REPORT OF THE DIRECTOR AND STATE GEOLOGIST 1903 99 



paragraph, it seems best to the writer to attribute the formation 

 of the deep depression to preglacial streams, though the forma- 

 tion of the present slight depression is due to postglacial 

 streams. 



In order to prove that the original valley was formed in post- 

 glacial time, it would be necessary to show that streams with 

 the power of Niagara had passed through this valley for a much 

 longer time (than has elapsed since the formation of the present 

 Niagara gorge began. It also, would be necessary to show from 

 what source the large deposits of gravel, sand and clay which 

 fill this depression were derived and also why some of the lakes 

 in central New York were not filled with these same deposits. 



Geologists seem agreed in the supposition that, on the retreat 

 of the Laurentian glacier, this region was covered with a suc- 

 cession of lakes, so that the theory of a great river cutting this 

 valley in postglacial time may be discarded. 



In the same way, we may discard the hypothesis that this 

 valley was filled in postglacial time, as the filling material is 

 largely sand and gravel from the Medina formation;^ and, if it 

 were derived from glacial debris at the south, the Finger lakes 

 would show more traces of filling, and the deposits would be at 

 the source and along the sides of these lakes rather than at their 

 outlets; whereas there are no marked accumulations in these 

 places that can not be explained in other ways. 



The supposition that much of the cutting was accomplished 

 by the glacier itself seems hardly tenable, inasmuch as the 

 valley is at right angles to the line of glacial flow. 



Before considering the question of preglacial cutting, a few 

 facts must be noted, on which to base a theory that the valley 

 Avas formed prior to the advance of the Laurentian glacier. 



(1) Glaciers, in passing over a ridge like the Niagara escarp- 

 ment, cut the ridge down nearly to the level of the adjoining 

 strata. (2) The bottom of Lake Ontario is about 250 feet below 

 sea level and is nearly at right angles to the direction of the 



'Fairchild, H. L. Kame Areas in Western New York South of Ironde- 

 qiioit and Sodiis Bays. Jour, of Geol. no. 2, v. 4. 



