Paleo-Indians hunted Ice Age mammoths and ancestral bison in what is now Roxborough, Colorado. In 1820, Stephen A. Long's expedition traveled west across the plains into America's western frontier and camped along the South Platte River where water rushed into the Platte Valley from Rocky Mountain peaks. They discovered an area described by their geologist as "a range of naked and almost perpendicular rocks . . . resembling a vast wall, parallel to the base of the mountain . . . the whole ...