Mountain men, pioneer fur trappers and traders in the Rocky Mountains in the 1820s and 1830s. Early mountain men included John Colter, who stayed in the area after the Lewis and Clark expedition of 1804–6, Thomas Fitzpatrick, Jedediah Smith, and W.S. Williams. Many mountain men, including James Bridger, took part in William Ashley's expedition up the Missouri River in 1822. The mountain men were the first to begin opening up the Rockies and make the area's potential known. They were quickly followed by the big fur companies, such as the Rocky Mountain Fur Company and the American Fur Company.