The Sierra Nevada, North America's highest mountain range, contains one of the most awe-inspiring geological features on the planet: Yosemite Valley. Walled by sheer 3,000-foot granite cliffs and made from one of the toughest rocks on earth, it is home to the mighty El Capitan and iconic Half Dome. Yet how this extraordinary valley formed has been the subject of controversy for over 100 years. Was it carved by gigantic glaciers or a cataclysmic rifting of the Earth?