This is just speculation, but I imagine the tendency of the redwall layer to act as an aquifer is what leads to these caverns being created. Water comes through the cliffs in many places and must have taken advantage of a weakness in the cliff face here. Whatever the cause, there are many of these large caverns in the canyon in this layer.
This is an ancient granary far up over the river. It's a bit of a climb up, but has spectacular views. The fingerprints of the original builders are still visible on the walls. I believe this was on the downstream side of Nankoweep Canyon.
