BASINS OF THE GKEAT LAKES OF AMERICA. 529 



7. The Glaciation of the Region. 



At the present stage in the investigation this subject can be quickly 

 dismissed. The question whether glaciers can erode great lake- 

 basins is hardly pertinent, for nowhere about the lakes is the glaci- 

 ation parallel to the shores or vertical escarpments which are asso- 

 ciated with the lakes. Indeed, the direction of the striae is often 

 at high angles, even to 90°, to the trend of the vertical walls of rock 

 bounding or crossing the lakes. Nor are the faces of these great 

 walls of limestone polished by an agent moving along their faces. 

 That there are no striae parallel to some local inlet or valley would 

 be perhaps rash to assert ; but, if so, it is a mere coincidence, with no 

 bearing upon the origin or moulding of the Great Lake-valleys. 

 Hence we are forced back upon a conclusion that the lakes were 

 subaerial valleys in spite of the barriers, and the fact that the floors 

 of most of the basins are below sea-level — that of Ontario being 

 nearly 500 feet. 



8. The former High Continental Elevation of North America. 



If the lakes and valleys originated from atmospheric and river 

 erosion, then the continent stood at much greater elevation than at 

 present, as shown by the depths of the lakes themselves. But there 

 is much collateral evidence that in the later Tertiary days, probably 

 during the Pliocene, the continent was very high. This is shown by 

 the submerged valleys of the St.-Lawrence Gulf, of the Gulf of Maine, 

 off N'ew York, at the Mouth of the Mississippi Eiver, upon the Pacific 

 coast, and in Hudson Strait. These indicate that eastern America 

 stood for long ages at between 1200 and 1800 feet above its present 

 altitude ; and the whole continent in more recent times, but for a 

 briefer period, at upwards of 3000 feet *. Hence the former con- 

 tinental elevation was sufficient to satisfy all demands for the erosions 

 of the lake- valleys ; but the rocky barriers still demand explanation, 

 both on account of the present obstructions not having impeded the 

 erosion of the valleys, and on account of their subsequent closing 

 the valleys, in part, into lake -basins — ^the necessary observations for 

 the explanation having long eluded investigation. 



9. Deformation of liaised Shores and Beaches. 



At the close of the episode of the newest Till, the region of the 

 Great Lakes was submerged to a depth of at least 1700 feet, as is re- 

 corded in the beaches which overlie the Till. These high beaches 

 only remain as fragments about ancient islands ; but if we descend 

 to beaches of lower levels we find them well developed and contain- 

 ing all the necessary evidence for explaining the rock-barriers at the 

 outlets of the lakes. Gen. G. K. Warren, Corps of Engineers, 

 U. S. A., was the first to suggest the closing of the lakes by warp- 

 ings of the Earth's crust t. Portions of the high-level beaches 



* "High Continental Elevation preceding the Pleistocene Period," by J, W. 

 Spencer, Bull. Geol. Soc. Am. vol. i. 1889 ; and Geol. Mag., May 1890. 

 t Appendix 13, Eeport of Chief of Engineers, U. S. A, 1875. 



