156 ' ROCHESTER ACADEMY OF SCIENCE. 



meadows just south, is excessive, and affords the proof of an effi- 

 cient obstruction ah'eady adverted to. Without some barrier here 

 capable of producing strong rapids the channel must have been 

 notched back far more deeply into the silts of the meadow and a 

 great hole rent in these by meander swinging. Of the latter the 

 soundings give no inkling, and since it is unlikely that it could have 

 been refilled later so accurately it may be denied that it ever existed. 

 The question of the nature of this barrier, whether rock or only 

 heavy moraine, need not be attacked here as it pertains rather to 

 the companion investigation on the rock valley. The longitudinal 

 profile of this stage (figure 6) renders the problem clearer. 



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( P/x£-OAjrARlO FiLLINqS ) 

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Fig. 7. Lake Ontario Filling. Longitudinal profile of the lower Ironde- 

 quoit valley showing renewed silting on the return of Ontario waters, 

 producing the shallows at each end of the Bay and alluvial marshes up 

 the valley to Zarges Mills. Vertical exaggeration 17.5. Compare Map 

 G (Plate X) and Figures 4 to 8; and observe how the sea-level has been 

 steadily "dropping" as the land rose. 



We come back at this juncture to the remarkable triple depres- 

 sion on the west wall of the Bay opposite its deep section, whose 

 three ravines (one of which breaches the "ridge" bar, sec page ijo) 

 show by their embayment that they were cut at this stage. The 

 form of this amphitheatre suggests that a meander loop of the Lake 

 Irondequoit outlet may have insinuated itself here during the Em- 

 mons stage. But this suggestion alone is inadequate to explain the 

 later history, and so does not help much if at all. The circ-like heads 

 cf these three gullies, and the brooks mapped in two of them, indi- 

 cate that they are powerfully spring fed. Their position between 

 the Ridge Road and Newfane beaches of Iroquois may have some- 





