[Illustration: 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).]
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 30° N. and 30° 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.
[Illustration: A dry community of vegetation grows among the dunes of
the Nebraska Sand Hills (photograph by N. H. Darton).]
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 57,000-square 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.