Which posed the greatest challenge to pioneers crossing the westerns mountains?

The greatest challenge faced by pioneers crossing the western mountains was the treacherous terrain and harsh weather conditions. Passes like the Sierra Nevada and Rocky Mountains presented difficult routes, with steep ascents, rocky paths, and potential avalanches. Additionally, extreme temperatures, snow, and rain could lead to delays and hardships, making the journey perilous. These factors combined made the crossing not only physically demanding but also life-threatening at times.