BY KEVIN FEDARKO
“If you break loose here, you can’t stop. You’re going into the abyss,” barks Rich Rudow. Normally he is unflappable, but as he knows too well, this is no place to let down one’s guard. We’re on a cliff roughly 3,500 feet above the Colorado River at the tip of the Great Thumb Mesa, a spectacular formation that thrusts out from the South Rim of the Grand Canyon like the bow of an immense ship. It is one of the canyon’s most remote spots, rarely seen even by the most hard-core backpackers. If you come this far out on the Thumb, there is no way to get down to the river without climbing gear, and the dwindling food in your pack won’t allow you to make the eight-day trek back the way you came. You have to move forward...