MODE OF DEPOSITION OF THE LOESS. 183 



immediate borders of the valleys, i. e., to the portion of the loess which 

 may have been deposited after the retirement of the floods from the uplands. 

 The most notable exceptions are their occurrence in the paha ridges and in 

 the heavy loess bordering the lobes of Iowan ice in eastern Iowa and west- 

 ern Illinois. It seems difficult to refer the latter deposits to a much later 

 date than the culmination of the glacial flood, though it is possible that the 

 filling was continued along the immediate borders of the ice lobes to a time 

 considerably later than the culminating stage of the flood and even to a 

 time when vegetation had obtained a foothold on the neighboring silt- 

 covered tracts. If the deposition of the portion of the loess containing 

 fossils can be shown to have continued down to a time when the floods had 

 retired to the limits of the main valleys, objections drawn from the char- 

 acter of the fossils against the aqueous deposition of loess on the bordering 

 uplands would be fully met. It becomes, therefore, a matter of much 

 importance to settle definitely the age of the fossil-bearing deposits with 

 reference to the culminating stage of the glacial water. 



Before leaving this subject a few remarks seem in place concerning the 

 deposits of loess which are evidently aeolian. The first seolian loess to attract 

 the writer's attention, as such, is found along the east border of the Missis- 

 sippi Valley opposite Burlington, Iowa. The loess there has accumulated 

 in dunes which give it a relief of 25 to 75 feet above the neighboring 

 uplands on the east. When viewed from the uplands it appears as a bil- 

 lowy ridge fringing the river bluff. This loess is found to be fossiliferous 

 and calcareous, and were it not for its topograph)- and for a slight admix- 

 ture of sand recently drifted to it from the broad bottoms of the Mississippi 

 River it would present no essential difference from the flat-surfaced portions 

 of the loess along the borders of the river elsewhere. A similar relief of 

 the loess on the brow of the bluff, above uplands to the east, has been since 

 noted just below Alton, Illinois. It also occurs to a slight degree on the 

 east border of the Mississippi above Burlington. It is now known that the 

 general thickness of the loess on the east side of the Mississippi Valley, from 

 the Driftless Area southward through the entire length of the State of Illi- 

 nois, is markedly greater than on the west side of the valley in Iowa and 

 Missouri, probably twice as great. A similar difference is found on the 

 east and west sides of the Wabash Valley; on the Illinois the difference is 

 not so marked. It is thought that this extra thickness of the loess on the 



