GLACIAL AND INTERGLACIAL BEDS 309 



present, and river valleys were eroded through 150 or 250 feet 

 of sand and clay and widened so as to have gentle slopes. 



It will be observed that the damming of the intercdacial 

 waters is held to be due to epeirogenic changes and not to the 

 presence of ice, since it is inconceivable that an ice dam should 

 hold its place at the Thousand Islands during the ages of mild 

 climate required for the growth of the luxuriant Don forests, 

 largely composed of trees that now barely reach the southern 

 edge of Canada. 



It is not unfair to assume that the time after the Iowan ice 

 retreated until the commencement of the Toronto Formation was 

 as long as from the retreat of the Wisconsin ice to the present, 

 a time variously estimated at from 7000 to 30,000 years. The 

 raising of the northeastern barrier of the Scarboro' lake to a 

 height at least 150 feet above that of Lake Ontario may also 

 have required thousands of years, if the results of Dr. Gilbert's 

 investigations as to the rate of tilting of the present lake basins 

 furnish the standard. These two stages cover only the first half 

 of the interglacial time, and probably an equal number of thou- 

 sands of years were required for the depression of the outlet 

 below that of Lake Ontario and the cutting of wide and deep 

 valleys through the Toronto Formation. 



To arrive at the total length of the interglacial period it is 

 not extravagant to double or even triple the number of years 

 since the last Ice age, giving estimates of from 14,000 to 60,000 

 years or more. It will of course be understood that the length of 

 time since Niagara began to cut its gorge can be estimated only 

 vaguely and that the guess at the length of the interglacial period 

 given here is still less certain. 



How long a time the later series of bowlder clays and inter- 

 stratified materials, more than 200 feet thick at Scarboro', 

 required in their formation one can hardly even guess ; but one 

 of the glacial retreats amounted probably to more than 50 miles 

 and may alone have demanded centuries of recession and ad- 

 vance. 



The time element in the series of events described has been 



