IOWA ACADEMY OF SCIENCES. 65 



Iowa, and hence the numerous problems with which more 

 recent investigators have been chiefly concerned, were not 

 considered. 



It remained for McGee, from 1875 to 1880, to introduce meth- 

 ods of investigation that finally furnished the key to the inter- 

 pretation of the complex system of deposits embraced in the 

 Pleistocene of this part of Iowa.* He pointed out that the drift 

 was certainly dual and not single as had been previously sup- 

 posed. He insisted that the interval between the two glacial 

 invasions was one of enormous length, going so far in one of 

 his earlier papers as to claim that the older drift might prob- 

 ably be Miocene Tertiary in age, and that the interval which he 

 so clearly recognized might represent the whole of the Pliocene. 

 He took account of the forest bed which he regarded as lying 

 between the earlier and later drift — the lower and upper till, 

 as he finally called them. He furnished the criteria for dis- 

 criminating the two till sheets by their color and contents. He 

 led the way to a rational and satisfactory classification of the 

 Pleistocene beds of the Mississippi valley. His insight and 

 farsight in the 23resence of such complex problems in an 

 unworked field betokens genius of a high order. 



McGee looked upon the forest bed as the plane of division 

 between his lower and upper till, but later investigators fol- 

 lowing the lines which he pointed out, have reached the conclu- 

 sion that his lower till embraces two distinct drift sheets, and 

 that it is between these two that the forest bed invariably lies. 

 Thus there are three drift sheets in northeastern Iowa, and in 

 the recent literature referring to Pleistocene geology they are 

 known respectively as sub Aftonian or Albertan, Kansan {j,nd 

 lowan. 



No forest material has been observed between the Kansan 

 and the lowan, but in this situation there occur extensive beds 

 of stratified sands and gravels. 



The forest bed between the first and second drift sheets is 

 frequently accompanied by beds of peat that range from one 

 inch or less to three or four feet in thickness, and cover areas 

 of considerable extent. When peat is absent there is often 

 evidence of an ancient soil, humus-stained and weather-stained 



*Ou the Relative Positions of tiie Forest Bed and Associated Drift Formailons in 

 Northeastern Iowa. Am. Jour. Sci., II[, Vol. XV, pp 339-341, 1878. 



Oa the complete series of the Superficial Geol. Formations ia Northeastern Iowa. 

 Proc Am. As'n. Ad. Sci., Vol. XXVII, pp. 198-231, 1879. 



See also The Pleistocene History of Northeastern Cowa, U 3. Geol. Sur.. Eleventh 

 Ann. Kept., pp. 190-577. Wash., 1893. 



5 Lla. Acad. Sci, Vol. vj [April 29. 1898.1 



