24 , PHYSIOGRAPHY 



may get some idea of the extent to which dunes migrate, in various 

 ways. When the dune which buried a forest moves on, the trees 

 which were covered and killed may be seen again, as shown in Fig. 

 2, PL VI, p. 11. So disastrous is the migration of dunes along some 

 coasts, that steps are taken to prevent it. If a dune is covered with 

 vegetation, its position is not likely to be changed so long as the 

 plants remain, for the plants hold down the sand. Trees, shrubs, 

 etc., which will grow in sand are sometimes planted on dunes, as 

 soon as they are formed, to prevent further drifting (Fig. 14). This 

 is done at various points on the western coast of Europe, where land 



Fig. 16. Erosion columns in Monument Park, Colo.; partly the product 

 of wind erosion. (Fairbanks.") 



of wind erosion. (Fairbanks.) 



is valuable, and it has been done to some extent in our own country, 

 as at San Francisco, where the westerly winds drift sand in from the 

 shore. Fig. 15 shows the effect of a clump of trees in holding sand. 

 All the dune except the part held by the roots of the trees has been 

 blown away. 



Eolian (wind-blown) sand is not always heaped up into dunes. 

 It is sometimes spread somewhat evenly over the surface where it 

 lodges. Eolian sand is therefore more widespread than dunes are. 



Abrasion by the wind. Sand blown against a surface of rock 

 has the effect of a sand-blast, and wears the rock away. If some 

 parts of the surface against which sand is driven are harder than 

 others, the softer parts are worn the more rapidly. Where abundant 

 sand is driven by the wind, projecting rocks are often carved into 



