<P> The next two periods of geologic history, the Ordovician and the Silurian, are missing from the Grand Canyon sequence . Geologists do not know if sediments were deposited in these periods and were later removed by erosion or if they were never deposited in the first place . Either way, this break in the geologic history of the area spans about 65 million years . A type of unconformity called a disconformity was formed . Disconformities show erosional features such as valleys, hills and cliffs that are later covered by younger sediments . </P> <P> Geologists do know that deep channels were carved on the top of the Muav Limestone during this time . Streams were the likely cause, but marine scour may be to blame . Either way, these depressions were filled with freshwater limestone about 385 million years ago in the Middle Devonian in a formation that geologists call the Temple Butte Limestone (see 4a in figure 1). Marble Canyon in the eastern part of the park displays these filled purplish - colored channels well . Temple Butte Limestone is a cliff - former in the western part of the park where it is gray to cream - colored dolomite . Fossils of animals with backbones are found in this formation; bony plates from freshwater fish in the eastern part and numerous marine fish fossils in the western part . Temple Butte is 100 to 450 feet (30 to 137 m) thick; thinner near Grand Canyon Village and thicker in western Grand Canyon . An unconformity representing 40 to 50 million years of lost geologic history marks the top of this formation . </P> <P> The next formation in the Grand Canyon geologic column is the cliff - forming Redwall Limestone, which is 400 to 800 feet (120 to 240 m) thick (see 4b in figure 1). Redwall is composed of thick - bedded, dark brown to bluish gray limestone and dolomite with white chert nodules mixed in . It was laid down in a retreating shallow tropical sea near the equator during 40 million years of the early - to - middle Mississippian . 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 . In late Mississippian time, the Grand Canyon region was slowly uplifted and the Redwall was partly eroded away . A Karst topography consisting of caves, sinkholes, and subterranean river channels resulted but were later filled with more limestone . The exposed surface of Redwall gets its characteristic color from rainwater dripping from the iron - rich redbeds of the Supai and Hermit shale that lie above . </P> <P> 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 . This formation only exists in isolated lenses that are 50 to 400 feet (15 to 122 m) thick . Surprise Canyon was unknown to science until 1973 and can be reached only by helicopter . Fossil logs, other plant material and marine shells are found in this formation . 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 . </P>

What type of rock is in the grand canyon