CENTRAL STABLE REGION OF THE UNITED STATES 



43 



I 



TRENTON AND BLACK RIVER LIMESTONES 



» 30 tfo ^j, 



XATION OF SECTION 



Fig. 5.4. Cross section from Illinois to western Ontario showing the unconformity at the base of 

 the St. Peter sandstone and the Trenton and Black River limestones. Top of Trenton is taken as 



horizontal datum. Younger formations and present structures not shown. By George Cohee, 

 U.S. Geological Survey. 



crop then swings eastward through the basins of Lake Erie and Lake 

 Ontario. The salt would emerge mostly under water, and since the 

 aggregate thickness of salt beds that once may have cropped out was 

 several hundred feet, it has been suggested ( Newcombe, 1933 ) that the 

 depressions of the Great Lakes (excepting Superior) may be due to salt 

 solution and consequent subsidence. The basins do not correspond to 



faults or folds, and were probably existent long before the Pleistocene 

 ice lobes occupied them. The theory of salt solution seems the most logi- 

 cal explanation yet advanced. 



The Lake Superior depression is north of the belt of salt outcrop and is 

 mostly in Precambrian rocks. The northwest shore may correspond to a 

 fault, and the lake bottom topography suggests fault scarps. Because the 



