Over the Edge

A black basalt forms the rim of the plateau here. This hard basalt, called the Bidahochi Formation, was deposited by local volcanoes between 16 and five 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 weather like an umbrella, while the rest of the desert is left exposed and vulnerable to erosion, carving the badland topography seen today. In the nearby cliff face, the black basalt of the Bidahochi Formation stands in stark contrast to the red sedimentary rocks of the Chinle below. The boundary between these two formations divides rocks of vastly different ages and marks an unconformity, a gap in the physical rock record. Geologists believe that more than 1,000 feet of rock layers were deposited and then eroded over 200 million years between these formations.

Arizona · United States