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USGS CMG InfoBank: Past Deserts

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Comment: 07:38 - 08:29 (00:51)
Source: Annenberg/CPB Resources - Earth Revealed - 22. Wind, Dust and Deserts
Keywords: "Southwestern United States", desert, "fossil dune", "Grand Canyon", sandstone, "Zion National Park", "cross-bedding", oxidation, iron, "dry land", plant, animal, fossil, forest, "geologic time", "plate tectonics", orogeny, mountain, "rain shadow"

Our transcription: In the Southwestern United States, too, there's evidence of a once widespread desert that existed 200 million years ago.
Fossil dunes are preserved in the upper wall of the Grand Canyon and in the sandstones of Zion National Park.
The buried surfaces of the shifting sand dunes appear as crisscrossing sets of beds.
Their large size and coloration from the oxidation of iron show that they formed on dry land.
Since that time, plant and animal fossils indicate that this region became moist and forested.
But in recent geologic time conditions have become dryer in response to plate motions, mountain building, and the development of rain shadows.

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