356 THE ICE AGE IN NORTH AMERICA. 



It is fair to Professor Newberry, however, to state that 

 he never took extreme grounds upon this point, but that his 

 fullest statement of the theory, written for the second vol- 

 ume of his " Report of the Geological Survey of Ohio," was 

 well matured, and gave weight to all the agencies which 

 combined with ice-action to produce these basins. The facts 

 which have to be borne in mind with reference to the Great 

 Lakes are briefly these : 1. Lakes Ontario, Erie, Huron, and 

 Michigan, being surrounded by sedimentary rocks whose 

 strata at the present time lie nearly horizontal, evidently 

 occupy valleys of erosion. The western end of Lake Supe- 

 rior, however, occupies a synclinal trough, and is doubtless 

 partly due to an early warping of the earth's crust. 2. The 

 bottoms of all these lakes, except Erie, are lower than the 

 present sea-level, the depression in the case of Superior 

 being 375 feet ; Michigan, 286 ; Huron, 127 ; Ontario, 507. 

 If, then, these lakes occupy valleys of erosion, it is an inter- 

 esting question to determine how any erosive agency could 

 have operated to such a depth below sea-level. 



Professor Newberry's theory is that, previous to the Gla- 

 cial period, the region to the south and southwest of Hudson 

 Bay was considerably elevated above its present position, 

 and that from early ages the lines of drainage had been 

 established in pretty much the same general course as at 

 present, forming valleys of considerable extent where now 

 the lake-basins exist ; that, when the Ice age came on, the 

 country to the north was still at a higher elevation than 

 now, and as the local glaciers increased, they occupied, en- 

 larged, and in some cases deepened, these old river-valleys, 

 both by direct action in eroding the rocky basin and by 

 the deposition of great quantities of glacial detritus. But 

 his own statement of the theory is so perspicuous and 



Progress for 1869" ; "The Surface Geology of the Basin of the Great Lakes 

 and the Valley of the Mississippi," " Lyceum of Natural History Society, New 

 York, 1869"; "The Surface Geology of Ohio," "Report of Geological Survey 

 of Ohio," vol. ii, 1874 ; " The Geological History of New York Island and Har- 

 bor," "Popular Science Monthly," 1878. 



