94 



PHYSICAL GEOLOGY 



as roads, railroads, towns, boundary lines, and the like, indicated 

 in black. 



FIG. 84. Model of ideal landscape. (U.S. Geol. Surv.) 



FIG. 85. Topographic map of ideal landscape. (U.S. Geol. Surv.) 



Migration of dunes. If winds strike the base of a dune 

 with less than they are able to carry, as happens frequently, 

 they pick up sand from the dune surface and move it toward 

 or to the crest, beyond which it falls. By this transfer of sand 



