Jackson Wyoming Geology, Rock Formations



Rocky Mountain landscapes typically offer picturesque views, but few are more impressive than the Teton skyline.

The Tetons are the youngest of the Rockies. The range was born some nine million years ago by intermittent, but violent, seismic activity. Because of the way the mountains formed, no foothills hide Jackson Hole's jagged peaks and broad canyons. At the base of the range, large lakes mirror the mountains on calm summer days, doubling their prominence.

What's in a name?

Prior to the 1820s, Native Americans referred to the mountains as "the Hoary Brothers." Later, they were known as "the Pilot Knobs." Around 1920, however, French Canadian trappers named them "Les Trois Tetons," or "The Three Breasts."

Today, the peaks are named the Grand, the Middle and the South Teton. The Grand Teton towers above the rest at 13,770'.

Before the Mountains there was a Sea

During the immense span of time before the mountains' rise, vast seas repeatedly advanced and retreated, leaving behind a thick blanket of sedimentary rock. Between 60 and 70 million years ago, ancestral mountains rose as a broad, northwest-ending arch and the last seas retreated eastward. East of this arch became the site of enormous sheets of gravel interspersed with thick volcanic ash, lava and freshwater lake sediments. Enormous tensional faults fractured these formations and today's Teton Range started rising. Broken sedimentary layers of ancient sandstone, shale, dolomite and limestone still cap each end and the backside of the range.

The Glaciers Went to Work

Mountain glaciers of the last major ice age shaped the Tetons more than any erosional force. At upper elevations, where most of the snow accumulated, the heads of huge, slow-moving glaciers scooped out depressions. At they crept, frost wedged in the rock cracks, increasing their quarrying. Sheer cirque walls, rugged ridges and jagged peaks reflect the dynamic carving of these great masses of moving ice. Rocks of all sizes, falling onto and picked up by these glaciers, furthered their grinding power.

The flanks of the range display scoured canyons that dive toward the valley. Upon leaving these canyons, the larger glaciers spread onto the valley floor, melting at a speed equal to their flow. Tons of unsorted rock, transported and dumped by these glaciers, formed natural dams forming the lakes called Leigh, Jenny, Taggart, Bradley and Phelps.

Similarly, a lobe of the extensive Yellowstone snowcap extended south as a broad glacier which deposited rock as morainic ridges, damming meltwaters and creating Jackson Lake. South of Jackson Lake, torrential meltwaters washed away moisture-holding clay that previous glaciers had spread down the valley.

What we See Today

Sagebrush identifies these washed areas of less soil where rain and melting snow percolate rapidly. Lodgepole pines and sub-alpine firs stand on morainal ridges that contain more fine-grained soil. On alpine slopes, trees and flowers struggle in fragile soil, where weather limits growth. At all elevations, geology and water determine vegetation, which in turn control the variety and distribution of wildlife.

The Snake River originates in the wilderness near the south boundary of Yellowstone before it meanders into Jackson Lake. From its exit through the dam, the Snake runs swiftly eastward along a path gouged by massive glaciers. From Jackson Lake, the Snake winds in braided channels for 27 miles while in Grand Teton National Park. At Moran Junction, the river turns abruptly southwest and flows down the broadened bed of the ancestral Snake, which diagonally bisects Jackson Hole.

Slower moving water insures essential habitat for numerous aquatic animals, including North America's largest waterfowl, the trumpeter swan, and plants. Beavers, otters and trout make their way past moose, who feed on primarily on plants. Native Snake River cutthroat trout, a distinct subspecies of the cutthroat, depend on the park's natural aquatic system for survival. Trout consume aquatic insects, invertebrates and small fish. Bears, eagles, ospreys and otters in turn feed on the trout.

Low-growing willows thrive among tall cottonwood, spruce and aspens trees. Riverbanks show an ever-changing checkerboard of plant communities because of flooding and channel shifting. Were it not for these continual natural changes, spruce would dominate along the banks, crowding out the willows and cottonwoods favored by moose and beavers.

The summer Snake teems with diving, wading and woodland birds. During darkness, owls and other predators hunt in this riparian ecosystem. Born of wilderness snow pack, the Snake River swells with meltwater. Trappers and settlers called it the Mad River, more than a challenge to cross during the spring. Now, the river erodes with only a fraction of its original force but the new challenge is to protect this powerful, life-supporting river.

Share Your Thoughts