AGE: 250 million years
(Permian Period, Paleozoic Era)

THICKNESS: 300 feet

COMPOSITION: Mostly limestone; some sandstone and shale.

NOTES: The Kaibab Formation combines with the two layers immediately below it to form the whitish cliffs at the top of the Grand Canyon. These sedimentary layers are the canyon's youngest, but they are still old enough to predate the dinosaurs.