 In 1848, during the American occupation of Mexico after the Mexican-American War, Raynolds and other U.S. Army personnel were the first confirmed to have reached the summit Pico de Orizaba, the tallest mountain in Mexico, and inadvertently set what may have been a 50-year American alpine altitude record. In 1859, Raynolds was placed in charge of the first government-sponsored expedition to venture into the upper Yellowstone region that was later to become Yellowstone National Park. Heavy winter snowpack in the Absaroka Range of Wyoming prevented the expedition from reaching the Yellowstone Plateau, forcing them to divert to the south and cross Union Pass at the northern end of the Wind River Range. After negotiating the pass the expedition entered Jackson Hole and surveyed the Teton Range, now within Grand Teton National Park.
