BRIDGING THE AGES OF ICE 
355 
cumstances it is not at all surprising that some of the Iowa facts 
were misinterpreted and that their true significance was for a 
time overlooked. Then, too, the prevailing notion concerning 
lacustrine origin of the loess tended to obscure a proper under¬ 
standing of data accurately recorded. 
Notwithstanding the fact that Doctor McGee was inclined at 
the time to attach rather slight importance to his really monu¬ 
mental observations and to regard the phenomena which he had 
noted as indicating perhaps mere local advance of the ice-sheet 
it soon became manifest that the two till deposits, separated by 
a thick loess bed, was impeachable testimony in support of two 
distinct and great ice movements within the period of what pre¬ 
viously was regarded as a single one. So far as is known this 
appears to be the first and most important recorded evidence 
proving 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 the Iowa Academy of Sciences 
in 1890, it was shown that there were still other thick members 
to be reckoned with beneath the tills underlying the loess. In 
recent years officers of the State Geological Survey were inclined 
to regard it as representing the pre-Kansan Aftonian beds, and 
the basal till or Nebraska Drift. 
Conclusions reached by McGee may be reiterated as follows: 
'Tn order to state intelligently the working hypothesis suggested 
by the foregoing facts, it will be necessary to state briefly the 
conclusion as to the formation of asar and the determination of 
river courses, reached by the author after practically exhaustive 
survey® of the cenology of the northeastern quarter of Iowa. 
“The ice-sheet over this region was thin; not more than 500 feet 
in average thickness. Each pre-existing plateau, or ridge, ac¬ 
cordingly produced’a relatively considerable attenuation of the 
sheets. Three results followed: (1) The motion of the ice was 
retarded along the ridge, and its vertical pressure was reduced, 
thereby not only diminishing the rate of erosion, but heaping up 
an unusual thickness of morainic debris along the ridge in what 
9 A decade afterwards the results of this investigation were published in a sumptu¬ 
ous monograph by the Federal Government, under the title of “Pleistocene History of 
Northeastern Iowa,” in the Eleventh Annual Report of the United States Geological 
Survey, pages 190 to 577, 1893. 
