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In page Geology of the Grand Canyon area:
"Surprise Canyon Formation is a sedimentary layer of purplish-red shale that was laid down in discontinuous beds of sand and lime above the Redwall (see 4c in figure 1). It was created in very late Mississippian and possibly in very earliest Pennsylvanian time as the land subsided and tidal estuaries filled river valleys with sediment.[2] This formation only exists in isolated lenses that are 50 to 400 feet (15 to 122 m) thick.[10] Surprise Canyon was unknown to science until 1973 and can be reached only by helicopter.[8] Fossil logs, other plant material and marine shells are found in this formation.[2] 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.
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