Building a Mountain Range

The precipitous Teton Range has perhaps the most complex geologic history in North America. Although the Tetons are ancient by any human scale, they are the youngest mountains in the Rockies, less than 10 million years old (versus 60 million years for the nearby Wind River Mountains).

The Tetons are a fault-block range, formed when the earth’s crust cracked along an angled fault. Forces within the earth have pushed the western side (the Tetons) up, while the eastern part (Jackson Hole) dropped down like a trapdoor.

Geologists believe the fault could slip up to 10 feet at a time, producing a violent earthquake. All this shifting has created one of the most dramatic and asymmetric mountain faces on earth.

Unlike in typical mountain ranges, the highest parts are not at the center of the range but along the eastern edge, where uplifting continues. The western slope, which drops gently into Idaho, is much less dramatic, although the views are still impressive.

This tilting-and-subsidence process is still going on today, pushed by the movement of a plume of magma beneath Yellowstone as the continental plate slides over the top. Because of this subsidence, the town of Wilson in Jackson Hole now lies 10 feet below the level of the nearby Snake River; only riverside dikes protect the town from flooding.

As the mountains rose along this fault, millennia of overlying deposits were stripped away by erosion, leaving three-million-year-old Precambrian rock jutting into the air above the more recent sedimentary deposits in the valley. Because of this shifting and erosion, sandstone deposits atop Mt. Moran match those 24,000 feet below Jackson Hole. Although the most recent major earthquake on the Teton Fault was at least 2,000 years ago, geologists are convinced that Jackson Hole could experience a major temblor at any time.

Rivers of Ice

In counterpoint to the uplifting actions that created the general outline of the Tetons, erosional forces have been wearing them down again. Glaciers, which are created when more snow falls than melts off, have proven to be one of the most important of these erosional processes.

After a period of several years and under the weight of additional snow, the accumulated snow crystals change into ice. Gravity pulls this ice slowly downhill, creating essentially a frozen river that grinds against whatever lies in the way, plucking loose rocks and soil and polishing hard bedrock. This debris moves slowly down the glacier as if on a conveyer belt, eventually reaching the glacier’s terminus.

When a glacier remains the same size for a long period, large piles of glacial debris accumulate at its end, creating what glaciologists call a terminal moraine. One of these created Jackson Lake, when a huge glacier dumped tons of rock at its snout. After the glacier melted back, this terminal moraine became a natural dam for the waters of the Snake River.

Similar mounds of glacial debris dammed the creeks that formed Jenny, Leigh, Bradley, Taggart, and Phelps Lakes within Grand Teton National Park.

The earth has experienced cyclical periods of glaciation for hundreds of thousands of years, probably because of changes in the earth’s orbit around the sun. During the colder parts of these cycles, glaciers appear and advance. The entire Yellowstone region has undergone a series of massive glaciations, the last of which is called the Pinedale Glaciation. It began about 70,000 years ago and had essentially disappeared by 15,000 years ago. At its peak, the Pinedale Glaciation covered all of Yellowstone and reached well into Jackson Hole.

Streams flowed from the ends of these glaciers, carrying along gravel, sand, silt, and clay. The cobbles and sands from these streams were dropped on the flat valley below, while the finer silts and clays continued downstream, leaving behind soils too rocky and nutrient-poor to support trees. Only sagebrush grows on this plain today, while the surrounding hills and mountain slopes (which were spared this rocky deposition) are covered with lodgepole and subalpine fir forests. Trees can also be found covering the silty terminal moraines that ring the lakes.

Other reminders of the glacial past are the “potholes” (more accurately termed “kettles”) that dot the plain south of Signal Mountain. These depressions were created when large blocks of ice were buried under glacial outwash. When the ice melted, it left a kettle-shaped pond surrounded by glacial debris. Only a dozen or so small glaciers remain in the Tetons; the largest is the 3,500-foot-long Teton Glacier, visible on the northeastern face of Grand Teton.

For a far more detailed picture of Teton geology, read Interpreting the Landscape: Recent and Ongoing Geology of Grand Teton and Yellowstone National Parks, by John Good and Kenneth Pierce (Grand Teton Association, www.grandtetonpark.org).