Figure 5. The Lower Gros Ventre slide, air oblique view south. The top of the scar is 2,000 feet above the river; the slide is more than a mile long and one-half mile wide. It dammed the Gros Ventre River in the foreground, impounding a lake about 200 feet deep and 5 miles long. Gros Ventre Mountains are in the distance. Photo by P. E. Millward.
Now and then the range is deluged by summer cloudbursts. Water funnels down the maze of gullies on the mountainsides, quickly gathering volume and power, and plunges on to the talus slopes below, as if from gigantic hoses. The sudden onslaught of these torrents of water on the saturated unstable talus may trigger enormous rock and mudflows that carry vast quantities of material down into the canyons. During the summer of 1941 more than 100 of these flows occurred in the park.
Wherever water moves, it carries rock fragments varying in size from boulders to sand grains and on down to minute clay particles. Erosion (wearing away) by streams is conspicuous wherever the water is muddy, as it always is each spring in the Snake, Buffalo Fork, and Gros Ventre Rivers. Clear mountain streams likewise can erode. Although the volume of material moved and the amount of downcutting of the stream bottom may not seem great in a single stream, the cumulative effect of many streams in an area, year after year and century after century, is enormous. Streams not only transport rocks brought to them by gravitational movement but also continually widen and deepen their valleys, thereby increasing the volume of transported debris.
The effectiveness of streams as transporting agents in the Tetons is enhanced by steep gradients (slopes); these increase water velocity which in turn expands the capability of the streams to carry larger and larger rock fragments.
Glaciers scour and transport
Mountain landscapes shaped by frost action, gravitational transport, and stream erosion alone generally have rounded summits, smooth slopes, and V-shaped valleys. The jagged ridges, sharply pointed peaks, and deep U-shaped valleys of the Tetons show that glaciers have played an important role in their sculpture. The small present-day glaciers still cradled in shaded recesses among the higher peaks ([fig. 6]) are but miniature replicas of great ice streams that occupied the region during the Ice Age. Evidence both here and in other parts of the world confirms that glaciers were once far more extensive than they are today.
Glaciers form wherever more snow accumulates during the winter than is melted during the summer. Gradually the piles of snow solidify to form ice, which begins to flow under its own weight. Rocks that have fallen from the surrounding ridges or have been picked up from the underlying bedrock are incorporated in the moving ice mass and carried along. The ability of ice to transport huge volumes of rock is easily observed even in the small present-day glaciers in the Tetons, all of which carry abundant rock fragments both on and within the ice.
Figure 6. The Teton Glacier on the north side of Grand Teton, air oblique view west. Photo by A. S. Post, August 19, 1963.