132 IOWA ACADEMY OF SCIENCE 



AGE OF THE PLAINS. 



, The question of the age of the two plains is not relevant to the 

 purpose of this discussion and only brief mention is here made 

 of the two sets of interpretations which have been advanced. The 

 earlier workers and some of the later ones consider the upper 

 plain to be of Cretaceous age and the lower one to be Tertiary. 

 On the other hand Salisbury has called the upper plain Tertiary, 

 on the basis of a tentative correlation of the gravels on the plain 

 with the Lafayette formation of the gulf coast. Work by Trow- 

 bridge and Williams in Iowa has placed the lower plain tenta- 

 tively as early Pleistocene in age. 



SUMMARY. 



(1) There are two and only two upland plains in the region. 



(2) Both plains are old peneplains. 



(3) Both plains slope in a direction south by southwest and 

 converge toward the south and southwest. 



(4) The dip of the plains is less than that of the underlying 

 strata and they cut across the bevelled edges of dipping strata, 

 rising stratigraphically to the south. 



(5) The upper plain shows more evidence of stream erosion 

 than the lower. 



(6) The Dodgeville plain belongs to the upper plain and the 

 Lancaster plain of Grant and Burchard is a part of the lower 

 plain. 



Geological Laboratory, 

 State University of Iowa. 



