The first known writings about Great Sand Dunes appear in Zebulon Pike's journals of 1807. As Lewis and Clark's expedition was returning east, U.S. Army Lt. Pike was commissioned to explore as far west as the Arkansas and Red Rivers. By the end of November 1806, Pike and his men had reached the site of today's Pueblo, Colorado. Still pushing southwest, and confused about the location of the Arkansas River, Pike crossed the Sangre de Cristos just above the Great Sand Dunes.