44 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Fig. 5.5. Thickness of Upper Cambrian and Lower Ordovician Rocks in the Michigan Basin. 

 After Cohee, 1948. 



SURFACE EVIDENCE OF COLLAPSE 

 x SUBSURFACE EVIDENCE OF COLLAPSE 

 o WELLS WITH NO EVIDENCE OF COLLAPSE 



Fig. 5.6. Map of Mackinac Straits areas showing zone of collapse and exposures of Mackinac 

 breccia. Reproduced from Landes, 1945. 





faults have been regarded as either Precambrian or late Paleozoic in age, 

 they are very ancient, and any scarps would be erosional features of the 

 fault-line variety. Previous conjecture places the Grenville front in the 

 position of the lake, and later subsidence along this zone may have oc- 

 curred to form the lake basin. It must be conceded, however, that the 

 origin of the Lake Superior basin, over 1000 feet deep in places, has not 

 yet been worked out satisfactorily. 





Eastern Interior Basin 



The Eastern Interior or Illinois-Indiana-Kentucky basin is deepest in 

 Wayne, White, and Hamilton counties where the base of the Mississippian 



