CENTURY OF IOWA GEOLOGY 421 



regarded as a single one. So far as is known this appears to be the 

 first and most important recorded evidence showing conclusively 

 the complex character of the Ice Age. 



Of similar import was the somewhat later description of a great 

 drift section several miles farther south on the Des Moines river. 

 In a paper read before our Academy, in 1890, it was shown that 

 there was still another thick interglacial member to be reckoned 

 with below the till-sheet underlying the loess. In later years of- 

 ficers of the Geological Survey were inclined to regard it as repre- 

 senting the pre-Kansan Aftonian sands and gravels. 



As it is, our fellow lowan narrowly escaped making one of the 

 great half dozen geological generalizations of the nineteenth cen- 

 tury — the establishment of the fact of the complexity of the great 

 Ice Age. 



PHYSIOGNOMY OF ANCIENT CONTINENTAL GLACIERS 



The landscape expression of our state during glacial times is 

 vividly set forth in fancy by W. J. McGee, in his great memoir^'' on 

 the "Pleistocene History of Northeastern Iowa." x^t the time when 

 the fieldwork was chiefly done the idea of a possible duality of the 

 Glacial Period was new. Suitable criteria for correlating observa- 

 tions had yet to be formulated. The character of the phenomena 

 were also unique in the annals of geological science. Here are 

 McGee's own words as he pictures the conditions : "The most 

 startling induction of geology, if not of modern science, is the gla- 

 cial theory ; but in the solution of the problem it is necessary to do 

 more than assume the existence and action of the great sheet of 

 ice hundreds or thousands of feet in thickness and hundreds or 

 thousands of miles in extent. In order to explain the sum of the 

 phenomena it is necessary to picture the great ice-sheet not only 

 in its general form and extent but in its local features, its thickness, 

 its direction and rate of movement over each square league, the 

 inclination of its surface both at the top and the bottom, and the 

 relation of these slopes to the subjacent surface of the earth and 

 rock ; and all this without a single stria or inch of ice-polish, save in 

 one small spot, in the whole tract of 16,500 square miles. It is 

 necessary to conceive not only the mode of melting of the ice at each 

 league of its retreat, but also every considerable brook, every river, 

 and every lake or pond formed by the melting, both at its undcr 

 surf ace and on its upper surface ; it is necessary to restore not only 

 the margin of the mcr dc glace under each minute of latitude it oc- 



"Eleventh Ann. Rept. U. S. Geol. Surv., pp. 110-577, 1893. 



