In 1848 news of the discovery of gold in California triggered an enormous wave of emigration toward the Pacific. Lured by the promise of riches, thousands of settlers left behind the forests, rain, and fertile soil of the eastern United States in favor of the rough-hewn lands of the American West. The dramatic terrain they struggled to cross is so familiar to us now that it is hard to imagine how frightening-even godforsaken-its sheer rock faces and...