57° 



CHARLES KEYES 



In casting about for a favorable locality in which to begin 

 inquiry it is but natural to turn to some spot nearest to the high- 

 lands that displays a considerable section of the later geologic for- 

 mations. There appears to be a section of this description off to 

 the southwest of the middle angle of the Canadian shield, on the 

 south side of the great tongue of pre-Cambrian rocks which extends 

 from Lake Superior into northwestern Iowa where the old terranes 

 are known as the Sioux quartzite. 



On the general stratigraphic scheme of lowa,^ which is the latest 

 and most complete of any now available, the erosion intervals may 

 be readily indicated, together with the taxonomic values of each. 

 Since some of these lines represent peneplanation sufficiently wide 



Fig. I. — Relative attitudes of later base-levelings in Lake Superior region. 



in extent to affect the Lake Superior region, they may be critically 

 examined in turn, eliminated, or further compared in order that the 

 parallelism with the highland plain under consideration may be 

 exact. 



Of the dozen or more conspicuous planes of unconformity which 

 mark the geologic column of the region the Arkansan, Comanchean, 

 and Eocene horizons are the most important. These represent 

 most assuredly peneplains of great extent. Their intersections with 

 the present ground-surface trend north-northwest through west- 

 central Minnesota and Iowa. Along this line the intersections of 

 all of them chance to be very close together. In diagram they may 

 be represented as in Fig. i . 



^ Geol. Surv. Iowa, XXII (1913), 154. 



