image: Sandstone cliff in Vermillion Cliffs National Monument, Arizona composed of the deposits of about 15 large, Jurassic sand dunes (person for scale in upper right). Each dune migrated toward the present southeast (toward the left in this photo). The dune sand accumulated about 200 million years ago, just above sea level in a slowly subsiding sedimentary basin. Circulating groundwater cemented the sand into sandstone. Uplift of the region in the last 10 million years led to erosion of the rocks, forming canyons and cliffs. Climate models constructed for the Supercontinent Pangaea suggest that the sands accumulated near the equator, and were swept by strong monsoonal winds that reversed direction every six months. Evidence based on the magnetic properties of rocks; however, indicate that the sand accumulated much farther north--about 20 degrees north of the equator. This image relates to an article that appeared in the Nov. 23, 2007, issue of the journal Science, published by AAAS. The study, by Dr. Clinton M. Rowe and colleagues at University of Nebraska, Lincoln in Lincoln, Neb., was titled "Inconsistencies Between Pangean Reconstructions and Basic Climate Controls." view more
Credit: Image © Science