About 350 million years ago, a shallow inland sea covered most of the present day United States. This resulted in this massive limestone cliff.
Limestone is always gray, being composed of calcium carbonate. The Redwall has been dyed red by mud washing down from the Supai above it. It is, in fact, gray, as can be seen in those areas where the patient action of running water has washed it free of the red patina.
The trail tends to be steep as it wends its way through the Redwall, as the trail builders had to confine their efforts to whatever break they could find in this stubborn barrier. In this ocean there were fish, and above the seas there were insects.