Late Miocene into the Early Pliocene
Mammals, birds, fish, and mollusks have been found in the Bidahochi Formation in the region, particularly near White Cone on the Navajo Nation. Much of this geological formation has eroded away, leaving only a very small outcropping within park boundaries. The largest deposit of the Bidahochi can be found to the north and east of Petrified Forest National Park. Not as well researched as the Chinle Formation, the Bidahochi Formation has still intrigued researchers. A variety of interpretations of the environments represented by the layers of the Bidahochi exist. Most theories point towards a volcanically active landscape with large lakes, many of which were ephemeral. Many of the volcanoes erupted under lake water, creating a specific type of crater called a maar. In the park, the middle member of the Bidahochi shoes interbedded layers of basalt, tuff, claystone, and mudstone.