892 GEOLOGY 



gorge cutting of the Niagara Falls could not have begun until the 

 Mohawk outlet of the lakes (p. 883) was abandoned, because the 

 escarpment through which the cutting subsequently took place was 

 submerged while Lake Iroquois discharged through the Mohawk 

 valley. The time measured by the Niagara cutting is only that 

 which has elapsed since the ice retired from the north flank of the 

 Adirondacks far enough to permit the waters of the ancestral Lake 

 Ontario to find an outlet lower than the Niagara escarpment, and 

 no very effective cutting could take place until the waters were with- 

 drawn to something near their present level. 



If the border of the ice-sheet at this stage (Fig. 588) is compared 

 with the border of the ice at the maximum Wisconsin stage, it 

 will be seen that it had retreated some 600 miles, and it cannot be 

 assumed that the retreat was an uninterrupted one. 



Before attempting to place a value upon the period so repre- 

 sented, the time at which the gorge below St. Anthony Falls began 

 to be cut may be considered. It is to be presumed that for a time 

 after the retreat of the ice-edge north of the site of these falls, the 

 Mississippi valley was being aggraded, for the outflowing drainage 

 must be presumed to have been overburdened with glacial detritus. 

 In support of this assumption is the abundant evidence that the 

 Mississippi valley, as far down as the mouth of the Chippewa River, 

 was filled with detritus to a depth of more than 100 feet. Farther 

 south the glacial filling appears to have been 80, 70, 60, and 50 feet 

 above the river, the last in the latitude of central Illinois. 



When fluvio-glacial aggradation of the Mississippi valley ceased, 

 it was necessary for the river to clear out its trench before effective 

 cutting of the gorge below the falls could begin. Under any prob- 

 able hypothesis, there must have been a retreat of the ice some 700 

 to 800 miles from its extreme extension (to Des Moines) during the 

 last glacial epoch, before the cutting began. The rate of recession 

 is unknown, but 200 feet per year is an improbably high rate. At 

 this rate, the ice must have been receding well toward 20,000 yours. 

 before the falls began. If the retreat of the ice previous to the I >e- 

 ginning of the cutting of the Niagara gorge is taken at 600 miles. 

 the time occupied, on the assumption of a retreat of 200 feet per 

 year, is about 15,000 years. 



