EVOLUTION OF IRONDEOUOIT VALLEY 



127 



passes under Lake Ontario in this region at about 240 feet, though 

 there is some reason to think that its higher and weaker beaches 

 may reach up to 260 feet. Indeed some of the wavework herein 

 attributed to (Ontario may have been initiated by this its predecessor. 

 Finally the evacuation of the St. Lawrence Valley by the glacier let 

 in the salt (or brackish) waters of Gilbert Gulf, whose levels "fell" 

 steadily through land uplift until the Thousand Islands emerged 

 from the waters. Then and thus came Lake Ontario into existence 



■LAKE WARRBiN 



LAKE IROQUOIS 



Fig. 2. Vertical Diagram of Glacial Lake Levels in the Rochester Region, 

 showing deformation (tilting) by land uplift. Exaggeration of vertical 

 scale 82.5 times. 



but not in its present dimensions, since by the same land ui)lift still 

 in progress its surface has been steadily rising on the south shore 

 until it has reached its present point. There are no published esti- 

 mates for the altitudes of initial Gilbert Gtilf or of mininuim 

 (transition) Gilbert Gulf-Lake Ontario oft' the Irondequoit outlet. 

 Rough calculations suggest about 175 feet A. T. for the highest 

 Cjilbert beach and 110 feet A. T. for the point of farthest retreat. 

 These are respectively 72 and 137 feet below the present stand of the 

 lake, but they are merely approximations. The fact they stand for 

 is, however, that for a time immediately before the present the base 

 level of our region (represented by the lake) was considerably lower, 

 perhaps by hundreds of feet, than at present, the shore lines were 



