238 MINNESOTA BOTANICAL STUDIES 



sions which are so numerous in certain parts of the hills and which 

 really exhibit the unique blow-sand conditions of the region. This 

 association includes the "Blow-out Formation" and the "Sand-draw 

 Formation" of Pound and Clements. 



Since the prevailing winds of the region are westerly and since 

 the blow-outs are the direct products of wind action, these peculiar 

 land forms are mostly confined to the western sides of the hills and 

 ridges. Blow-outs usually occur below the summit of the hill but 

 occasionally one finds a case where the whole top of a hill or ridge 

 has been blown away and the position is then occupied by an enor- 

 mous blow-out. The greatest number and usually also the largest 

 blow-outs occur upon the northwest exposures, but blow-outs range 

 from this position to the southwest exposures. The position de- 

 pends somewhat upon the shape of the particular hill in question as 

 well as its relation to the neighboring hills which may deflect the 

 wind currents toward a certain slope or crest. A single hill some- 

 times shows only one blow-out although this one may be so large 

 that the whole side or top of the hill is occupied. Frequently sev- 

 eral blow-outs originate upon a single hill or ridge and these may 

 coalesce rather early so that a single large crater is formed which 

 possesses several secondary depressions. 



The almost complete restriction of blow-outs to the westerly 

 slopes of the uplands results in the production of very different 

 appearing landscapes according as to whether you face eastward or 

 westward. Facing eastward you look directly into the many circu- 

 lar or crescent-shaped craters with always much bare sand exposed 

 over their inner slopes. These alternate in varying proportions 

 with hills without blow-outs which naturally show one of the other 

 upland plant associations. As you face westward the blow-outs 

 are never so conspicuous since you view the opposite sides of the 

 hills and even when blow-outs are seen in this view it is their highest 

 or eastern rim that one sees and as a consequence the depression is 

 not visible. In the case of some of the larger more active blow-outs, 

 when such sand is being hurled out of the depths and scattered upon 

 the lee slope of the hill or ridge, blow-out conditions are quite 

 readily appreciated even as one looks westward. These areas of 

 comparatively bare sand upon east- facing slopes are not common 

 however and when present they serve to break up the remarkable 

 continuity of the bunch-grass landscape. 



Blow-outs do not occur upon all hills. Even in those portions 



