This layer averages about 285 million years old and is composed primarily of shale that is intermixed with some small amounts of limestone and capped by sandstone. The limestone features become more and more prominent in the western regions of the Canyon, leading one to believe that that region was more marine. The eastern portions where probably a muddy river delta that fed into an ancient sea. The color of this layer varies from red for the shale to tan for the sandstone caps. Numerous fossils of amphibians, reptiles and terrestial plants exist in the eastern portion which are replaced by marine fossils as you move westward.
 
The layers contained within the Supai Formation are, from top to bottom, as follows:

• Esplanade Sandstone
• Wescogame Formation
• Manakacha Formation
• Watahomigi Formation

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