THE DATE OF THE GLACIAL PERIOD. 545 



lake there was a rise of 400 feet in the northern part while 

 there was a rise of but a few feet at the southern end. 



These facts give us a chance to estimate the rate of this 

 differential rise in the land lying north of the glacial border 

 which has been going on up to the present time, and thus 

 furnishes data from which to calculate the length of time dur- 

 ing which the depression at North Bay was sufficient to divert 

 the water of the Upper Great Lakes from Niagara. From the 

 data collected by Dr. Upham during his investigations of 

 Lake Agassiz he concludes that its entire existence could not 

 have been more than 2,000 years and probably was not more 

 than 1,000 years. 



The facts upon which Dr. Upham relies, are: 1. The small 

 size of the deltas deposited on the margin of the lake by the 

 great rivers entering from the west; 2. The .small size of the 

 ridges themselves; 3. The limited extent of the dunes about 

 the southern end of the lake. 



1. The most important rivers which formed deltas in the 

 lake are the Cheyenne, the Assinniboine and the Saskatchewan, 

 which all come in from the region to the westward which was 

 free from ice during the greater part of the time of the exist- 

 ence of the lake. The gradient of these streams is rapid, and 

 the supply of sand and gravel within their reach is abundant. 

 Yet their delta deposits at the level of the beaches is small, 

 and entirely inconsistent with the continuance of the lake for 

 more than 1,000 or at most 2,000 years. 



2. The shore lines, or beaches, are very much smaller than 

 those around Lake Erie, which, as we elsewhere show, could 

 not have been more than 2,000 to 3,000 years in forming. 



3. The dunes at the south end are not over one-tenth the 

 size of those at the south end of Lake Michigan, which demon- 

 strably were not over 10,000 to 15,000 years in forming. 



In explanation of this point it is necessary to call attention 

 to the facts concerning the dunes south of Lake Michigan. 

 These are very prominent features along all the railroads 



