The flat top of Mt. Moran coincides with this surface of erosion between the Precambrian and younger rocks, and on the summit, a small remnant of the oldest Paleozoic rocks (the Flathead sandstone) is preserved as a light-brown-colored outcrop on the southern half of the peak. The black dike, which traverses the summit of Mt. Moran, does not cut across this sandstone, indicating that the sandstone is younger in age than the dike.
Figure 3. Mt. Moran, in an aerial view from the east. The dark-colored exposures of the Mt. Moran gness dominate the peak, and are cut by the vertical black diabase dike which extends upward to the summit. The southern (left) side of the summit is capped by a 50-foot thick exposure of the Early Paleozoic (Cambrian) Flathead Sandstone.
Geologic Structure
The Tetons are among the youngest ranges in the Rocky Mountains. Although there was structural uplift of an ancestral Teton Range during) Paleocene time (about 50 million years ago), the modern range is the product of uplift along the Teton Fault which began 9 million years ago. The Teton Fault, which parallels the eastern front of the Teton Range from north to south, is an eastward-dipping fracture in the earth's crust along which movement has occurred (Fig.2). The block on the western side of the fault (called the footwall because it lies beneath the inclined surface of the fault) was uplifted, while the block on the eastern side of the fault (called the hanging wall because it lies above the inclined surface of the fault) moved downward. This sort of motion defines a type of structure called a normal fault. Such faults are the result of extension or stretching in the earth's crust.The amount of uplift, or structural relief, can be determined by comparing the elevation of a geologically recognizable horizon on opposite sides of the fault. The Precambrian-Cambrian unconformity atop Mt. Moran is at an elevation of about 12,500 feet, while the same boundary is buried far beneath the surface of Jackson's Hole, at an elevation of 22,500 feet below sea level! The amount of structural relief is 35,000 feet, which indicates a rate of uplift of 4.5 inches per hunred years!
Glaciation
The details of the Teton landscape were developed on the uplifted Teton block by alpine glaciation of preexisting deeply incised stream valleys (Fig. 4).
Figure 4. Aerial view of the Teton Range from the north. The Grand Teton, together with Mt. Owen on the right and Mt. Teewinot on the left foreground, form a group of summits often referred to as the "Cathedral Group". The details of this topography have been molded by glacial erosion during the Pleistocene Epoch (the past 2 million years).
Several cycles of climatic cooling followed by warming during the past 2 million years resulted in the advance and retreat of alpine glaciers in the Teton Range. Modern glaciers in the park are but a vestige of the former extent of the ice. Glaciers such as Falling Ice Glacier on Mt. Moran (Figure 3) and the Teton Glacier (Figure 6) are restricted to shaded portions of cirques at the head of alpine valleys, and are not remnants of the Pleistocene ice, but rather reformed during the so-called "Little Ice Age as recently as 5,000 years ago.
Alpine glaciers formed at the heads of stream valleys, and erosion by the ice created bowl-shaped ampitheaters called cirques (Fig. 5). These cirque glaciers eroded the bedrock in headward and lateral directions, and as the volume of ice expanded, the toes of the glaciers advanced down the valleys, sculpturing the v-shaped stream valleys into u-shaped glacial forms called glacial troughs. The great canyons (e.g., Cascade, Paintbursh, Garnet, and Death Canyons) that facilitate access by hikers to the interior of the Teton Range are excellent examples of u-shaped valleys or glacial troughs.
Figure 5. The bowl-shaped amphitheater at the head of Avalanche Canyon is an excellent example of a cirque. The glacier which carved this cirque has long-since melted, and a cirque lake or tarn now occupies the bedrock depression.
Cirque glaciers eroding headward on three or more sides of a divide created pyramid-shaped peaks called glacial horns. The summit of the Grand Teton is an excellent example of a glacial horn (Fig. 6). Erosion on two opposite sides of a divide created knife-edged ridges called aretes, and when glaciers eroding headward on opposite sides of a ridge coalesced, passes through the ridge called cols were created. The Lower Saddle between the Grand and Middle Tetons is an excellent example of a col.
Figure 6. The Grand Teton is an excellent example of a glacial horn. The Teton Glacier is nestled beneath the sheer 3,000 foot-high North Face of the Grand and occupies a cirque at the head of the valley of Glacier Gulch. Headward and lateral erosion by the glacier results in expansion of the cirque. When three cirques on opposite sides of the mountain block are undergoing erosion, a pyramid-shaped peak called a horn is created.
Glaciers in the Teton Range advanced down glacial troughs and spilled onto the floor of Jackson's Hole. During the two earliest identifiable glacial episodes (referred to as the Pre-Bull Lake and Bull Lake episodes), the alpine glaciers merged with a great piedmont ice sheet which advanced southward and southwestward into Jackson's Hole from the Yellowstone and Absaroka centers of glaciation. At the maximum advance of the ice, nearly all of Jackson's Hole was engulfed, and the peaks of the Teton Range rose above the glaciers as islands in a sea. Even Signal Mountain and Blacktail Butte, near Moose, were buried by the piedmont glaciers.
During retreat of the piedmont glaciers, poorly sorted and unstratified material called till was deposited during standstills at the margins of the ice. The till accumulated in ridges called moraines. Today, drainage in the Teton Range and Jackson's Hole is often impounded behind these moraines, forming jewel-like lakes such as Phelps, Bradley, Taggert, Jenny, String, Leigh and Jackson Lakes.
Beyond the moraines, braided streams draining the retreating ice spread deposits of sorted and stratified gravel called outwash across the floor of Jackson's Hole. The outwash plain is flat compared to the morainal topography, except for occasional kettles and terraces (Figure 7).
Figure 7. Jackson's Hole looking south from Signal Mountain. The nearly flat sage-brush covered terrane is a glacial outwash plain, broken only by kettle holes (shallow depressions with small lakes, marshes, or growths of evergreens) and stream terraces.
Kettles are depressions in the outwash plain created when blocks of ice were detached from the melting glaciers. Terraces are step-like flats along drainage courses, created during alternating episodes of stream erosion (during glacial retreat) and stream eposition (during glacial advance). Well-developed terraces are especially visible along the Snake River (Figure 8) and along the former course of Cottonwood Creek just north of Moose (Figure 9).
Figure 8. Terraces along the Snake River, viewed from the Snake River overlook along U.S. Highway 26. The oldest terrace stands at the highest level, and successively younger terraces are at successively lower levels.
Figure 9. Low-relief terraces along an abandoned course of Cottonwood Creek, north of Moose along the park road. The terrace levels represent episodic downcutting by the ancestral Cottonwood Creek during episodes of glacial retreat. During glacial advances, the stream deposited sands and gravels in its channel and floodplain.
Figure 10. Jackson's Hole viewed from the summit of Disappointment Peak.
Outwash plain and moraines are easily distinguished from one another on the basis of vegetation. Evergreen forests grow on the till of the moraines because the unsorted sediment is less permeable and can support a relatively high water table. Outwash is very porous and permeable because there is no fine-grained sediment to plug the pore spaces. Hence, in these areas the water table is at greater depth. Here, mainly sagebrush grows, because it has a long tap root that can reach water at depths as great as 40 feet.
Credit by : http://www.winona.edu/geology/travels/tetons/travel.html
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