220 



THE POPULAR SCIENCE MONTHLY 



Fig. 7. Detail in Blow-out shown in Fig. 5. The only plant here is 



Redfieldia flexuosa. 



as more and more sand is carried away and the up-rooted plants are 

 swept on with the gale, the embryonic blow-out comes rapidly into ex- 

 istence. At this early stage it appears as an area of bare sand a few feet 

 or yards across, over which the wind sweeps and continues to eat its way 

 deeper and deeper into the sand. During this early stage tlie deep- 

 seated roots of woody plants frequently appear strewn over the surface 

 of the shallow depressions until the wind has finally eaten its way far 

 below the point of penetration of the deepest rooted plants. At last the 

 whole rounded or conical hill top is blown away and a deep crater is de- 

 veloped in its stead. 



The two chief factors that enable the wind to begin this work of de- 

 struction are fire and over-grazing. Both factors frequently result in 

 reducing the vegetation to a point below effective wind resistance and as 

 soon as this is done, if the exposure be right, wind erosion begins. 

 !N"othing is quite so terrible as a prairie fire in paving the way for shift- 

 ing sands and the development of blow-outs, since in such cases abso- 

 lutely everything above the surface is destroyed. And so if cattle are 

 allowed to run for too long a time over a given range the grasses are 

 seriously reduced and the soil is tramped bare of plants for considerable 

 distances, making it very readily possible for the wind to strike at the 

 open sands. The effects of over-grazing are contrasted to a striking de- 

 gree in the Sand Hills, where a fence separates the over-grazed pasture 

 from the ungrazed range. Such sights have resulted in the enactment 

 of grazing laws which naturally do not in all cases please the cattlemen, 

 but they do usually protect the range and make it more stable. 



When the young blow-out is no more than a foot in depth the sand 

 begins to slide into the depression from the sides. This sand is blown 



