CHAPTER IV. 

 THE ILLINOIAN DRIFT SHEET AND ITS RELATIONS. 



GENERAL STATEMENT. 



Relation to outlying and underlying drift. — The first recognition and separation of 

 the Illinoian drift sheet from an older sheet of drift which underlies it on 

 the borders of the Mississippi was made by the writer in southeastern Iowa 

 in the spring of 1894. The occurrence of a bowlder of red jasperv con- 

 glomerate in Lee County, Iowa, had been noted some ten years earlier, 

 but its significance was not recognized at that time. It is now found that 

 bowlders of this class are not rare, and that they are in all probability 

 derived from the ledges north of Georgian Bay. If so, they are an indica- 

 tion that the southwestward movement of the ice from that region extended 

 somewhat beyond the Mississippi River. The studies in 1894 developed 

 other evidence that the invasion which brought in these bowlders from the 

 northeast reached beyond the Mississippi, and that it occurred at a much 

 later date than the general glaciation of southern Iowa and northern Mis- 

 souri. The western limits of this ice invasion are plainly indicated by a 

 marginal ridge. The evidence of a long interval is found in the greater 

 erosion of the drift sheets outside the limits of the Illinoian, and in the 

 prevalence of a soil horizon and weathered zone beneath the Illinoian sheet 

 where it overlaps the earlier one. This interval appears to be of sufficient 

 importance to be termed an interglacial stage and to justify the reference 

 of the two sheets to distinct stages of glaciation. The intervening stage of 

 deglaciation has recently been named the Yarmouth interglacial stage, 1 and 

 is discussed farther on. 



Relation to the iowan drift sheet. — The question arises whether the Illinoian sheet 

 should be classed with the Iowan sheet, which, as shown by McGree in the 



1 In a paper presented by the writer at the twelfth annual meeting of the Iowa Academy of 

 Sciences, December, 1897: Proc. Iowa Acad. Sci., Vol. V, 1898, pp. 81-66; also Jour. Geol., Vol. VI. 1898. 

 pp. 238-243. 



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