June 13. The Redwall Limestone

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

Fairly soon after the deposition of the Redwall – and this applies to the Madison limestone in many places, too – gentle uplift brought the limestones above sea level. Then the action of moving water, percolating through the rock, did what it does to limestone – it dissolved out caves. In a good number of places, caves collapsed, and sinkholes and other features typical of karst topography developed. This was a pretty widespread development, because we can see the evidence of the collapses in the Black Hills, in Montana, and in the top of the Redwall limestone too. This was not remotely related to the cave formation that made the modern Lewis and Clark Caverns we talked about yesterday, which are at most a couple million years old, but was something that happened probably quite soon after these Mississippian limestones were lithified. How do we know that? Because the collapse structures don’t include much of the overlying rock layers. The collapsing had to take place before those layers were laid down.

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