THE WEATHERED ZONE 243 



base of a ridge of Illinoian drift. The gray clay here rests upon 

 a gravelly bed instead of a sheet of till, but appears to be of 

 similar origin and age to the other beds referred to the Yarmouth 

 stage. 



Within a few miles south from this exposure the border of 

 the Kansan drift emerges from the edge of the Illinoian and 

 passes southward into Missouri. The driftless peninsula found 

 by Professor Salisbury here sets in and occupies a narrow strip 

 west of the Illinois from Pike county to the mouth of that 

 stream, 1 beyond which the margins of the Illinoian and Kansan 

 sheets take widely divergent courses. Fortunately there was 

 sufficient overlap north from this driftless peninsula to make 

 clear the interpretation that the Illinoian is a markedly younger 

 sheet than the Kansan. This difference in age was suspected to 

 occur from a comparison of maturity of valleys in the two dis- 

 tricts, but the testimony of the weathered zone preserved below 

 the Illinoian was necessary to confirm it. 



Frank Leverett 

 Denmark, Iowa. 



'See Proc. A. A. A. S., Washington meeting, 1S91, pp. 251-253. 



