94 Tfte American Geologist. Feb. i89i 
lake-basins is hardly pertinent, for nowhere about the lakes is the 
glaciation parallel to the shores or vertical escarpments which are 
associated 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 at 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 conclu- 
sion that the lakes were subaerial valleys in spite of the barriers, 
and the fact that the floors of most of the basins are below the 
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 New York, at the mouth of the Mississippi 
river, upon the Pacific coast, and in the Hudson strait. These 
indicate that eastern America stood for long ages at between 1.200 
and 1,800 feet above its present altitude ; and the whole continent 
in more recent times, but for a briefer period, at upwards of 
3,000 feet*. Hence the former continental 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 ex- 
planation having long eluded investigation. 
9. Deformation of raised shores and beaches. 
Ai the close of the episode of the newest till, the region of the 
great Lakes was submerged to a depth of at least 1.700 feet, as is 
recorded in the beaches which overlie the till. These high 
*" High Continental Elevation preceding the Pleistocene Period," by 
.1. W. Spencer, Bull. Geol. Soc. Am. vol. i. 1889; and Geol. Mag., May, 
L890. 
