412 GEOLOGY. 



The fact that the chief loess formation of the drift region is related, 

 in the way above described, to the area of the Iowan drift, has led 

 to the conception that at the time of the Iowan ice invasion, the glacial 

 streams were more sluggish and widely wandering than in most other 

 stages, and that by fluctuations between flood and recession, and by 

 shif tings, they exposed more extensive silty flats, while at the same 

 time the climate was more arid, the silt flats more quickly dried, and 

 the dust more freely picked up by the winds and distributed over 

 the adjacent uplands. It is a singular fact that the out wash from 

 the ice edge during the Iowan and at some other stages, has left little 

 record of itself, unless the loess be its record. Gravel trains of moment 

 have not been found. The loess deposits seem to be, in some way, re- 

 lated to these stages, and both phenomena, perhaps, imply aridity, 

 strange as that may seem in a glacial epoch. 



Opposed to the idea of a strict correlation with an ice stage, Shimek 

 has urged that the mollusks whose shells are the chief fossils of the 

 loess are such as inhabit the region to-day, and do not indicate, by 

 pauperate forms or otherwise, such climatic conditions as might natu- 

 rally be assigned to the near presence of an ice-sheet. A notable 

 dwarfing of the fossil species in the loess had previously been announced, 

 and regarded as an evidence of rigorous climate. Shimek suggests 

 the interglacial accumulation of the loess, and a careful test of this 

 hypothesis is merited. It is consistent with the fact that there is often 

 an aggregation of stones, pebbles, etc., on the surface of the till, below 

 the loess (Fig. 525). The concentration of stony matter here has been 

 interpreted as the result of surface wash, after the deposition of the 

 till below, and before that of the loess above. 



The deposits in the West called loess seem to be in part fluvial and 

 in part eolian. 



Bull. Phil. Soc. of Wash., Vol. IV., Bull. Geol. Soc. of Am., Vol. V, Science, New Ser., 

 Vol. V; Shimek, Am. Geol., Vols. XXVIII and XXX, Bull. la. Lab. Nat. Hist., Vols. 

 I, II, and V, Proc. la. Acad. Sci., Vols. Ill, V, VI, and VII; Leverett, Am. Geol., 

 Vol. XXXIII, and Mono. XXXVIII; Calvin, Bull. Geol Soc. Am., Vol. X, p. 119; 

 Chamberlin, Jour, of Geol., Vol. V, 1897; Hershey, Am. Geol., Vols. XII and XXV, 

 1900; Fuller and others, Patoka and Ditney, Ind., Folios, U. S. Geol. Surv.; Davis; 

 Explorations in Turkestan, 1905 



