Once the United States had acquired new Western territories, the government desired to learn about the geography of the region and, even more important, to determine the value of its natural resources. To this end, the government sponsored the systematic exploration of the Trans-Mississippi West. Representative of these efforts were the expeditions of Lewis and Clark, Zebulon Pike, Stephen H. Long, and John C. Fremont. Lewis and Clark's Corps of Discovery made its way upstream to the headwaters of the Missouri River, then crossed the Rocky Mountains and floated down the Columbia River to the Pacific Ocean. In 1806, the same year that the Corps of Discovery returned from exploring the Northwest, Zebulon Pike departed on an expedition to learn more about the Southwest. Major Stephen H. Long ventured west in 1820. A map compiled from information gathered on this expedition labeled the area lying between the Mississippi River and the Rocky Mountains the "Great American Desert." John C. Fremont made numerous trips to the West during his lifetime and, among other things, is credited with documenting and naming the Great Basin. Each of these expeditions helped to record details of the geographic and cultural diversity of the American West. Perhaps more important, however, published accounts of their journeys advertised the vast natural resources of the region and served to attract the armies of settlers that would follow in their footsteps.