Data on ancient sand seas (vast regions of sand dunes), changing lake basins, archaeology, and vegetation analyses indicate that climatic conditions have changed considerably over vast areas of the Earth in the recent geologic past. During the last 12,500 years, for example, parts of the deserts were more arid than they are today. About 10 percent of the land between 30Š N. and 30Š S. is covered now by sand seas. Nearly 18,000 years ago, sand seas in two vast belts occupied almost 50 percent of this land area. As is the case today, tropical rain forests and savannahs were between the two belts.
Fossil desert sediments that are as much as 500 million years old have been found in many parts of the world. Sand dune-like patterns have been recognized in presently nonarid environments. Many such relict dunes now receive from 80 to 150 millimeters of rain each year. Some ancient dunes are in areas now occupied by tropical rain forests.
The Nebraska Sand Hills is
an inactive 57,000square kilometer dune field in central Nebraska. The
largest sand sea in the Western Hemisphere, it is now stabilized by
vegetation and receives about 500 millimeters of rain each year. Dunes in
the Sand Hills are up to 120 meters high.
 This aerial photograph of the Nebraska Sand Hills paleodesert shows a well-preserved crescent-shaped dune (or barchan) about 60 to 75 meters high. (Photograph by Thomas S. Ahlbrandt) |
 A dry community of vegetation grows among the dunes of the Nebraska Sand Hills. (Photograph by N.H. Darton) |
Information provided by A
. S . Walker
http://pubs.usgs.gov