"Yosemite Valley in July of 1967 would have had to be seen to be believed. There was never an empty campsite in the valley; you had to create a space for yourself in a sea of cars, tents, and humanity.... The camp next to ours had fifty people in it, with rugs hung between the trees, incense burning, and a stereo set going full volume." Scenes such as this will probably never be repeated in Yosemite or any other national park, yet the urgent...