PT156 S1 P4 Q23 ExplanationSand Fossils

`Discovered in 1993, the site known as Ukhaa Tolgod, in the Gobi desert of Mongolia, is one of the world's best sources of fossils from the Late Cretaceous period, which ended about 65 million years ago. The dinosaur, lizard, and mammal fossils from this area, including skeletal structures less than 2 millimeters scientists analyzing the geological formations of Ukhaa Tolgod indicates that this sandstorm hypothesis is probably mistaken.

The scientists found that there are three distinct types of sandstone at the site. The first exhibits a well-defined structure of layers tilted at 25 degrees and sorted by particle size. Such arrangements are typical of deposits created from windblown sand. While this sandstone contains dinosaur footprints, it contains no skeletal remains. this third type of deposit that all the vertebrate skeletal fossils of Ukhaa Tolgod are found.

This third type of sandstone exhibits a structure similar to that caused by a phenomenon in which an otherwise stable sand dune becomes drenched with water from heavy rains, triggering a sudden debris flow. The resulting avalanche of sand can be as powerful and violent as a snow avalanche or mudslide. Such were in its path, resulting in the pristine quality of the remains.

The cause of these sandslides is not well understood. However, there is evidence that clay plays an important role. When windblown clay particles deposited by dust storms are moistened with rain, the wet clay filters into the dune and adheres to individual sand grains. The buildup of clay eventually inhibits drainage of time was not a sterile desert, but a stable dune field with plant life and rain.