Existing comment:
A Gap in the Geologic Record:
The black basalt that caps the cliffs behind you stands in stark contrast to the colorful Chinle Formation visible throughout the Painted Desert.
Below this layer of basalt, a horizontal line cuts across the face of the mesa and separates rocks of two different geologic periods. The pink mudstone below this line belongs to the Chinle Formation deposited about 225 million years ago. The brown mudstone and basalt layers above the line represent the Bidahochi Formation deposited only 5 to 8 million years ago.
How is it that only this thin line represents more than 200 million years of geologic history? This gap is known as an "unconformity." Geologists believe the missing layers were more than 1,000 feet thick and that they were eroded over the years by running water and wind -- forces that continue to shape this landscape. The emplacement of the basalt has temporarily stopped erosion on the Painted Desert rim, while it has continued in the valley below. |