Glaciation


The sculpturing influence of ice has provided a final spectacular touch to a scene that already boasted mountains rising sharply from a broad, flat valley. About 150,000 years ago this region experienced a slight cooling that allowed an accumulation of more and more snow each year. Eventually glaciers (masses of ice) began to flow from higher elevations. Over two thousand feet thick in places, the ice sheet flowed from north to south through Jackson Hole. The glacier finally halted south of the town of Jackson and melted about 100,000 years ago.

About 60,000 years ago the glaciers returned, first surging from the east down the Buffalo Valley, stopping near the Snake River Overlook. The most recent ice advance flowed from the Yellowstone Plateau south down the Snake River drainage and east from the canyons in the Teton Range, about 20,000 years ago. The Yellowstone ice mass gouged out the depression occupied today by Jackson Lake. Smaller glaciers flowing eastward down the Teton Range broadened the V-shaped stream canyons into U-shaped canyons, typical evidence of glaciation. Ice flowed from the canyons into Jackson Hole, then melted to form the basins that small lakes occupy today. Glacial lakes include: Phelps, Taggart, Bradley, Jenny, String, and Leigh. As glaciers flowed down the canyons, rocks and ice smoothed and polished canyon floors and walls. Look for glacial polishing today in Cascade and other canyons. Other telltale signs of glaciation include cirque lakes high up in the canyons, such as Lake Solitude in the north fork of Cascade Canyon.

The peaks of the Teton Range became more jagged from frost-wedging, where water freezing in the rocks exerted a prying force, eventually chiseling the rocks free, leaving the sharp ridges and pinnacles seen today.

Although the last great ice masses melted about 15,000 years ago, a dozen re-established glaciers still exist in the Teton Range. Mt. Moran exhibits five glaciers: Triple Glaciers on the north face, prominent Skillet Glacier on the east face and Falling Ice Glacier on the southeast face. Teton Glacier lies in the shadow of the Grand Teton. One way to view a glacier up close involves a ten-mile hike (twenty miles round trip) up the south fork of Cascade Canyon to Schoolroom Glacier. It demonstrates all the features of a classic glacier.

Moraines (deposits of glacially-carried debris) accumulated at the terminus of each ice surge. Because moraines contain a jumble of unsorted rocks and soil that retains water and minerals, glacial debris today supports dense lodgepole pine forests. To locate moraines, look for large stands of pines on ridges projecting above the valley floor, such as Timbered Island and Burned Ridge. Glacial moraines also surround the lakes at the base of the peaks. Where glacial meltwater washed away most of the soil, the cobbles and poor, thin soil left behind cannot retain moisture or nutrients. Sagebrush, certain wildflowers and grasses can tolerate such desert-like growing conditions. Thus the geologic history of a region determines the vegetation and ultimately the wildlife, too.

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