Complex sentence: The Shoshone agreed to barter horses to the group, and to provide guides to lead them over the treacherously cold and barren Rocky Mountains, where they were reduced to eating tallow candles to survive.
Split into simpler sentences: The Shoshone agreed to barter horses to the group, and to provide guides to lead them over the cold and barren Rocky Mountains. The trip was so hard that they were reduced to eating tallow candles to survive.