The Painted Desert is an outdoor museum of fossilized plants and animals. Its striking colors come from the Chinle Formation of the Late Triassic, eroded by the Little Colorado River drainage system. From above, it appears as tie-dyed corrugated hills of highly colored sedimentary rock, mostly soft, fine-grained mudstone and claystone, with harder beds of more somber-colored siltstone, sandstone, and conglomerate. The wide range of reddish color in these rocks comes from iron minerals. The Chinle Formation is a storehouse of plant and animal fossils that provides evidence of a time when giant amphibians and reptiles ruled the Earth, and deep in the Painted Desert large fragments of petrified wood may be seen.