The glacial material that fills the valley is largely quartzite rock rounded by tumbling in running water into softball- to basketball-sized cobbles, supplemented by gravel, sand, and silt. This rock came from long-vanished mountains to the northwest. The depth of the valleys cobble material is estimated at perhaps about 2,000 feet. This cobble material has been washed by glacial runoff so often that it lacks the clay content that is essential for the ground to retain water. Melting snow and rain rapidly percolate through, so that only grasses and other plants adapted to arid conditions can thrive in the valley’s coarse-textured soil. This is why sagebrush dominates the valley floor, except where streams and ponds provide enough water for willow bushes, spruce, and cottonwood trees. Lodgepole pines grow atop the recent glacial moraines that contain sufficient nutrients and clay, such as the one surrounding Jenny Lake.
The Snake River meanders through the surprisingly flat valley called Jackson Hole.
Jackson Hole was settled around the turn of the century, initially by homesteaders.
John D. Rockefeller, Jr., donated more than 32,000 acres of valley land, which were added to Grand Teton National Park in 1950.
Local residents were divided over the park issue. Two local park proponents, Mardy and the late Olaus J. Murie, are shown at a 1949 annual council meeting of the Wilderness Society. The Muries advocated park enlargement.
Geologic forces have not always been subtle influences in the valley. Melting snow and heavy rains in June 1925 saturated a layer of clay sandwiched between sedimentary rock layers that form the north end of Sheep Mountain, near Kelly. An earthquake, probably, triggered an enormous landslide, and thousands of tons of debris raced down into the river, damming it and backing up a lake 5 miles long and 200 feet deep. Two years later the top 50 feet of the dam broke off and a wall of water rushed down through Kelly, leveling all buildings except the church and school. Six lives were lost, despite ample warning. The landslide scar on Sheep Mountain’s north end is more than 5,000 feet long and 2,500 feet wide. Geologists say that more of Sheep Mountain is perched to slide, given similar spring rainfall and an earthquake trigger.