In 1855, geologist Ferdinand V. Hayden explored the upper Missouri River for the United States Geologic Service in an area that was then the hunting grounds of the Lakota, Blackfeet, Atsina, and River Crow Indians. Though a lone white man in Indian Country was often fair game to the tribes, Hayden's passion for rocks and fossils earned him the name "He Who Picks Up Stones While Running" and a reputation for madness, and the Indians left him alone. He explored what later became known as the Judith River Formation, a large area of sedimentary materials deposited in the lowland area bordering the Colorado Sea during the Late Cretaceous Period 78 to 74 million years ago. Near the mouth of the Judith River, Hayden collected many fossilized teeth, bones, and shells from the formation and took them to anatomy professor Joseph Leidy in Philadelphia for identification, and Leidy classified the teeth as belonging to a new species of hadrosaur. The teeth and bones were the first dinosaur fossils found in North America, and Hayden's discovery drew paleontologists to Montana in search of dinosaur fossils, making the Judith River Formation a significant paleontological destination.