TREATMENT OF SEEDING AND PLANTING SITES 193 



2. By the reduction of the gradient. Any reduction in the gra- 

 dient causes a corresponding decrease in the flow of water and 

 a consequent decrease in soil erosion. The only method of reduc- 

 ing gradient is by a system of terraces which can be undertaken 

 only at large expense. 1 



Ordinarily, works of the character mentioned cannot be under- 

 taken by private enterprise. In the full utilization of many of 

 our present denuded mountain lands, however, they will be neces- 

 sary. After a forest has been once established, the vegetation 

 itself prevents further soil instability. 



9. Overcoming Soil Instability of Dunes. Along the Atlan- 

 tic and Pacific coasts and on the shores of the Great Lakes are a 

 number of extensive areas of sand dunes. Inland sands of shift- 

 ing nature also occur in widely separated parts of the country, 

 but more particularly in the semiarid Southwest and in Nebraska 

 and the Dakotas. For the most part, these areas of shifting sand 

 were originally covered with vegetation. Through excessive 

 grazing, frequent fires or forest destruction, the surface soil became 

 exposed to the wind and weather, and, as a result, their instability 

 increased. The soil must be deficient in binding material, such as 

 clay and humus, and composed of moderately fine particles in order 

 to form dunes. When such soils become dry, they blow about and 

 form moving or wandering dunes in the form of ridges or hills. 

 The form and extent of these accumulations depend upon the 

 soil and the force and direction of the prevailing winds. 



Sand dunes have been comprehensively treated by Gerhardt 2 

 and many other writers in recent years. Extensive dune areas 

 in France, 3 Germany, and other European countries have been 

 reclaimed and covered with productive forests by artificial means. 

 Dune regions in the United States have been studied by Scribner, 4 

 Hitchcock, 5 and Westgate. 6 The most extensive attempts at rec- 

 lamation have been in the Cape Cod region of Massachusetts. 



1 Wang, Ferdinand: Fortschritt und Erfolg auf dem Gebiete der Wild- 

 bachverbauung. Wien, 1890. 



2 Gerhardt, Paul: Handbuch des deutschen Dunenbaues. Berlin, 1900. 



3 Brown, J. C.: Pine plantations on the sand-wastes of France. Edin- 

 burgh, 1878. 



4 Scribner, F.L.: Sand-binding grasses. (Yearbook, U.S. Dept. of Agr. 1898.) 



5 Hitchcock, A. S.: Methods used for controlling and reclaiming sand 

 dunes. (U. S. Bui PI. Ind., Bui. 57. 1904.) 



6 Westgate, J. M.: Reclamation of Cape Cod sand dunes. (Ibid., Bui. 

 65. 1904.) 



