17 



H. A, Gleason presented an illustrated paper of which the 

 following is an abstract. 



THE VEGETATIONAL HISTORY OF A BLOWOUT. 



Blowouts are saucer-shaped or bowl-shaped excavations 

 caused by the action of wind on sand which is not sufficiently 

 protected by a plant covering. They may reach in Illinois a 

 length of 200 yards or more, and a depth of 10 to 30 feet. 



They consist of four physiographic divisions, with each of 

 which a definite association is correlated. The windward slope, 

 occupied by the windward slope association, is situated at the 

 west end of the blowout. On it the sand is being removed 

 by wind and is also sliding down by gravity. The vegetation 

 is composed principally of grasses derived from the vegetation 

 outside the blowout. The deepest part of the blowout is the 

 basin, from which sand is being removed by wind alone. The 

 basin association is composed of a few individuals of deep- 

 rooted perennials. The upward slope at the east side of the 

 basin is called the lee slope. On it the sand is merely in 

 motion, without any essential change in level. It is occupied 

 by the blowsand association, consisting of large numbers of 

 slender, quick-growing annuals. The east end of the blowout 

 is occupied by the deposits, on which sand is being piled by 

 the wind. The vegetation of the deposit association is princi- 

 pally composed of sand-binding perennials, through the agency 

 of which the sand is accumulated into dunes. 



The successional relations of these four associations are com- 

 plex, but lead as a rule to the ultimate dominancy of the blow- 

 sand association. When the sand has ceased blowing, stabili- 

 zation of the blowout begins in all four parts, and the area is 

 soon reoccupied by a more luxuriant vegetation. 



