276 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



for these terraces and concluded with good reason that the soundings 

 did not permit any such generaUzation as Andrews had made. He 

 suggested that the terraces and vallej^s on the lake floor at some 

 places were submerged land features. However, some of these val- 

 leys cited have been shown by the recent surveys to contain deep, 

 rimmed depressions so that they can hardly be ascribed to river ero- 

 sion, and near them are similar features more than 600 feet deep (fig. 



ii'iuuBE 6.— Contour map of tlie nortliern portion of Lake Micliigiin. 



6, lower left comer). Is it not more reasonable to assume that these 

 features were produced by glacial erosion, especially as some of the 

 submerged valleys have distinct U-shaped transverse profiles? 



The irregular topography of the lake bottoms, especially that of 

 Lake Superior, suggests that it has not been greatly modified by 

 postglacial deposition. The "rock bottom" ^^ reported in surveys 

 from various parts of the lake floors and glacial till reported from 

 Lake Michigan '^ and from Lake Erie ^* corroborate this conclusion. 



" These may be largely boulders rather than bedrock. 



" Hough, 3. L., The bottom deposits of southern Lake Michigan. Journ. Sed. Pet., vol. 6, pp. 57-80, 

 1035. 

 ■' Kindle, E. M., personal communicatloa 



