DUNES 



21 



Distribution of dunes. Dunes are likely to be formed wherever 

 dry sand is exposed to the wind. They are especially characteristic 

 of the dry sandy shores of lakes and seas, of sandy valleys, and of 

 arid, sandy plains. Along coasts, dunes are developed extensively 

 only where the prevailing winds are on shore. Thus about Lake 

 Michigan, where the prevailing winds are from the west, dunes are 

 abundant and large on the east shore, and few and small on the 

 west. Along valleys, dunes are most numerous on the far side as the 

 prevailing winds blow. The dunes may be in the valleys, but in quite 

 as many cases the sand is blown up out of the valley, and the dunes 

 are on the bluffs above. Dunes probably reach their greatest 

 development in the Sahara, but they are conspicuous in other arid 

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

 and in parts of Wyoming. 



Eolian sand is not all piled up into dunes. It may be spread 

 somewhat evenly over the surface where it lodges. Eolian sand is 

 much more widespread then 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. 13). While the ripples are, as a rule, but a fraction of an inch 

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





Fig. 13. A ripple-marked sand dune in a western valley. (U. S. Geol. Surv.) 



