364 A. P. COLEMAN — IROQUOIS BEACH IN ONTARIO 



a considerable extent by the evidence given earlier of erosion and shal- 

 low water shells, found 70 feet below the Iroquois level at Toronto, and 

 the presence of unworn mammoth tusks and bones 83 feet below the 

 Iroquois gravel bar at Hamilton. The peculiar wall-like character of 

 Burlington heights also may be considered as pointing in the same direc- 

 tion, suggesting a beginning in shallow water and steady upgrowth as 

 the water rose. 



Taking the evidence as a whole, it will be admitted as almost a cer- 

 tainty that the deformation of the land and the corresponding tilting of 

 the basin progressed during the existence of lake Iroquois, and that the 

 amount of differential elevation toward the northeast was important, 

 possibly as much as 250 feet between the extreme ends of the basin and 

 certainly not less than 140 feet. The total deformation, including that 

 which has taken place since Iroquois times, can hardly be less than 500 

 feet in a distance of 108 miles along a line running north 20 degrees east, 

 and something should no doubt be allowed for differential elevation to 

 the southwest of the Ontario basin, though the amount of the latter is 

 uncertain. 



The whole amount of deformation must, however, be less than the 

 elevation of the highest part of the Iroquois shore above sealevel, about 

 744 feet at the West Huntingdon island, otherwise the surface of .the 

 Iroquois water at its earliest stage would have been below the sea, which 

 is impossible with the Niagara river pouring into it. 



It is probable that lake Iroquois began its existence much nearer to 

 sealevel than lake Ontario is now, and some day its' initial elevation may 

 be determined approximately by tracing the slope of the Mohawk river 

 channel to its outlet at some marine terrace in the Hudson valley. 



Time Estimates 



As shown by Professor Fairchild, the amount of warping of the land 

 during and since Iroquois times is so great as to demand a very long time 

 if the present rate of differential elevation worked out by Doctor Gilbert 

 for the Great lakes is made the standard.* Taking the estimates given 

 above and assuming the rate of 0.42 feet per hundred miles per century, 

 the minimum differential elevation of 140 feet in 108 miles implies more 

 than 30,000 years for the duration of lake Iroquois, and the maximum 

 of 250 feet requires nearly 55,000 years. If the total differential eleva- 

 tion of not less than 500 feet from the beginning of Iroquois times to the 

 present is taken as correct, more than 100,000 years must have elapsed 

 since the ice set free the Mohawk outlet. 



♦Pleistocene Geol. Western New York, 1900, p. r 112. 



