Lieutenant Zebulon M. Pike led a U.S. Army expedition to explore the United States' new southwest boundary with New Spain, accompanied by Doctor John H. Robinson, Sergeant William Meek, Corporal Jeremiah Jackson, and privates John Brown, Jacob Carter, William Gordon, Theodore Miller, John Mountjoy, Alexander Roy, and Freegift Stout. Along with the careful logbooks of early explorers such as Lewis and Clark, Pike's records helped Washington mapmakers create new images of the mostly unknown Rocky Mountains, including an original map published in 1807. In late 1806, Pike wrote of unbounded prairie under stormlike clouds and bitter cold, with the thermometer falling to four degrees below zero, and on December 25 he wrote that the party was eight hundred miles from the frontiers of their country in the most inclement season, with no one clothed for winter and many without blankets. On January 28, 1807, he recorded discovering through a lengthy vista another chain of mountains, with prairie between the two mountains running nearly north to south, and described sand hills extending along the White mountains for about fifteen miles and about five miles in width.