PAPERS ON GEOLOGY 115 



Supporting the view that the loess which mantles the slopes 

 below the highest level of the drift is an original deposit, is the 

 general relation of such loess covered slopes to areas of flood 

 plain and to the direction of prevailing winds. 



Bordering the east bank of Spoon river, in Sees. 19 and 20, 

 T. 9 N., R. 3 E., and Sees 24 and 25, T. 9 N., R. 2 E., and 

 farther south in Sees. 10, 15, and 22, T. 8 N., R. 2 E., the till 

 is entirely concealed down to the level of the flood plain 

 by a covering of wind blown material, in some places loess and 

 in others sand. The only evident source of the sand in this 

 region is the adjacent flood plain of Spoon river to the west- 

 ward, and this sandy alluvium would furnish a ready supply 

 both of the sand and the finer loess material. Observations 

 show that both sand and loess are at present being deposited 

 on the slopes on the east side of the valley by the winds which 

 gather the most of the material from the surface of the adja- 

 cent flood plain. 



Relations similar to those in the vicinity of Spoon river 

 generally exist between the thicker sand and loess deposits, 

 not only on the bluffs, but also on the more gentle slopes bor- 

 dering the windward side of the valleys and the areas of flood 

 plains along the Sangamon and Illinois, and the other rivers, 

 as well as many of the smaller streams, in Iowa and Illinois. 



Some of the fine material gathered from the flood plain of 

 the river may be carried by the wind for several miles, and 

 this material, together with the dust swept by the winds from 

 exposed portions of the intervening uplands, may be lodged on 

 the bluffs and opposing slopes along smaller streams at a dis- 

 tance from the river and thus supplement the local supply. 



The general relations of the loess covered slopes to the 

 direction of the prevailing winds and the possible source 

 of supply of the material together with the lack of any 

 trace of removal or diminution in the quantity of the 

 loess material at the tops of the hills in such places, make it 

 practically certain that the loess occurring on the slopes below 

 the uppermost level of the drift has been mainly deposited by 

 winds since the present, gentle gradient of the slopes was de- 

 veloped, and that it is largely in its original position. 



CLIMATAL IMPLICATIONS OF THE LOESS 



Wind deposits are characteristic of arid climates, hence some 

 geologists have assumed that arid conditions must have pre- 

 vailed in the upper Mississippi Valley during the time of max- 



