In the Grand Canyon area, the rocks equivalent to the Madison Limestone that we talked about yesterday are called the Redwall Limestone. It forms one of the highest near-vertical cliffs in the canyon, as much as 500 to 800 feet high. The Redwall is a typical gray to tan limestone – so why is it called Redwall? It is stained red by erosion of the overlying rocks, especially the Supai group and Hermit Shale, which is red because of oxidized iron in the shale. The shale was deposited in a low mud flat that was periodically exposed to air and at other times underwater, which allowed the iron in the rock to oxidize. Shale erodes quite easily so it washed down the cliff faces and stained the underlying Redwall.
|Redwall Cavern, Grand Canyon|
I can assure you that the Redwall does make really steep cliffs. Probably the most difficult hike of my life was on a perfectly smooth, level trail about three or four feet wide along the Redwall’s face. The problem was there was a drop-off on the left that went straight down about 400 feet, and the wall on my right went straight up another 400 feet. If you’re interested in the travelogue of that 1987 backpack trip, here’s a link to my report on it.
—Richard I. Gibson
Photo in Redwall Cavern, Grand Canyon, by Richard I. Gibson