AGE AND MODE OF DEPOSITION OF THE LOESS. 299 



It is certain that the loess on the south border of the lowau drift in eastern 

 Iowa and western Ilhnois is of the same age as that drift, for the two 

 deposits connect completely at their borders. The loess sets in abruptly at 

 the south border of the lowan drift like an outwash from the low an ice 

 sheet. The silt appears to hold the same stratigraphic position throughout 

 its entire extent, and no reason has been found for excluding any part of 

 it from the lowan stage. It is not certain, and perhaps it is scarcely 

 probable, that the deposit is everywhere an outwash from the lowan ice 

 sheet. But it seems to have been deposited at a single epoch of general 

 lew altitude and slack drainage. Its wide distribution on the uplands may, 

 however, be due immediately to the agency of winds. 



MODE OF DEPOSITION. 



The mode of deposition of the loess and associated silts has been and 

 still remains one of the most puzzling problems of Pleistocene geology. 

 At present little more can be done than to state the several hypotheses and 

 discuss the difficulties of interpretation. This has been attempted by the 

 writer in Monograph XXXVIII of this Survey, and but little can be added 

 to that discussion. The leading hypotheses are known as the aqueous and 

 the eolian. But as a portion of the deposit in southeastern Ohio has been 

 attributed to organic agencies,' that interpretation also should be considered. 



It is generally recognized that difficulties attend the application of any 

 one hypothesis to the entire deposit. Probably no one questions the view 

 that the influence of the wind has been important, and nearly all will concede 

 that water, or at least imperfect drainage, has been influential. The division 

 of opinion, therefore, is concerned with the relative importance of wind and 

 water in the distribution of the loess. 



The question of the influence of the atmosphere as an agent of erosion, 

 transportation, and sedimentation has been carefullj^ examined and ably 

 discussed by Udden, with the result of showing that it is competent to per- 

 form as much work as is required in producing this deposit." Chamberlin 

 has recently discussed the peculiarities of distribution and considered the 



1 See Edward Orton: Geology of Ohio, Vol. 1, 1873, p. 445; also G. F. Wright: Bull. U. S. Geol. Sur- 

 vey No. 58, 1890, p. 104. 



^ The main results of Udden 's investigations are presented in the following papers: Erosion, 

 transportation, and sedimentation performed by the atmosphere: Jour. Geol., Vol. II, 1894, pp. 318-331 ; 

 Loess as a land deposit: Bull. Geol. Soc. America, Vol. IX, 1897, pp. 6-9; The mechanical composition 

 of wind deposits: Augustana Library Publications, No. 1, 1898, Lutheran Augustana Book Concern, Kock 

 Island, 111. 



