What metaphors are used in Undaunted Courage by Stephen Ambrose?

The Bitterroot Mountains are a metaphor for the hardship of travel in this era. The Bitterroot Mountains are a small spur range of the Rocky Mountains; they run along the border of Montana and Idaho. The mountains were the area of Lewis and Clark's crossing of the Rocky Mountains and provided the most difficult terrain encountered by the expedition. The lack of game or other food, the lack of forage, and the rocky, freezing terrain made both the east-to-west and the return west-to-east passages miserable and dangerous.