Eolian Loess. 203 



Theoretical Explanation. The seventh case may be suf- 

 ficiently accounted for by conceiving it to be quite analogous 

 to the formation of common snowdrifts. In the other cases 

 there is the difficulty of raising the material to greater height 

 and spreading it out as a thin sheet. The conditions appar- 

 ently essential to these are the following : 



(1) Broad, bare areas of sand bars or mud flats, where the 

 wind has free sweep at them. 



(2) A precipitous slope lying athwart the course of the 

 wind, steep enough to turn the stream of air sharply upward, 

 with velocity sufficient to prevent any wind striking the top of ■ 

 the bluff horizontally ; in fact, so as to produce a still area with 

 a partial vacuum there. Such areas are easily found on flat- 

 topped buttes, or on flat-roofed buildings on windy days. I 

 have frequently found a difference of .02 inch air pressure, on 

 the same level, between the verge of the level in the wind and 

 back a few feet in the still air. 



(3) A flat area of some extent, covered with vegetation of 

 some sort, which will prevent the dust which settles there from 

 being blown away by the. gentler breezes which will strike the 

 surface horizontally, and from the wash of the rains. Grass 

 is probably best suited to do this. 



Under such conditions the wind gathers the sand and dust 

 from the bars on windy days. They may be often seen rising 

 like smoke along such streams as the Missouri. When the 

 air is turned upward by the steep bluff, the sand is mostly 

 arrested at lower levels. The dust goes on, and may be aug- 

 mented by more wTenched from the side of the bluff where the 

 friction is great. 



After the wind regains its horizontal direction after its up- 

 ward turn, the dust will gradually and quite evenly settle out 

 of it into the still space below, where it is apt to rest protected 

 by the vegetation from the rains and horizontal breezes, until 

 it is undermined and carried away by ravines cutting back 

 from the adjacent slopes. 



University of Kansas, Lawrence. 



