THE DATE OF THE GLACIAL PERIOD. 543 



dences of a post-glacial flow of water in enormous quantity are 

 perfectly clear. At Mattawan there is an enormous delta 

 of bowlders where the valley of the Mattawa joins that of the 

 Ottawa. Many of the bowlders are several feet in diameter, 

 and thej r are all waterworn. Moreover, the bowldery delta 

 has been pushed out into the Ottawa Valley so as to dam the 

 river and create a deep lake-like expansion above and a long 

 series of rapids below. In confirmation of the theory that 

 there was a flow of water through this channel for a consider- 

 able time Mr. Taylor found miniature pot-holes worn in 

 some of the large bowlders of the delta terrace at Mattawan. 



Of course while the drainage of the Upper Great Lakes 

 was diverted around to the St. Lawrence by way of the Ottawa 

 Valley, the recession of the Niagara gorge was practically at 

 a standstill. It is important therefore to ascertain how long 

 a time this continued. Calculations upon this point will 

 largely depend upon the question of how rapidly the post- 

 glacial re-elevation of the region went on. Happily we have 

 much evidence upon this point, all of which indicates a rapid 

 rate as compared with that which is now going on. 



The most important evidence comes from Dr. Upham's 

 study of the shore lines of the glacial Lake Agassiz which 

 spread over the valley of the Red River of the North (see 

 page 401) . This temporary lake covered more than 100,000 

 square miles and its shore lines are easily traced for hundreds 

 of miles, like railroad embankments, across the prairie coun- 

 try of that region. There are several series of these shore 

 lines, at successively higher levels. But at the head of the 

 valley where the outlet was through Big Stone and Traverse 

 lakes into the valley of the Minnesota River, the beaches are 

 approximately at the same level. On proceeding northward, 

 however, while the lower beach remains nearly horizontal, the 

 upper one rises until in latitude 51° 52/ 200 miles north of 

 the international boundary, it is 400 feet above the lower 

 shore line. Thus it appears that during the existence of this 



