With the Louisiana Purchase treaty signed, President Thomas Jefferson wanted to know what he had bought, so he sent the Lewis and Clark Expedition overland to the Pacific in 1803 to find out. On the return trip John Colter left the expedition along the Yellowstone River to stay in the West and join a trapping venture. He is considered the first white person to discover what is now Jackson Hole. Colter supposedly wandered through this high, mountain-encircled valley—trappers called such valleys holes—in the winter of 1807-1808. Colter was soon followed by other trappers, and 40 years later the trappers were followed by homesteaders. Several homesteaders became dude ranchers, and their dudes were followed by vacationers, who now number nearly three million each year. The flatness of Jackson Hole comes as a surprise, considering that the Teton Range was formed by a fault-block process. You would expect a deep valley, but it has been filled repeatedly by rock debris transported by glaciers and their meltwaters. The Snake River does little cutting into the valley floor today. The flat areas above the river, called benches, were carved out when the river had the torrential force of glacial meltwater. The river’s north-south flow shows that the valley slopes southward. The valley also tilts westward, toward the fault that gave rise to the Teton Range. For reasons not fully known, the valley has sunk more than the mountains have risen. We know this because a sedimentary cap of rock atop Mount Moran—nearly 6,000 feet above the valley floor—was once connected to the same rock layer that now lies an estimated 24,000 feet below the valley surface. 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. [Illustration: _The Snake River meanders through the surprisingly flat valley called Jackson Hole._] [Illustration: _Jackson Hole was settled around the turn of the century, initially by homesteaders._] [Illustration: _John D. Rockefeller, Jr., donated more than 32,000 acres of valley land, which were added to Grand Teton National Park in 1950._]