420 IOWA ACADEMY OF SCIENCE Vol. XXVI, 1919 



is neither so complete nor so widely applicable as was at first sup- 

 posed. What is even more important to its scientific value than the 

 mere statement of the conception alone is the recognition of the fact 

 that there is not one, but many, glacial epochs in the earth's his- 

 tory. Of course CroU's hypothesis provides the necessity of suc- 

 cessive glacial periods, but it soon becomes apparent that his astro- 

 nomical dates are too far apart to satisfactorily account for the 

 vicissitudes of the epoch which we are now mainly studying. So 

 we have to go back to the testimony of the glacial deposits them- 

 selves for our fundamental data. 



The arguments for a dual glacial period, and at the time of its 

 proposal, for a multiple ice age, were based chiefiy upon the fact of 

 the presence in certain till sections of thin black soil-streaks, replaced 

 here and there by thicker peat-beds. That there might be such a 

 thing as extensive interglacial sands or loams was not thought of. 

 Yet they were actually observed, recorded and fully described a 

 complete decade prior to the time when their true significance was 

 pointed out. Such an interglacial loam deposit, intercalated between 

 two thick till-sheets, is the one exposed on Capitol Hill, in the city 

 of Des Moines. It was fully described^^ in great detail by W. J. 

 McGee so far back as 1882. It seems to be the first expression of 

 the phenomenon ever recorded the stratigraphic relations of which 

 were unmistakable. 



At the time when these observations were made the possible com- 

 plexity of the Glacial Period was not even yet surmised. Possibili- 

 ties of a Second Glacial Epoch were only vaguely being considered. 

 The prolix and bitter controversy on the duality versus the unity 

 of the Glacial Period was just beginning. Under these circum- 

 stances it was not at all surprising that some of the phenomena ob- 

 served on Capitol Hill were partially misinterpreted ; and that the 

 true significance of others was for a considerable time overlooked. 

 Then, too, the prevailing theory of the lacustrine origin of the loess 

 tended 1 1 obscure the proper understanding of the accurately re- 

 corded data. 



Notwithstanding the fact that McGee was inclined at the time to 

 attach rather slight importance to his observations and to regard the 

 phenomena as indicating merely local advance of the ice-sheet, it 

 soon became manifest that the two till-blankets separated by a thick 

 loess deposit was ini]:)eachable testimony in support of two distinct 

 and great ice movements within the span of what was previously 



"American. Jour. Sci., (3), Vol. XXIV, iip. 202-223, 1882. 



