The Lolo Trail west of here differed from other east-west 19th century American trails because it did not carry a flood of cross-country migration and saw no covered wagons. Unmapped and shifting over time, it crossed such formidable terrain that travelers could pass only with the help of people who already knew the way, knowledge handed down from generation to generation. Long before it became an explorers' route, it was an American Indian trail, and Lewis and Clark would have been lost there without their Indian guides. On June 27, 1806, William Clark called these mountains "... Stupendous Mountains principally covered with snow like that on which we stood; we are entirely serounded by thos mountains from which to one unacquainted with them it would have Seemed impossible ever to have escaped..." The Bitterroot Mountains formed the most difficult part of a trail linking the plains of the Columbia River with those of the Missouri. Their unyielding topography and dense timber resisted improvement for wheeled vehicles until the 1960's, and railroads were never able to penetrate the mountains to the west. U.S. Highway 12 now roughly parallels the Lolo Trail, which mostly lies above it along ridges and saddles north and south of the highway. Aside from changes in vegetation, the trail looks much as it did hundreds of years ago, and Congress preserved these settings as the Nee Mee Poo National Historic Trail and the Lewis and Clark National Historic Trail, which together make up the Lolo Trail.