On river left, Redwall Cavern gapes at mile 33. The Colorado River carved this large cave from the Redwall limestone formation at river level. Explorer John Wesley Powell, clearly impressed at its vast size, wrote that it could hold 50,000 people (unlikely, but it is big). River runners often stop here for lunch on the beach or a game of Frisbee inside the cavern’s sandy expanse, but no camping is allowed.

Grayish Mauv limestone, the oldest of the Paleozoic rocks (formed about 500 million years ago), begins to appear at mile 34. Five miles downstream is the Marble Canyon dam site, proposed and test-drilled in 1963. (Imagine, for a moment, if all the natural beauty you’d just floated through had been drowned by water impounded behind the proposed dam.) The river continues to meander gently until mile 41, when it bends sharply east around Point Hansbrough, toward President Harding Rapids, before turning west toward Saddle Canyon.

This hairpin is an entrenched meander, a river bend deepened by downcutting. Above is Eminence Break, a northwest-facing escarpment formed along a fault line. A challenging route leads to the rim from the camp below President Harding Rapids.

On his second attempt at running the canyon, six months after hiking out of South Canyon, Robert Brewster Stanton discovered the body of Peter Hansbrough near this rapids. He buried Hansbrough here, leaving an inscription on the cliff as an epitaph. The rapids were named by a later expedition, a U.S. Geological Survey mapping trip led by Claude Birdseye, with Emery Kolb as chief boatman. The 1923 expedition carried a radio, and when they heard that President Harding had died, they camped here for a day and named the rapids in remembrance.

At mile 50, Bright Angel Shale appears, a late-Palezoic series of mudstone, sandstone, and limestone in shades of green, tan, and lavender. Easily eroded, it forms fantastic shapes farther downriver. In the central canyon, it is found on top of the Tonto Platform, where it has mostly eroded away from the underlying Tapeats sandstone, creating the rim of the inner canyon.