The Devils Hall Trail ends, reasonably enough, at Devils Hall, a narrow slot canyon of sorts carved through the limestone. The hall is about 300 feet long, running between cliffs that rise a hundred feet on both sides.
This almost had the look of a roadcut, but it obviously is natural. It's hard for me to work out how this happened, though. It's not like an actual canyon, as the level of the trail is the same on either side. It's more like somebody cut through the hill. It may be something like a water gap, in which streems cut through either side of the hill until one captured the other. Or there might have been a natural break in the rock, and one side slid away from the other.
Edit: I've since worked this out. Pine Springs Canyon marks the location of a minor fault formed during regional deformation probably related to the Laramide Orogeny that formed the Colorado Plateau and the Rocky Mountains. The canyon and its fault does a little jog here, running to the left of the rock wall you see. The hall began as a fracture in the rock during this faulting which has since weathered.