On June 24, Prince Maximilian and his expedition members arrived at Fort Union. Bodmer made several studies of an Assiniboine summer camp, including portraits of prominent tribal members. On their walk on the prairie they saw a sacred rock formation described by Maximilian in his journal as "a kind of medicine device. Large rocks with an oblong shape stand upright...and on top there is a bison skull. Mr. Bodmer sketched one of these... They build these pyramids to attract the buffalo."

The bison was the main food source for Northern Plains tribes such as the Assiniboine, providing nearly all that they required in terms of nutrition, clothing, and shelter. The Assiniboine are also known as Nakota (or Nakoda). Their language is part of the Siouan language family. Originally they inhabited the region of the headwaters of the Mississippi River in today's Minnesota and were members of the Yanktonai Sioux, but separated after a battle sometime before 1640 and developed into a distinct tribe. Like the Dakota, they were gradually forced by the Ojibwe to move west on the plains.