96 



PHYSICAL GEOLOGY 



from their windward to their leeward sides, dunes shift slowly 

 in position (Fig. 86). From the nature of the migration it is 



apparent that only 

 an extremely small 

 fraction of the sand 



FIG. 86. Diagram showing successive positions 



of a migrating sand dune. 1S ln motion at any 



given time. Dunes 



have often invaded and destroyed farm lands and forests (Fig. 

 87), and have sometimes buried towns. The easiest and 

 surest method of stopping 

 migrating dunes is to plant 

 vegetation upon them (Fig. 

 88). This fastens the sand 

 and protects it from the wind. 

 Topography of dune 

 areas. Figures 89, 90, and 

 91 show typical dune topog- 



ISS99 



FIG. 87. Dune advancing up- 

 on a forest. Dune Park, Ind. 

 (Cowles.) 



raphies. Depressions are 

 sometimes as character- 

 istic of dune areas as are 

 the elevations (Fig. B, 

 Plate I; depressions 

 shown by hachures within 

 closed contours), and are 

 formed in a variety of 

 ways. They may be 

 scooped out by the wind. 

 More sand may be de- 

 posited around than on a 

 given place, which there- 

 fore forms a depression. 



v 8 --. Plant t in e'^h grass to stop g hifting dunes may fitf 

 the drifting OP sand. Near Province- , 



town, Mass. (U.s. Dept. Agr.) parts of valleys, whose 



