A chockstone is a rock that gets caught between two opposing walls of a canyon. The most important one in this book is the boulder that traps Aron Ralston in the canyon after dislodging while he’s maneuvering over it. As it falls, it hits his left hand and then pins his right arm against the wall as it settles into place. As the immediate cause and symbol of his captivity, the chockstone is the focal point of Ralston’s anger and frustration; he feels incredibly unlucky that it moved exactly as it did. For most of the narrative, it represents an immovable, insurmountable obstacle, larger and more devastating than any he has faced before.

His epiphany about using the boulder to break the bones of his arm, allowing him to free himself, turns the boulder from an enemy to a tool. By the time he returns to civilization and completes his recovery, the boulder is no longer the enemy; rather, it’s a key player in an experience that has shaped him for the better.