AGE OF LAKE IROQUOIS 209 



each lamina represented an annual deposit. Whatever be the date, there- 

 fore, which we assign to the upper beach of Lake Warren, that of the 

 Iroquois beach around Lake Ontario must be 2,000 years less. This, 

 according to my calculation, would bring the date of the 200-foot shelf 

 at Toronto at about 10,000 years. 



5. To account for the high level of the water in Lake Iroquois, which 

 eroded the 200-foot shelf at Toronto, it is sufficient to note that the shelf 

 has almost exactly the same elevation as that of the col from the Ontario 

 basin into the Mohawk Valley at Eome, New York. Lake Ontario is 247 

 feet above the sea. The col at Rome is 445. Moreover, as Professor 

 Fairchild has abundantly detailed, ice obstructions, in the Mohawk Valley 

 raised the level of the drainage lines into the Mohawk Valley for an in- 

 definite period.^ One of the clearest evidences of this exists a few miles 

 southeast of Syracuse, where a stream comparable in size to that of Ni- 

 agara, but in fact doubtless much larger, has eroded a rock gorge of such 

 length and proportions that its formation must have occupied many cen- 

 turies. This stream must have been kept up at this level by the ice 

 bordering it on the north side and filling the valley. 



6. All these considerations show that there must be some error in the 

 assumptions which underlie the calculations which assign a date of from 

 20,000 to 40,000 years to the formation of Lake Iroquois, to the erosion 

 of the 200-foot shelf at Toronto, and we may add to the age of the Ni- 

 agara Gorge. It should be remembered that all these calculations are 

 based on assumptions underlying the interpretation of very complicated 

 phenomena. Among these assumptions the most misleading are those 

 which unduly minimize the rate at which glacial movements and torren- 

 tial erosion may take place. By way of caution it is sufficient to call 

 attention to two facts : 



a. Within 25 years the front of the Muir Glacier, in Alaska, has re- 

 treated seven miles, while the ablation from, its surface during that time 

 amounts to fully 700 feet, thus confirming my original estimate that 

 since Vancouver's visit in 1794 the glacier has retreated more than 30 

 miles, and that the ablation of the surface during that time amounts to 

 more than 2,000 feet. This certainly is a movement on a scale sufficiently 

 large to be considered in speculating concerning continental glacial ice- 

 sheets.^ 



&. Dr. Warren Upham in his exploration of the shoreline of Lake 

 Agassiz obtained evidence which is ample to show that the retreat of the 

 ice-front from the Canadian boundary in the Red River of the North so 



s See Fairchild : Bull. Geol. Soc. Am., vol. 10, pp. 27-68 ; "Pleistocene geology of west- 

 ern New York," 20tli Rep. of State Geol., 1900, pp. 13-139. 



« See Wright's "Ice Age in North America," 5th ed., chap. 3, with references. 



