CHAPTER X 



THE IOWAN ICE INVASION AND THE PEORIAN 

 INTERGLACIAL INTERVAL 



I. The Iowan Ice Invasion 



"The Iowan ice invasion is recorded in a thin sheet of till, marked by an 

 exceptional profusion of large granitoid bowlders which lie chiefly on the sur- 

 face and are somewhat aggregated into a bowlder belt on the easteVn border of 

 the tract. The typical Iowan drift was formed by a lobe of the Keewatin ice 

 sheet, occupying the north-central part of Iowa. ni 



The Iowan was once thot to enter the northwestern portion of Illinois 2 

 but later researches have shown 3 that the till in this region is to be classed as 

 Illinoian. Taylor 4 maps the Iowan as extending southeasterly from beneath 

 the Iowa lobe of the Wisconsin till, well toward, but not reaching the Mississippi 

 River. A small area fringes the Wisconsin till in Wisconsin, north of the drift- 

 less area. This is also the area of the Iowan given by the Iowa geologists. 

 Whether there is a lobe from Labrador corresponding to the Iowan beneath 

 the Wisconsin till in Michigan, and farther east, is not definitely known. A 

 till shown in sections near Niagara Falls and at Toronto has been doubtfully 

 referred to this stage. The existence of this drift as a separate till sheet has 

 been questioned by some geologists. 



Leverett 5 says of this drift, as found in western Wisconsin overlying Kansan 

 drift: "The so-called Iowan drift may stand in about as close relation to the 

 Illinoian as do the later Wisconsin moraines to the earlier Wisconsin. It does 

 not seem to be separated from the Illinoian drift by a definite interglacial stage 

 but instead to represent a substage or stadium of the Illinoian. It may, there- 

 fore, be advisible, pending further study, to apply to it the double name Later 

 Illinoian or Iowan. " 



The Iowan geologists, however, affirm the existence of the Iowan drift as a 

 distinct till sheet, unrelated to either the Kansan or Illinoian drift sheets. 

 Calvin, 5a in an analytical paper, summed up the case as follows: 



i. The Iowan drift is. 



2. The Iowan drift is young compared with the Kansan. 



1 Chamberlin and Salisbury, Geology, III, p. 391. 



2 Leverett, Mon. 38. 



3 Alden, Journ. Geol., XVII, pp. 694-709. 



4 Smith. Rep., 1912, p. 326. 



6 Bull. Geol. Soc. Amer., XXIV, p. 698. 



5a Bull. Geol. Soc. Amer., XXII, pp. 729-730. 



