154 



ROCHESTER ACADEMY OF SCIENCE. 



as to enable them to commence lateral swing or meander, and they 

 completed a good bit of valley-widening during that brief episode. 

 (Sec map D). The detritus so derived perforce found lodgment in 

 the unfilled basin of Lake Irondequoit, which thus tardily became a 

 locus of sedimentation. (Compare figure 6.) 



The further withdrawal of the waters after the Emmons epi- 

 sode, had its next interruption during the "'Vermont" stage. In 

 Northern New York this stage is marked by a series of beaches dis- 

 tributed through about a hundred feet of vertical range, instead of 

 by a single ])lane. The higher beaches are weak, however, whereas 



( ^ 



LACAL DRIFT AND If^OdUCS 



5/LT3 ) 



100 ft. ■ 



STAGE £"■ 



' XI7 - 



Fig. 6. Lake Iroquois Filling. Longitudinal profile of the lower Irondequoit 

 valley showing valleys newly channeled through the silts by the Ironde- 

 quoit river upon the draining away of the glacial waters, and the tardy 

 flooring up with silts of the unfilled section. The stream supposedly 

 encountered drift obstacles at the five points indicated. Vertical scale 

 17.5 times the horizontal. Compare Map E (Plate VIII) and Figures 

 4 to 8. 



the lower ones and especially the deltas are conspicuous. This lower 

 phase, marking the most prolonged stillstand, is calculated at ap- 

 proximately 205 feet present altitude for Rochester, or forty feet 

 below the existing level of Lake Ontario. Referring back to page 

 141 it will be seen that this is precisely the average depth of the re- 

 markably flat floor of the Bay in its widest portion, — between Glen 

 Haven and the Newport House. 



Since we are unable to-day to trace the shorelines of Lake Ver- 

 mont beneath the enveloping waters of Ontario and so prove beyond 

 a doubt that our figuring is correct, it would be unsafe to assert 

 that this coincidence is more than casual. It is equally thinkable that 

 the downcutting of the Lake Irondec|Uoit outlet in pace with the 



