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The South Kaibab Trail descends past O'Neill Butte. The Supai Group is subdivided into four formations in the Grand Canyon area-Watahomigi Formation, Manakacha Formation, Wescogame Formation, Esplanade Sandstone, from bottom to top respectively. The Supai Group consists of Upper Mississippian to Lower Permian sandstone, mudstone, and shale beds originally deposited as coastal flood-plain deposits (river and stream beds, marsh, bay, and other sedimentary environments in a subtropical setting). The Supai Group yields rare vertebrate fossils and trackways preserved in some beds. Thick red beds of the Esplanade Sandstone form the massive top of O'Neill Butte (Beus, 1987).