THE WORK OF THE ATMOSPHERE 



23 



as shown in Fig. 1, PI. VI, p. 11. Caravans have been destroyed 

 in the African desert by sand storms. 



Fig. 14. Dune sand. held by brush fences on the Kurische Nehrung. 



Migration of dunes. Sand is being blown from the windward 

 side of a dune and dropped on the leeward side much of the time. 



Fig. 15. The remnant of a dune held by roots. The surrounding sand not 

 so held has been blown away. Head of Lake Michigan. 



This continued shifting of sand to the leeward side, results in a slow 

 migration of the dune in the direction of the prevailing winds. We 



