300 C. R. Keyes — Eolian Origin of Loess, 



sippi River from Minnesota to southern Mississippi ; along the 

 Illinois and the Wabash from the points of their emergence 

 from the territory of the later glacial sheets to their mouths. 

 Along these valleys the loess is thickest, coarsest and most typi- 

 cal in the bluffs bordering the rivers, and grades away into 

 thinness, fineness and non-typical nature as the distance from 

 the rivers increases. In some instances the loess mantle rises 

 to the divide and connects with the similar deposit of an adja- 

 cent valley, but the law of progressive fineness and thinness 

 still holds. This relationship is such as to create a very strong 

 conviction that the deposit of the loess was in some vital way 

 connected with the great streams of the region." 



The Bluff loess is not to be confounded with other similar 

 fine silts that are found mingled with the glacial drift occurring 

 in many localities and that are, by some writers, called loess. 

 Along the Missouri River the Bluff loess forms a belt fifteen 

 to twenty miles wide. From the mouth of this river to the 

 Iowa line at least, the deposits appear to be much heavier and 

 the belt much wider on the left bank than on the right side of 

 the stream. The same seems true of the Mississippi River, at 

 least south of St. Louis. The characteristic great thickness and 

 coarseness at the river's edge, and, away from the stream to 

 the margin of the belt, the gradual change of the deposit to 

 greater fineness and less depth is everywhere apparent. 



Missouri's great river, in its course across the State, passes 

 from the drift-covered region to the driftless, and crosses and 

 recrosses from one area to the other. The belt of Bluff loess 

 lies sometimes on driftless areas, sometimes on what appear to 

 be older silts, and then on drift and sands having a glacial 

 origin. It appears, to occupy the tops of the bluffs irrespective 

 of underlying formations. 



The Missouri River has long been known to be a stream 

 that is heavily ladened with silt. Vast bars exist along its 

 course, often a mile or more wide, and continuous on one side 

 or the other the whole length from Dakota to its mouth. 

 These bars are bare for a period of two or three months in the 

 spring. During this time and immediately after the June 

 floods they constitute boundless mud-flats which soon dry. 



During certain periods of the year, marked by high winds, 

 great u dust storms " prevail on the Missouri and middle Mis- 

 sissippi rivers. Down or across the valley sweep the strong 

 currents of air, catching up the light silt particles from the 

 river bars, whirling them about, and rolling them in dense yellow 

 clouds up and out of the valley, and over the high bluffs into 

 the open country beyond. The heavy dust'-clouds rise high 

 into the air. A score or more miles away from the stream, 

 the latter's course is marked by the dark pall that hangs over it. 



