270 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



He calls attention to islands of weak rock in the Lake Superior Basin, 

 He notes that the basins are too wide to be properly described as 

 U-shaped, and he questions the existence of hanging valleys along 

 the margins. Furthermore, he refers to similar basins which extend 

 across the direction of ice movement and to neighboring basins in 

 Wisconsin which are "explicable only as depressed parts of river 

 valleys." He calls attention to the numerous glaciated surfaces 

 where ice did not even remove the weathered material. Finally, he 

 expresses doubts on theoretical grounds of the capacity of conti- 

 nental glaciers to erode deeply. The objections of other geologists 

 are adequately covered by these same points. 



ALTERNATE HYPOTHESIS 



That glacial deposition blocking the outlet of preglacial depres- 

 sions is the principal cause of the Lakes has been suggested. Spencer * 

 believed that the basins were cut below present sea level by streams 

 as a result of uplift prior to the glacial period. Attention has also 

 been called by Thwaites and others to the warping in the Great 

 Lakes region which might suggest a diastrophic origin. Other deep 

 basins, such as the Rift Valley lakes of Africa, the large lakes of 

 Asia, and the basins of Nevada and Utah have been caused by 

 dias trophism. 



TESTS OF THE HYPOTHESES 



Though it is recognized that in all probabihty several factors have 

 contributed to the development of the Great Lakes basins, the ques- 

 tion arises whether some one of them has not been of major impor- 

 tance. Tests may be applied which should make a decision on this 

 point possible. 



DRIFT-OBSTRUCTED PREGLACIAL VALLEYS 



In the first place, if the basins are largely drift-obstructed valleys, 

 cut originally by river erosion, the bottoms of the basins should be 

 higher than the rock floors of the seaward extension of these old 

 valleys. Also, the basins should show cross-sections of a character 

 suggestive of river erosion and in keeping with the relation of the 

 valleys to the drainage divide. 



These expectations are not fulfilled. It is well loiown that all the 

 Great Lakes except Lake Erie extend below sea level (for maximum 

 depths see fig. 1), and their rock bottoms may be at a much greater 

 depth than shown by the soundings because the retreat of the ice 

 may have left considerable drift coverings in these basins. Thwaites 

 notes that "well records fail to show any wide preglacial vaUeys 

 leading out of the basins of the Great Lakes," and "* * * the 

 rock bottoms of the Mississippi and other preglacial valleys are much 



» Op. cit., p. 94. 



