As motorists drive along old Route 66 through northeastern Arizona’s badlands, they will likely notice a peculiar phenomenon occurring on the landscape. The farther west they go, the beiges, tans, and browns that characterize New Mexico’s Bluff Country to the east give way to a wider array of colors, including deep oranges and reds, bright silvers, even blues and purples.
This is Arizona’s Painted Desert, a region of the Colorado Plateau characterized by exposed shale, siltstone, mudstone, and other layers of the Chinle Formation—a group of sedimentary strata formed over 200 million years ago when the region was covered by a large tropical river system. High concentrations of iron and manganese compounds within these strata are responsible for the diverse hues that define the Painted Desert.
In the Painted Desert’s southern reaches, Petrified Forest National Park (previously Petrified Forest National Monument until 1962) protects an area of badlands peppered with permineralized trunks and logs from ancient trees, many of which are now extinct species. Geologists have determined that these remnants of the tropical forests that once covered the area were at some point rapidly buried by volcanic ash, preventing them from decomposing. Over time, mineral-laden water seeped through the ash and into the pores of the buried trees, replacing their organic matter with crystals to create stunningly accurate casts of their internal and external structure.
