170 



cerned, the erosion should have l^een uniform throughout the period of 

 the growth of the valley. 



Whether or not the stream flowing over the falls at present is as great 

 as in the past is a problem rather difficult of solution. The falls are in 

 the main valley, yet as they have retreated through the two and a twelfth 

 miles, several tributary valleys have been left to work back their heads, 

 and because of this element it may be that there is a smaller volume of 

 water flowing over the falls than in the i)ast, and hence a somewhat slower 

 retreat. 



Again, the valley above the falls has certainly been gi'owing longer, 

 and developing tributaries, and hence has been adding to its volume of 

 water during these milleniums, and because of this factor the falls may be 

 retreating more rapidly than during the earlier period of its growth. 



Considering all the possible factors which may have influenced the 

 erosion of Cliffy Valley, it is iirobable that the present rate of sapping 

 beneath the falls, and hence the retreat of the falls up the valley, is very 

 approximately that which has held throughout the history of its growth. 

 Whether the valley has l>een entirely or only itartially eroded since reti'eat 

 of the ice sheet, pr(»l)ably the lUinoian, which at one time covered the 

 entire region concerned, is an open question. 



