On August 25, 1804, several members of the Lewis and Clark Expedition left the river to visit a high hill believed to be a place of "Deavels" with remarkable large heads that killed anyone who tried to approach it. They apparently found no evidence of little spirits, but from the hilltop they saw large herds of bison and elk on the surrounding plains and large flocks of birds feeding on insects blown up the slopes. The weather was hot and humid, and the nine-mile walk proved too much for Seaman, Lewis's Newfoundland dog, who had to be taken back to the campsite near the Vermillion River before they reached the mound. On his map, William Clark labeled it the "Hill of little Devils," and it is known today as Spirit Mound.