Pool: THE VEGETATION OF THE SANDHH^LS OF NEBRASKA 237 



A. Iiallii may be more or less common and may be of either relict or 

 invading nature. 



Occasionally other typical upland plants make their appearance 

 in this association as secondary species but the typical nature of the 

 association is expressed in the dominion of Muhlenbergia pun gens 

 with the presence of relatively few associates. When the principal 

 and secondary species increase notably in frecjuency and abundance, 

 the association rapidly approaches the more extensive bunch-grass 

 association. 



THE BLOW-OUT ASSOCIATION 



The most striking and peculiar habitats of the uplands in the 

 sandhills of Nebraska are the blow-outs. The present blow-outs 

 appear to be the remnants of a condition that was probably general 

 and much more extreme over the whole region hundreds of years 

 ago when the first plants began to migrate into the region. These 

 blow-outs are quite different in form and ecological relation from 

 those that occur in other parts of the world, Cowles (30), Gleason 

 {2>7), Cockayne (26). Our blow-outs are rounded or more com- 

 monly irregularly conical depressions of varying depth and diam- 

 eter, formed by the blowing of the sand and vegetation from certain 

 areas on the upper slopes and crests of the hills and ridges.- The 

 more or less conical depression is sometimes almost circular in out- 

 line. An irregular form is, however, more common both as to the 

 perimeter of the top and the configuration of the inner slopes. One 

 side of the rim is usually considerably lower than the opposite side 

 because of the slope of the hill or ridge within which the blow-out 

 has been formed. Landslides, the presence of stratified sands, the 

 coalescence of neighboring blow-outs, and the encroachment of 

 vegetation are factors that frequently unite in the production of 

 bizarre forms characteristic of the rim and inner surfaces of blow- 

 outs. 



The size and depth of blow-outs vary with the position and the 

 age of the depression. In the beginning blow-outs cover a few 

 square yards and are but a few inches in depth, but in the extreme 

 cases they become hollowed out to a depth probably exceeding 100 

 feet while the greatest circumference may be more than 600 feet. 

 I am fully aware of the presence of plane sand-sweeps that are 

 much broader than this and of sand-draws sometimes more than a 

 mile long, but these should not be confused with typical blow-outs. 

 The term blow-out should be restricted to the crater-form depres- 



