Ashley went up alongside the Missouri River, then up the Platte River and finally the South Platte River until it reached the Front Range of the Rockies. At this point, he headed in a northwesterly direction until he reached the wide and sandy pass that is now known as South Pass in southwestern Wyoming. This pass at 7,000 ft. remained snow free longer than the mountains on either side.
The Rocky Moutains presented a major detriment to western settlement. Much of Ashley's route from St. Louis and through South Pass later became the Oregon Trail which led to the California Trail. The tracks left by the wagon that Ashley had on the expedition were later used by the first wagon trains as they made their way across the country in a migration that saw hundreds of thousands of settlers move west. The Pony Express came through South Pass in 1860 as well as the Mormons that settled Utah earlier in 1847.
All of this trailblazing was being accomplished by the ambitous, but reluctant adventurer, William Ashley. Lewis and Clark had crossed the Rockies, but they made their crossing farther north near where Yellowstone is located and much too far north to be considered a viable route for the settlers.
William Earl Cook