164 



tually balanced by that deposited, and the whole remaining 

 approximately in a state of equilibrium. On the crests of the 

 dunes, however, where the sand is more exposed to the wind 

 and more is blown away than is deposited, the plants are fre- 

 quently separated by shallow excavations, and may sometimes 

 be uprooted, entirely, thereby permitting the free action of the 

 wind, and leading to the formation of a blowout. In fields 

 that have been pastured, there is frequently a black crust an 

 inch or so deep formed over the surface, probably by the decay 

 of the grass and leaves trampled into the sand by cattle. In 

 dry weather this crust is quite hard, and effectually checks the 

 shifting of the sand. 



The resistance offered by the vegetation is, indeed, very 

 effectual, and some old settlers say that there were no blowouts 

 in the sand region in the middle of the last century, when set- 

 tlements were first begun, but that they have all been formed 

 since then. The report of the Illinois Geological Survey pub- 

 lished in 1S70 does not mention them, and it is improbable that 

 such conspicuous blowouts as exist to-day could have escaped 

 the attention of the State Geologist, who traveled over the 

 country on horseback. There are, however, a few large fields of 

 blowouts and blow-sand that have been in existence as long 

 as the neighboring residents can remember, such as the Devil's 

 Neck (PI. XII., Fig. 1; XIV., Fig. 1), north of Topeka, where a 

 field of about eighty acres is entirely covered with blow-sand. 



Plowing is generally believed to be the cause of the forma- 

 tion of blowouts, the natural vegetation being thus destroyed, 

 and the sand left exposed to the winds of winter and spring 

 after the cultivated crops are removed. All of the Miami loam 

 between the sand deposits is under cultivation, and the fre- 

 quent efforts of the farmers to extend their fields upon the dunes 

 have often led to disastrous results. Adjacent fields of fertile 

 soil are known to have been ruined in that way (PI. XL), and 

 at present most farmers let the sand stand unused, or use it for 

 pasture only. It is also said that cattle may destroy the veg- 

 etation, and that blowouts may be started in this way. 



When the protective covering of vegetation is once broken, 



