The Supai Group was deposited in Pennsylvanian and early Permian time in swampy and riparian environments from clastic sediment mostly derived from the Ancestral Rocky Mountains (the average age of this group is 285 million years). The Supai in the western part of the canyon contains limestone, indicative of a warm, shallow sea, while the eastern part was likely a muddy river delta. This formation consists of red siltstones and shale capped by tan-colored sandstone beds that together reach a thickness of 600 to 700 feet (180 to 210 m). Shale in the early Permian formations in this group was oxidized to a bright red color. Fossils include amphibian footprints, reptiles, and plentiful plant material in the eastern part and increasing numbers of marine fossils in the western part. The formations of the Supai Group are (from oldest to youngest; an unconformity is present at the top of each):
Watahomigi: Slope-forming gray limestone with some red chert bands, sandstone, and purple siltstone that is 90 to 175 feet (30 to 50 m) thick.
Manakacha: Cliff- and slope-forming pale red sandstone and red shale that is 200 to 275 feet (60 to 85 m) thick.
Wescogame: Ledge- and slope-forming pale red sandstone and siltstone that is 100 to 225 feet (30 to 70 m) thick.
: Ledge- and cliff-forming pale red sandstone and siltstone that is 225 to 300 feet (70 to 90 m) thick.
An unconformity marks the top of the Supai Group.