LAKE SUPERIOR HIGHLANDS 



573 



Manitoba, and their gentle dip to the west point to their former 

 great eastward extension probably to Lake Superior and Hudson 

 Bay. There is little reason to doubt, therefore, the assumption 

 that the Cretaceous sediments once covered the Lake Superior 



Fig. 2. — Erosion of the Triassic Siouan mountains in southern Minnesota. 



highlands and that the peneplain which formed the Cretaceous 

 floor must have imparted the main topographic expression to the 

 highland plain. 



In Iowa, and Minnesota also, the Cretaceous strata themselves 

 are notably beveled, but at a lower angle than that of their floor. 

 There are good reasons for believing that this Tertiary base-leveling 

 took place in Eocene time. There appear to be important water- 



