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List of Grand Canyon Rock Layers
What is Grand Canyon Stratigraphy?
Kaibab Formation
Toroweap Formation
Coconino Sandstone
Hermit Formation
Supai Group
Redwall Limestone
Tonto Group
Muav Limestone
Bright Angel Shale
Tapeats Sandstone
Vishnu Complex
Redwall Limestone
Mississippian Period, 340 Million Years Old, 500 Feet Thick
Tall vertical red cliff about halfway to the canyon bottom.
The stately cliff of red-stained limestone is both a popular formation and a barrier to travel. Massive jutting
overhangs form alcoves that only birds may visit, as most of these shady stadiums are just under the top of the cliff. The Redwall
(and underlying Muav Limestone) are the chief cave-forming units of the Canyon, with small to large entrances visible from nearly
any rim viewpoint.
The Redwall Limestone has a gray color, but almost everywhere it is stained red by iron oxide washed down from
the red, iron-rich layers above (Supai and Hermit Formations). Like most limestones, the Redwall is formed mostly of the
calcium carbonate shells of sea creatures. When the animals die their shells fall apart in marine waves and currents or even
dissolve and re-precipitate as lime mud. But some of the creatures' remains are preserved as fossils. Corals,
brachiopods, bryozoans and foraminifera are common in the Redwall but the champion is the ubiquitous crinid..
Pure, thick-bedded limestones like those of the Redwall are usually deposited relatively far from shore. The sea had encroached far inland during this period. Yet shallow-water fossils are present throughout the formation. This shows that the water was never very deep--probably at maximum 300 feet. How can 450 feet of limestone accumulate in 300 feet of water? Offshore subsidence is the key. As the sea floor slowly subsides, lime muds slowly accumulate, keeping the water depth approximately constant during the entire deposition. In this way, a thick sequence of rocks can form entirely in shallow water.
The following story pretends that people were alive when the Redwall Limestone formed. But it was long before people.
Our Tribe in Redwall Time
One time I counted to ten thousand. I was sitting on our shore counting crinoid stems, the little beads that the storm waves cast up on our beach. Each stem fragment has a little hole in the center and we string them into necklaces. Once a year, far to the east, there is a clan gathering and we take our crinoid jewelry for trade. They give us a thing called dried amphibian--a rich protein source from a small fish-like creature that actually walks on the land and breathes air like us.