82 



The Blowout Formation 



The distinguishing feature of the blowout fomiation is the domi- 

 nance of the physical factors of the environment. The wind exer- 

 cises complete or almost complete control of the vegetation. The 

 formation is so open that its general appearance and color is that of 

 almost bare sand. The associations, being subordinated by the phys- 

 ical conditions, are distinguished from each other by differences in 

 physical environment ; the difference in plant and animal species i.'i 

 an effect rather than a cause. Changes in the life are determined by 

 changes in the physiography. In the prairie formation the normal 

 changes in the physical conditions are largely the work of the plants. 

 In the blowout formation, even the names of the associatiohs have 

 been taken from physical features. The prevailing tendencies are 

 continual shifting of the sand, which is thus kept in a sterile condi- 

 tion, the formation of blowouts and dunes in places where the vege- 

 tation is or has been of influence in modifying the action of wind 

 (PI. Ill, Figs. I, 2), and the formation of large, nearly level sandy 

 wastes, wherever the wind has long been the controlling factor (PI. 

 IV, Figs. I, 2, 3). This latter "blowsand'' (see p. 88) coniorma- 

 tion is due largely to the confluence of a number of blowouts, but is 

 very characteristic, and covers in the Havana region many times the 

 area of the isolated blowouts. Gleason's discussion of the forma- 

 tion of the 1>lowouts is without doubt the best we have ('10: 84-90). 



The blowout itself is a wind-formed excavation, normally origi- 

 nating in the FauiciDii pscudopiibcsccns association by gradual as- 

 cendency of the influence of wind, resulting in increasing openness 

 of the association. Blowouts sometimes start in the bunch-grass, 

 usually by accident. Figure 2, Plate II, shows the beginning of a 

 blowout on the slope of the dune to the left. A small bare expanse 

 from which the wind is gradually removing the sand is the first stage 

 of the blowout. It gradually deepens, the sand from the basin of the 

 depression being deposited above the general level on the lee side. 

 The deepening continues, the sides of the depression become steeper, 

 except the lee slope, up which the sand is drifted and deposited as 

 a low dune formation. The windward slope reaches the critical 

 angle, and from then on the removal of the sand from this side of 

 the depression is by gravity. There are thus four physiographic di- 

 visions of the blowout: (i) the basin, from which sand is being 

 removed by wind; (2) the windward slope, from which sand is be- 

 ing removed by gravity; (3) the lee slope, or blowsand division, 

 over the surface of which the sand is being merely drifted, without 



