58 F. W. Sardeson—What is the Loess? 



Art. VIIL— What is the Loess f by F. W. Sardeson. 



There is wide difference of opinion regarding the origin 

 of the American loess. Is it an aqueons or an geolian deposit ? 

 Many believe it to be partly aqueous, partly asolian, and thereby 

 raise the very important question of how to distinguish the one 

 kind of loess from the other. To those who view the loess as 

 seolian, this discrimination is easy, for the loess is all seolian 

 except locally where distinct sedimentary characters appear. 

 The loess is interpreted by them as wind-carried dust, deposited 

 on a land surface interrupted by streams and ponds. 



The loess viewed as a sediment is also easily distinguishable 

 from other sediments, but requires then the very difficult 

 explanation of how a little altered glacial sediment could be so 

 peculiar, — in fact like dust deposit. Negative evidence to this 

 theory is found in a region like Minnesota, where there are 

 thousands of existing and extinct lakes of glacial origin, in not 

 one of which is loess known to have deposited, — not even in 

 Lake Agassiz. They have in them gravels, sand and silt, and 

 around them are beaches. No small nor shallow lake could 

 have spread over the extremes of altitude on which the loess 

 lies, and a necessarily huge body of water must have left some 

 beaches and coarse sediments besides silt or loess. It was not 

 a lake that deposited the loess. The loess could also scarcely 

 be all a glacial sheet flood deposit. For example, the Iowan 

 loess lies beyond the Iowan drift sheet as it were its continua- 

 tion. Either the loess is the older and hence remains intact 

 only beyond the Iowan glacier's domain, or else the loess is con- 

 temporaneous and deposited by agency of the same glacier's 

 waters. The latter is the sedimentary theory. But the theory 

 does not explain why the glacial waters ceased depositing 

 loess when the glacial retreat began. Iowan loess does not 

 cover the Iowan drift,* and this fact the seolian theory alone 

 can explain. Partial or entire recognition of the seolian origin 

 is necessary. 



Partial recognition does not, however, remove all incon- 

 sistencies, because " no means of discriminating between the 

 two kinds of loess are yet known to be formulated,"f as the 

 advocates of the mixed hypothesis^ say. Prof. T. C. Cham- 

 berlin teaches this double origin of the loess, and his view, as 

 understood by the writer, is that the glacial waters flooded 



* Prof. S. Calvin discredits the reported slight occurrence of this loess on the 

 Iowan drift. Iowa Geol. Sur., vol. viii, p. 174; see also p. 339. 

 f C. R. Keyes, this Journal, vol. vi, p. 229. 

 % T. C. Chamberlin, Jour. Geol., vol. v, p. 795. 



