He entered the slot canyon section at approximately 2:45 in the afternoon. The interior of a slot canyon in late April receives limited sunlight; the narrowness of the passage and the height of the walls create a perpetual shade that makes the space feel both protected and confining. Ralston was moving downward through the canyon, negotiating the technical sections with the efficiency of a practitioner.
At a point in the descent, he encountered a chockstone — a boulder wedged between the canyon walls above a drop. This is a common feature of slot canyons: rocks of various sizes become lodged between the walls over geological time, and canyoneers typically use them as stepping stones or handholds to navigate sections that would otherwise require a full rappel. Ralston began to climb down from the boulder.
As he did, the boulder shifted.
What happened in the next fraction of a second was irreversible. The chockstone — later estimated at approximately 800 pounds, though this figure was not independently measured and should be understood as a reported estimate — moved with him as he descended, trapping his right hand against the canyon wall. The sandstone, the boulder’s weight, and the geometry of his position combined to pin him completely. He could not free his hand by pulling, pushing, or repositioning himself. He could not dislodge the boulder by any force available to him.
He was alone. He was in a slot canyon approximately three feet wide. The walls rose above him. The afternoon light moved across that narrow strip of sky far overhead.