A chockstone is a boulder that’s become lodged between the walls of a chimney, cave, or slot canyon, much like a cork jammed into the neck of a bottle. In 1931, the eminent mountaineers Robert Underhill and Fritiof Fryxell attempted the previously unclimbed north ridge of Wyoming’s Grand Teton. While ascending a chimney on the vertical face of the ridge, they encountered a giant rock wedged across the slot’s six-foot-wide throat, which halted their progress. The “chockstone [was] so big,” Fryxell reported, “that it extended well out over our heads. I could touch the chockstone but nowhere find the slightest hold.” With tremendous effort, the men managed to climb around the boulder and eventually reach the summit, completing what was then the hardest climb in the United States. The notorious passage that nearly thwarted their ascent was christened “Chockstone Chimney.”

Jon Krakauer