THE DYNAMIC NATURE OF SOU, PHENOMENA 21 



deposits on snow-capped peaks. Soil material on roofs and 

 similar inaccessible places has been observed many times, and 

 testifies to the continual activity of the wind. The burial of 

 objects even of considerable size by wind-borne soil gives like 

 testimony. 



Measurements of the amount of action of wind in trans- 

 locating soil material are rare and probably have a qualitative 

 value only. But Udden 1 in what appears to be a conservative 

 calculation, finds "the capacity of the atmosphere [over the 

 Mississippi Valley] to transport dust is 1000 times as great as 

 that of the [Mississippi] River." The wind seldom is carrying 

 anything like so great a load as it is capable of carrying. That 

 is, the wind in its attack upon the land surface does not ordinari- 

 ly obtain so large an amount of material capable of being wind- 

 borne as it is possible for the wind to carry when suitable ma- 

 terial is artificially provided. It should be remembered that, 

 speaking generally, the velocity of the wind is lower just at the 

 surface of the ground than at heights above, and it is necessary 

 to get the soil material above the surface before the wind can 

 exercise its full efficiency as a carrying agent. 



Moreover, wind-borne material is constantly being deposited 

 as well as being removed from the land surface. It is evident, 

 however, that this movement of soil material by winds is very 

 great, and there is no reason to believe that it is of any less 

 importance in other areas than in the Mississippi Valley. It is 

 also evident that the individual grains in any surface soil of any 

 particular field or area are continually and more or less rapidly 

 changing, and the farmer is not dealing to-day with just the 

 same soil complex he faced a few years back, or will face a few 

 years hence. 



But besides the movements of the solid components of the 

 soil by translocating agencies, other movements are constantly 

 taking place. Whenever a moderately dry soil becomes wetted, 

 it "swells up" until a certain critical amount of moisture is 

 present above which there is a shrinking. But as a wet soil 



1 Erosion, transportation and sedimentation performed by the atmos- 

 phere, by J. A. Udden, Jour. Geol., 2, 318-331 (1894). 



