242 MINNESOTA BOTANICAL STUDIES 



blow-out development, I have spoken (53) of this as the action of a 

 "sand mill." 



After many years of this sort of growth blow-outs at the end 

 of their maximum activity become depressions varying in size and 

 form as has already been indicated. The inner slope which faces 

 the wind is always the longest and most extensive side of the inte- 

 rior. The gradient of this slope is usually about 30°, although this 

 angle varies somewhat in different blow-outs according to the degree 

 of compactness of the sand. The opposite or lower side of the 

 blow-out is usually much steeper, often being nearly perpendicular 

 in places. This condition is due to the fact that the sand con- 

 tinually rolls from this side and is blown out over the opposite side 

 while the side toward the wind is never exposed directly to wind 

 action. The deflected currents aid in undermining this margin and 

 so land-slips are very common on this side of the blow-out but the 

 slope remains comparatively steep unless indeed the bottom of the 

 blow-out occurs on a level with the slope of the hill when the conical 

 form becomes less prominent. Very commonly under these condi- 

 tions the lateral sides of the depression are much steeper than the 

 wind-swept slope. 



The lee slope of a hill with an active blow-out often becomes 

 much steeper than any other exposure because of the influence of 

 sand-collecting plants which become established in such places. 

 These, usually grasses, keep above the accumulating sand deposits 

 by means of a combined horizontal and vertical growth and at the 

 same time hold the sand at a steeper gradient than it would assume 

 in the absence of the sand-collectors. Such slopes often become as 

 steep as 60°. 



Blow-outs were found in Garden County in rather low hills 

 near the lakes from which the sand had been removed until the 

 saturated zone was reached. The bottoms of such blow-outs were 

 covered by a few inches of clear water and in a single case I found 

 Lemna minor and a species of Potamogeton in such a place. The 

 same "blow-out pond" was furthermore interesting because of the 

 presence of a thin marginal belt of Eleocharis acicularis and Cyperus 

 iniiexus. At the time I saw this blow-out the sand was encroaching 

 upon the depression from a neighboring blow-out and it appeared 

 that in a few years this area in which pond-weeds and blow-out 

 grasses were so closely associated would be completely covered by 

 sand. 



