YOSEMITE NATIONAL PARK — The rumble began high on the sheer cliff wall, like faraway thunder before a storm. A slab of granite as big as a railroad boxcar had let loose 1,300 feet up Glacier Point's age-worn face. The million-pound rock cartwheeled and shattered, tracing a plume of dust downward toward Peter Terbush. In his last earthbound moments, Terbush turned to a long-ago climbing lesson taught by his dad. As a little boy first astride a mountain, he learned to always protect a partner at the end of the rope. Never let go. The broad-shouldered 21-year-old held fast to...