the cliff-forming Redwall Limestone, which is 450 to 525 feet (140 to 160 m) thick (see 4b in figure 1). The Redwall is composed of thick-bedded, dark brown to bluish gray limestone and dolomite with white chert nodules mixed in and was laid down in a retreating shallow tropical sea near the equator in early to middle Mississippian time (about 335 million years ago). Many fossilized crinoids, brachiopods, bryozoans, horn corals, nautiloids, and sponges, along with other marine organisms such as large and complex trilobites have been found in the Redwall. Caves and natural arches are also found. After this formation was deposited the Grand Canyon region was slowly uplifted, and part of the upper Redwall was eroded away in late Mississippian. The exposed surface of the Redwall gets its characteristic color from rainwater dripping from the redbeds of the Supai and Hermit shale that lie above.
The Surprise Canyon Formation is a sedimentary layer of purplish-red shale that was laid down in discontinuous beds above the Redwall (see 4c in figure 1). It was created by evolving tidal estuaries in very late Mississippian and possibly in very earliest Pennsylvanian time. This formation, which only exists in isolated lenses up to 40 feet (12 m) thick, can only be reached by helicopter. It was unknown to science until the 1980s. An unconformity marks the top of the Surprise Canyon Formation and in most places this unconformity has entirely removed the Surprise Canyon and exposed the underlying Redwall.