THE WISCONSIN ICE INVASION 367 



it is possible that many of the marl deposits found on the older Wisconsin till in 

 Illinois, and elsewhere, may be referable to the interval between the deposition 

 of the two drift sheets. The bodies of water in which these marl beds were 

 formed may have been inhabited by hardy mollusks and other animals while 

 the ice front was still in the vicinity of Chicago. An example of this possibility 

 is the marl deposit underlying the campus of the University of Illinois, des- 

 cribed on page 127 of this work. 



WABASH FAUNA 



Hay 5 has proposed the name "Wabash Beds" for the deposits laid down 

 subsequent to the retreat of the late Wisconsin ice sheet, and for the biota he 

 proposes the name "Wabash Fauna," believing that the period between the 

 waning of the ice sheet and the historical period should bear a name and be 

 equivalent in value to the interglacial periods between the different ice sheets. 

 This biota, lying in deposits formed on the surface of the late Wisconsin till 

 sheet, is described in Chapter IV of this volume. 



5 Smith. Mis. Coll., No. 20, p. 13, 1912. 



