The 1840s ushered in the beginning of the largest migration in US history. People in crowded Eastern cities and Missouri River towns were feeling the pull of the Western frontier. It was the dawn of a new era of expansion, and over the next few decades, the making of a new kind of pioneer. It was the birth of the cowgirl Welcome to the world of nimble equestriennes, hawkeyed sharpshooters, sly outlaws, eloquent legislators, expert wranglers and talented...