Most of the ones mentioned in this thread are great films. I have a particular liking for the
The Big Sky, Howard Hawks, 1952, starring a very young Kirk Douglas, Dewey Martin and an utterly un-PC blacked-up Elizabeth Threatt as the Native American princess. The action centres on a trading mission up the Missouri river from St Louis in 1832, into hostile unknown territory. It captures the vastness of the country, does justice to the French culture and history — the Louisiana Territory was sold to the USA by France in 1803 — and tells a rattling good tale with some excellent character actors who frequently steal the scenes. Arthur Hunnicutt, centre in the picture, plays frontiersman Uncle Zeb with what must be one of the greatest performances never to have won an Oscar. A DVD you will never regret buying — and that music will play along in your head for thousands upon thousands of miles.