At river mile 33, the Colorado River carved a limestone amphitheater big enough that John Wesley Powell guessed it could seat 50,000 people. The park's own rulebook disagrees with him on one point — technically, it insists, this thing isn't even a cave.

The short version

• Redwall Cavern is a river-carved alcove at River Mile 33 in Marble Canyon, cut into the Redwall Limestone — a roughly 340-million-year-old marine rock layer full of fossils.
• John Wesley Powell floated past in August 1869 and described a chamber that could theoretically seat tens of thousands of people. Modern accounts treat that number as classic Powell exaggeration.
• The current Superintendent’s Compendium explicitly defines Redwall Cavern as not a cave under park management rules, because its mouth is wider than the chamber is deep.
• It’s one of the most popular stops on a Grand Canyon river trip — a shaded, sandy flat used for lunch breaks, frisbee, and simply getting out of the sun.
• The rules are narrow but firm: day use only, no overnight camping, and wood fires are never allowed there.
• Glen Canyon Dam changed how often the cavern floods, which changed what the inside of it looks like.

A limestone amphitheater, cut by the river

Redwall Cavern sits where the Colorado River bends sharply through Marble Canyon, roughly 33 miles downstream of Lees Ferry. The chamber isn’t a passage that tunnels back into the rock — it’s an open, curved alcove, closer to a stage shell than a cave mouth, gouged out by the river undercutting the base of a limestone cliff. Descriptions put it at roughly 300 feet wide and 150 feet deep, with a vaulted ceiling that rises to around 100 feet at its highest point.

The rock doing the eroding is the Redwall Limestone, a Mississippian-age formation — on the order of 340 million years old — laid down as sediment on the floor of an ancient sea. It’s mostly calcium carbonate from marine shells, and the cavern’s walls still hold fossils from that sea floor: cephalopods, brachiopods, crinoids, and corals. The limestone itself is naturally gray; the “red” in Redwall comes from iron oxide washing down off the redder rock layers above and staining the cliff face, a coloring that’s cosmetic rather than structural.

Technically, it isn’t a cave

The park’s current Superintendent’s Compendium spends a full paragraph on what legally counts as a “cave” in Grand Canyon National Park, borrowing its base definition from the federal Cave Resource Protection Act of 1988: any naturally occurring void large enough for a person to enter. For management purposes, the park extends that definition to cover erosion features 50 feet or longer as well — but only if the entrance is no wider than the feature is deep.

Redwall Cavern fails that test on purpose, and the Compendium says so by name: “For management purposes, Grand Canyon National Park extends this definition to include any dissolution or erosion feature 50 feet or longer where the entrance is not wider than the cave is long (e.g. Redwall Cavern is not a cave by definition).” Its mouth is wide open — wider than the chamber runs back — so it doesn’t trigger the cave-access permitting rules that apply to Grand Canyon’s true caves. It’s an amphitheater under the compendium’s own logic, not a cave, even though every river guide on the water calls it one.

“Sitting to 50,000 people” — Powell’s exaggeration

Major John Wesley Powell passed this stretch of Marble Canyon in August 1869, on the first expedition to run the length of the Colorado River through the Grand Canyon. By most retellings, he was struck enough by the open chamber to guess out loud that it could seat 50,000 people, theater-style, if anyone ever put it to that use.

Whether that’s Powell’s exact wording is hard to pin down. The line gets repeated, with slightly different phrasing, across river-outfitter histories and guidebooks, and this session’s research couldn’t track it back to a verbatim passage in Powell’s own published account. What’s consistent across every retelling is the gist: an enormous number, framed as theater seating, and an acknowledgment — then and now — that Powell tended to exaggerate. Modern visitors don’t dispute that the cavern is huge. They just don’t buy the 50,000 figure.

From flood-scoured to bone dry — the Glen Canyon Dam effect

Before Glen Canyon Dam began regulating the Colorado River’s flow in the 1960s, the river ran through seasonal flood cycles. At high water, accounts describe the cavern filling completely with silty floodwater; as that water receded, it left fresh sand behind on the cavern floor. Since the dam started controlling releases, that natural flood-and-refill cycle mostly stopped, and the cavern now stays largely dry. It’s a small, specific example of a pattern that shows up across the length of the canyon: a structure upstream on the mainstem river reshaping what happens to a landmark dozens of miles downstream, long after the water itself has moved on.

A river-trip institution

Whatever Powell’s number was, the cavern earns its reputation honestly. For both commercial and private river trips floating the Colorado, it’s one of the standard stops — a wide, shaded, flat expanse of sand that offers a break from the sun and a place to stretch out that most beaches in the canyon can’t match. River parties use it for lunch, for games of frisbee or volleyball, for yoga, or just for looking at the fossils embedded in the walls. Its scale is part of the appeal: there’s enough open ground that a whole group can spread out without feeling cramped.

That popularity is also the reason the rules are as narrow and specific as they are. Heavy day-use traffic is part of why overnight camping isn’t allowed — a stop this well used doesn’t need an overnight crowd on top of the daytime one. The wood-fire ban is even more absolute: it isn’t seasonal or conditional, it’s a flat “never,” full stop.

What’s actually off-limits (and why)

The rule itself, straight from the current Superintendent’s Compendium: “Wood fires are never allowed at Redwall Cavern (River Mile 33). Restricted to day use only — no camping.” That’s the entire restriction — no permit process, no exceptions listed, just a hard day-use limit and a fire ban that applies regardless of season or river level.

It’s a lighter-touch closure than most of what shows up elsewhere in this project. Redwall Cavern isn’t fenced off for radiological hazards or closed outright the way a protected cultural site can be. It’s simply managed the way a heavily used, fragile piece of public land gets managed: come see it, use it during the day, and leave before dark. Two stops downstream, the same river corridor gets a lot stricter — Nankoweap limits camping to four numbered sites, and the Little Colorado River confluence bans camping outright near the water.

Sources

1. Grand Canyon National Park Superintendent's Compendium, effective June 16, 2026 — National Park Service
2. Redwall Cavern — An Immense and Spectacular Grand Canyon Cave — Western River Expeditions
3. Redwall Limestone — Wikipedia (flagged as last-resort source for formation background)
4. Redwall Limestone, Grand Canyon — Grand Canyon Natural History Association
5. Redwall Cavern — Intermountain Histories
6. Redwall Cavern — Pima County Public Library
7. What is Redwall Cavern All About? — Grand Canyon Whitewater