THE NEBRASKA SAND HILLS. 281 



southwest, depending somewhat upon the shape of the hill concerned 

 and its relation to the adjacent hills. Blow-outs do not occur on all 

 hills, nor does a single hill show more than a single blow-out, as a rule. 



On an exposed upper slope when the vegetation becomes broken or 

 seriously depleted from any cause, the wind as it sweeps up the slope 

 catches the sand and carries it over the crest of the hill a few yards 

 farther away and deposits it upon the lee face of the hill. In this way 

 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 the 

 deep-seated roots of woody plants frequently appear strewn over the sur- 

 face 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 

 developed 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 re- 

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

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

 Nothing 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 

 feeriously 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 

 away and more continres to slide in. and in this manner the blow-out 

 increases in area as well as in depth. These two processes continue for 

 a number of years until, in many cases, the well-developed crater-form 

 depression is blown out of the hill. Naturally with the increasing depth 

 of the blow-out the direct force of the wind becomes considerably checked 

 by the prominent rim of the crater. But peculiarly enough, as the wind 

 strikes the farther slope of the blow-out a reverse current is developed 

 which strikes beneath the rim and dips into the bottom of the crater. 

 In this way a spiral wind movement is fiequently developed and the 

 wind reaches to the very bottom of the blow-out, which may now be fifty 

 or more feet below the rim. This grinding action of the wind continues 

 to loosen more sand at the sides, cau.=ing it to slip more and more into 

 the bottom, where the wind catches it and hurls it up over the sloping 



