 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 . 
