File:Over the Edge (Petrified Forest NP).jpg

English: Below you lies a black basalt which forms the rim of the plateau on which you stand. This hard basalt, called the Bidahochi Formation, was deposited by local volcanoes between 16 and 5 million years ago, and forms a protective cap over the soft, red rocks of the much older Chinle Formation. The Bidahochi basalt shelters this area from the effects of weather like an umnbrella, while the rest of the desert is left exposted and vulnerable to erosion, carving the badland topography you see today.

As you look at the cliff face to your left, the black basalt of the Bidahochi Formation stands in stark contrast to the red sedimentary rocks of the Chinle below. The between Chinle and Bidahochi Formation highlights the change between these two formations which divides rocks of two vastly different ages. This thin line represents a gap in the physical rock record called an unconformity. Geologists believe there are more than 1,000 feet (305 meters) of missing rock layers that were deposited and then eroded over 200 million years between these formations.

@ Petrified Forest National Park, Arizona