LAKE SUPERIOR HIGHLANDS 571 



If the plane in which the Knes of the diagram lie be considered 

 as standing in a northeast-southwest direction the several pene- 

 plains of the Lake Superior region must have been successively 

 obliterated, leaving only remnants of the very latest ones. The 

 Arkansan peneplain, which constitutes the floor of the Coal Meas- 

 ures in the Upper Mississippi valley north of the Ozarks, may be 

 for the present neglected. For reasons which are to follow, the 

 Comanchean peneplain, which forms the Cretaceous floor in Iowa, 

 Minnesota, and Manitoba, may be particularly examined. 



Recent investigations about the point where the three states of 

 Iowa, Minnesota, and South Dakota meet clearly disclose some 

 instructive geologic structures bearing directly upon the problem 

 under consideration. The field data thus acquired are supple- 

 mented by numerous deep- well records. By reference to the gen- 

 eral geologic map of Iowa the Paleozoic formations are noticed to 

 be distributed in relatively narrow belts trending in a northwest 

 direction across the northeastern one-third of the state. Very 

 singularly, it has always seemed, these belts abruptly terminate at 

 the north soon after the state boundary is passed. This rather 

 peculiar circumstance appears never to have excited curiosity as to 

 its cause. Far to the north, about Winnipeg, in Manitoba, there 

 is the same narrow belting of the same formations and, as farther 

 south, the strike is northwest. The Canadian Paleozoic area is 

 separated in central Minnesota from the lowan Paleozoic field by 

 a broad expanse of pre-Cambrian rocks. 



Structurally these pre-Cambrian rocks form the core of a rather 

 notable arch the axis of which runs northeast and southwest. This 

 anticline is one of large proportions and extends from the east 

 shore of Lake Superior to central South Dakota, where, as a canoe- 

 shaped form, it plunges beneath the post-Paleozoic deposits of the 

 Great Plains. The exposure of Sioux quartzite constitutes the 

 western nose of the fold. 



It is against the south slope of the sharp Siouan anticline that 

 the belted Paleozoic terranes of northeastern Iowa are upturned, 

 and there cut off. The eastern margin of the vast Cretaceous field 

 crosses the same line, so that there is apparently no westward 

 extension of the five groups of formations, if it ever existed, at 



