Black and grizzly bears are roaming throughout the park--near roads, trails and in backcountry areas. Hikers and backcountry users are advised to travel in groups of three or more, make noise and carry bear spray. Visitors must stay 100 yards from bears.
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Learn about the geologic forces that continue to change the dramatic scenery of Jackson Hole. Find out why the Teton Range rises abruptly from the valley and how the Teton fault and glacial ice shape the landscape.
For many the first glimpse of the Teton Range inspires awe and wonder. What is special about these mountains? Is it their jagged profile in the evening sky? Is it the way the mountains rise abruptly from the valley floor? Fritiof Fryxell, Grand Teton National Park’s first naturalist said, “True appreciation of landscape comes only when one is alive to both its beauty and its meaning.” Understand the forces that shape this landscape, may help you appreciate it. Some of these forces began and ended long ago while some are active today.
The juxtaposition of old and young provides the essential elements for this landscape. The rocky outcrops of metamorphic gneiss and igneous granite found in the highest peaks formed roughly 2½ to 3 billion years ago, some of the oldest rocks in North America. The Teton Range, however, began rising only 10 million years ago, one of the youngest ranges in the world. If softer sedimentary rocks formed the core of the mountains or if the mountains were older, the skyline would be far less dramatic.
Looking southwest, layers of sedimentary rocks drape over the ancient bedrock on the southern end of the Teton Range. Seawater flooded this region around 500 million years ago, laying down sandy beaches, muddy marshes, and coral reefs that formed sandstone, shale, and limestone. The gray limestone cliffs you see above the ski area remind us of these inland seas.
What’s missing from this picture? No foothills block your view of the high peaks. Every few thousand years, movement on the Teton fault generates a large earthquake that fractures the Earth’s surface, forming two large crustal blocks. Each time an earthquake occurs, the mountain block swings skyward and the valley block hinges downward. Mountains formed this way do not produce foothills as they rise abruptly from the valley floor.
Beginning 2 million years ago as motion on the Teton fault continued to lift the mountains, ice age glaciers carved the landscape. Massive sheets of ice covered the land and flowed down from the mountains carving U-shaped canyons and gouging out beautiful lakes.
Imagine the power of the Earth to lift these mountains skyward and carve canyons and lakes. Take your time to find the meaning behind the landscape of this very special place.
I suppose I’m a bit biased. As a geologist, I think Mount Moran is the most incredible mountain in Grand Teton National Park. Formed from a massive block of metamorphic gneiss, cut by dikes of igneous granite and diabase, capped by sedimentary sandstone, and flanked by glaciers, this peak dominates the park’s northern skyline.
Like the other high peaks in the Teton Range, most of Mount Moran is metamorphic gneiss. This rock formed when two continents collided almost 3 billion years ago, much like India and Asia are colliding to form the Himalayan Range today. With burial, heat and pressure transformed, or metamorphosed, sediments and volcanic debris into layered gneiss. White layers, rich in quartz and feldspar, alternating with black layers rich in biotite mica and hornblende, produce this zebra-striped rock.
Look closely to see light-colored stripes slicing across the gneiss. These are veins or dikes of igneous granite. As these ancient continents collided, part of the crust melted, and magma squeezed into cracks crystallizing around 2½ billion years ago. Even though the minerals in granite are the same as gneiss, granite is speckled because of its molten origin rather than layered.
It’s hard to miss the 200-foot wide vertical Black Dike that transects the face of Mount Moran. This igneous dike is made of diabase. Nearly a billion years ago, iron-rich magma squeezed into cracks and cooled. Today, the dike protrudes from the face of the mountain because diabase resists erosion more than the surrounding gneiss.
If you stood on the summit and examined the small patch of rock at your feet, you would realize that it is different from the surrounding rocks. This rusty-tan rock is sedimentary sandstone deposited as a beach 500 million years ago. As the range began to rise roughly 10 million years ago, erosion stripped this sandstone from the other high peaks leaving this odd patch behind.
By late-summer, winter’s snow has melted, and the white patches you see on Mount Moran are glaciers. There are five small glaciers on the peak. These are not remnants of the Pleistocene Ice Age, but formed during a cold period called the Little Ice Age that ended in the 1850s. Currently these glaciers are shrinking, so appreciate them now, because they may soon be gone.
