GREAT LAKES BASINS— SHEPARD 277 



THICKNESS OF GLACIAL DRIFT 



One of the most convincing arguments favorable to glacial excava- 

 tion of the Great Lakes is the enormous quantity of glacial drift 

 found south of these basins. Despite postglacial erosion and the fact 

 that a large percentage of the debris was carried beyond the glacial 

 margins by glacial outwash, the surfaces are, nevertheless, covered 

 with drift which is commonly a hundred or even several hundred 

 feet in thickness. Furthermore, the nature of the drift is such as to 

 suggest derivation from adjacent lake basins as, for example, the 

 clayey till of Illinois which was presumably derived from the shales 

 of the Lake Michigan basin. The freshness of a large part of the 

 drift, particularly the younger sheets, shows conclusively that the 

 glaciers were digging into soUd rock rather than just removing the 

 mantle rock from old weathered surfaces, 



CONCLUSIONS 



The various tests which have been applied to the hypotheses of 

 origin of the Great Lakes appear to give overwhelming weight to 

 glacial erosion as the principal cause. Possibly such a conclusion is 

 not justified, but more significant evidence should be given before 

 assuming alternative modes of origin. If it is true that the Great 

 Lakes, the various other large basins in glaciated territory, and the 

 deep arms of the ocean along glaciated coasts are all largely the prod- 

 uct of glacial erosion, one is led to wonder whether the glaciers were 

 not much thicker than commonly assumed. An average thickness of 

 4 miles or more for the continental ice masses have been suggested 

 by the writer as a cause of great lowering of sea level to permit the 

 cutting of submarine canyons. If the ice was that thick, the cutting 

 of large basins might have been greatly intensified. 



