GEOLOGICAL WORK OF THE ATMOSPHERE 99 



the prevailing winds blow. Dunes may be formed in the valley 

 bottoms, but the sand is often blown up out of the valley, and 

 lodged on the bluffs above. 



Dunes probably reach their greatest development in the 

 Sahara, where some of them take the form of hillocks, and some 

 the form of ridges. They are also conspicuous in other arid, sandy 



Fig. 60. A resurrected forest. After burying and killing the forest, the 

 sand was blown away, exposing the dead trees. (Myers.) 



tracts, as in some parts of western Kansas and Nebraska, and in 

 parts of Wyoming. 



Eolian sand is not always built up into dunes. It is sometimes 

 spread somewhat evenly over the surface where it lodges. Eolian 

 sand is probably much more widespread than dunes are. 



Wind-ripples. The surface of the dry sand over which the 

 wind has blown for a few hours is likely to be marked with ripples 

 (Fig. 61). While the ripples are, as a rule, but a fraction of an inch 

 high, they throw much light on the origin of the great dune ridges. 

 If the ripples be watched closely during the progress of a wind- 

 storm, they are found to shift their position gradually. Sand is 

 blown up the gentler windward slope to the crest of the ridge, and 

 falls down on the other side. Wear on the windward side may 

 be about equal to deposition on the leeward, and the result is the 



