44 



NATURE STUDY REVIEW 



|9: :3— Feb., 19i; 



Fk;urk 1. — Dune advancing over the Calumet River. Partially buried 

 cottonwoods are shown on the active dunes and an oak forest on the 

 stable dunes. 



and is then blown about by the wind. Since the wind prevails 

 from the west in this latitude few dunes are found along the 

 west shore of the lake. 



Movement of the Sand. 



The great motive ix)wer in moving sand on the land is the 

 w^ind. Any small obstacle will cause upward ctu-rents in the wind 

 and it is these currents that enable the wind to pick up and keep up 

 the sand grains. But since each grain has a very small surface in 

 proportion to its weight it is not caried very high, hence the 

 movement consists primarily in rolling the grains along the sur- 

 face or, at best, in carrying them only a short distance and then 

 repeating the operation. The movement of sand, therefore, takes 

 place only in the lower air. If, for any reason, the wind is 

 checked the grains will fall and a pile of sand may accumulate. 



Work Done by Wind-Blown Sand. 



It is obvious, therefore, that a permanent or semi-permanent 

 checking of the wind will result in building a sand dune. Among 

 the most common obstacles that start a dune are young shoots of 

 trees, shrubs, and grasses. Some, as the cottonwood and dog- 

 wood, have the ability to send out roots from their trunks and 

 keep above the accumulating sand, thus formins: a constant ob- 

 struction to the wind and aiding rapid dune building. However. 



Figure 2. — Diagram illustrating the gentle windward 

 (a) and steep leeward (b) slopes of a dune. 



