the soil of the midwestern united states owes much of its fertility to what geologists call loess. loess is made up of fine particles of silt and clay that have drifted in over the millennia from barren lands uncovered by melting pleistocene ice. in eastern china, deposits of loess have reached remarkable proportions, hundreds of meters thick. from time immemorial, the chinese have carved cave dwellings out of this soft, but surprisingly cohesive material. windblown sand, too heavy to blow across oceans and continents, bounces and skips along the ground until it is caught by an obstacle in its path. here it begins to accumulate, forming an even larger trap for additional sand. wind plucks sand from the windward side of the dune, blowing it across the crest, where it settles on the quiet leeward side. in time, the entire dune shifts downwind, grain by grain, possibly migrating kilometers from its point of origin. a major source of desert sand is desert playas-- lake beds from more humid times which have long been dry. the same winds that build up dunes may also hollow out depression

most important to our modern civilization, however, are the windblown deposits of glacial silt, or loesswhich have weathered to form rich farmland soil. among the most profound effects of the ice ages on our world was the formation of land bridges. the dropping sea level exposed low-lying coastal areas, in some cases linking land masses that are presently separate. the most famous land bridge between asia and north america allowed animals, including humans, to migrate between the two continents. evidence of ice ages is not only recorded by the landscape. scientists have also discovered important clues to earth's past within glaciers themselves. glaciers contain within their stratographic layers all of the material that fell with the snowfall that it incorporated within it. one could liken glaciers as a depositional environment, a glacial environment. in the deposits are contained events of earth history that occur simultaneously during the depositional process. you see, ice sheets are essentially atmospheric processes. practically any material passing through the atmosphere may be trappe