GEOLOGIC WORK OF ATMOSPHERE 



Extensive deposits of wind-blown dust are known. Consider- 

 able beds of volcanic dust, locally as much as 30 feet thick, are 

 known in various parts of Kansas and Nebraska, hundreds of miles 

 from the nearest volcanic vents. In some parts of China there is 

 an extensive earthy formation, the loess (Fig. 5), in some places 

 reaching a thickness of hundreds of feet, much of which is believed 

 to have been deposited by the wind. The loess of other regions 

 has been referred to the same origin, and much of it is quite certainly 

 eolian. From the flood plains of such rivers as the Missouri, clouds 

 of dust are swept up and out over the adjacent high lands at the 

 present time, especially when the surface of the flood plain has 

 become dry after floods. This dust is very like loess, if, indeed, it 

 is not loess. 



The transportation of dust is important wherever strong winds 

 blow over dry surfaces free or nearly free of vegetation, and com- 

 posed of earthy or sandy matter. Its effects may be seen in such 

 regions as the sage-brush plains of western North America. The 

 roots of the sage-brush hold the soil immediately about them, but 

 between the clumps of brush where there is little other vegetation, 

 the wind has in many places blown away the soil to such an extent 

 that the base of each shrub stands up several inches, or even a foot 

 or two, above its surroundings. Some of the mounds in this posi- 

 tion are due partly to the lodgment of dust about the bushes (Fig. 6). 



Fig. 6. Shows the effect of sage-brush or other similar vegetation in holding 

 sand or earth, or in causing its lodgment, in dry regions. 



