The Missouri River Valley as far as this location was well known to French and British fur trappers, who for decades traveled down the Big Sioux River and the lower Missouri toward St. Louis, but the intimidating presence of the Yankton and Teton Sioux caused the fur trade to dwindle upriver from the mouth of the Big Sioux. President Jefferson ordered the Corps of Discovery to seek out Indian tribes and record their languages, populations, religions, customs, food, clothing, and especially their willingness to trade with the United States. With one desertion, one fatality, and four court-martials before even one fourth of the journey was complete, Meriwether Lewis and William Clark faced their first meeting with representatives of the Sioux Nations with anxiety. At the Yankton Sioux encampment, the Corps of Discovery first saw the conical tipis used by the nomadic Northern Plains Indians. In this unfamiliar country, strange stories also circulated, including a legend about a lone prairie hill twenty miles upriver, now called Spirit Mound, said to be inhabited by eighteen-inch-high "Deavels" armed with sharp arrows that could kill at a great distance, a place Indian tribes were said to avoid. On August 27, 1804, William Clark wrote that above the bluff the prairie was set on fire to let the Sioux see that the party was on the river and to signal for them to come to it.