As you travel through Grand Teton National Park, remember what you have learned about Mount Moran and contemplate the other peaks. Look for glaciers, layers of sedimentary rock, and dikes of diabase and granite cutting the faces of these majestic mountains.
Learn about the various glacial periods that swept over the landscape carving the high peaks, carving U-shaped canyons and gouging out depressions to form valley lakes.
As you stand at the Teton Glacier Turnout, turn around slowly. Would you believe that you can see evidence of three different glaciers from here? Two of these glacial advances happened during the Pleistocene Ice Age that ended thousands of years ago. The last glacial advance happened during a cool period called the Little Ice Age that ended in the 1850s.
The Pleistocene Ice Age began a little less than 2 million years ago when glaciers scoured North America numerous times. As ice sheets flowed south from Canada, ice sheets also flowed off of the high plateau of Yellowstone and the surrounding mountains. Evidence left by these glaciers includes sharp mountain ridges called arêtes, U-shaped canyons, ridges of glacial debris called moraines, and cobble-covered plains washed by melting glaciers.
The last two glacial advances during the Pleistocene draw their names from the nearby Wind River Range. The Bull Lake period ended roughly 120 thousand years ago, when ice blanketed the entire valley covering this spot with about 2,000 feet of ice. Look to the east, the ridge covered by lodgepole pines is a glacial moraine from the Bull Lake period. The Pinedale period ended 14,000 years ago, when alpine glaciers carved depressions along the base of the mountains now filled with lakes such as Jenny and Jackson. As you look to the southwest, the low hills covered in lodgepole pines and aspen trees are glacial moraines from a Pinedale glacier that created a natural dam around Taggart Lake.
During historic times, quite a number of small alpine glaciers formed during a cool period called the Little Ice Age that lasted from 1400 to 1850. Today, less than a dozen of these glaciers remain in the Teton Range. If you look high on the north flank of the Grant Teton, you can see the crevasses of the Teton Glacier. Crevasses or cracks in the glacier indicate that the glacier is flowing downhill due to gravity. Even though these glaciers are flowing downhill, they are melting and receding faster than they advance. Imagine the Teton Range once the glaciers are gone.
As you travel through Grand Teton National Park, imagine how glaciers have shaped this landscape. Look for moraines covered by lodgepole pines around Jenny Lake, for U-shaped canyons such as Cascade Canyon, and admire the jagged features of the Teton Range while appreciating the changing landscape.
Motion on the Teton fault began about 10 million years ago lifting the mountains skyward and hinging the valley down. It is only a matter of time before another earthquake lifts the mountains and drops the valley floor.
Look toward the Teton Range in the late afternoon, and you may notice an odd shadow low on the slope. This shadow is cast by the steep slope or scarp formed by the Teton fault. A scarp is a steep transition between two land surfaces and may form by water cutting into the Earth’s surface or an earthquake breaking the Earth’s surface.
Earthquakes formed this particular scarp by breaking through sediments deposited during the last ice age. The fault started moving roughly 10 million years ago and continues to this day. Regional stretching periodically builds up on the Teton fault until the fault reaches its breaking point. As the ground breaks, two crustal blocks slip past one another generating an earthquake and shaking the ground. The mountain block swings skyward as the valley block hinges downward, building this rugged landscape one-step at a time.
The vertical displacement of this scarp is about 100 feet generated by earthquakes during the last 14,000 years. Geoscientists estimate the Teton fault may produce earthquakes up to a magnitude 7.5 that would each break the ground by about ten feet. The scarp you are viewing probably reflects more than half a dozen major earthquakes, or one earthquake about every 2,000 years.
Research shows that the last major earthquake occurred nearly 5,000 years ago, more than twice as long as geoscientists think earthquakes have happened in the past. Imagine what might happen if an earthquake occurred now. How would the landscape change? When will the next one happen? Only time will tell, but for now we can enjoy the rugged scenery generated by these very powerful forces. As Ansel Adams said in 1950, “The grand lift of the Tetons is more than a mechanistic fold and faulting of the earth’s crust; it becomes a primal gesture of the earth beneath a greater sky.”
Did You Know?
Did you know that pikas harvest grasses so they can survive the long cold winter? These small members of the rabbit family do not hibernate, but instead store their harvest as “haystacks” under rocks in the alpine environment.
