What are the motifs in Undaunted Courage by Stephen Ambrose?

Man and nature are recurring ideas. The members of the Corps of Discovery, moving into areas uninhabited by whites, relied to a large extent on their environment for survival. Lewis and his men had to depend on their own abilities. There was no book knowledge that could ensure their safe trip. They hunted for food and ate roots. They constructed boats out of natural resources. Time after time, the men were challenged by nature, from making the upstream battle against the Missouri or crossing the snowy Rocky Mountains, to dealing with the geographical fact that an all-water route leading to the Pacific Ocean did not exist.