EVOLUTION OF IROXDEOUOIT VALLEY. 



12: 



valley beneath, whose partial effacement is due to glacial and post- 

 glacial agencies, including stream-fillings in bodies of standing 

 water. With these last mentioned deposits of the glacio-postglacial 

 lakes we are specially concerned, since they have played the major 



""^ ONTARIO 



CH/K8.L0JTE. 



,^^ '''''■.■ -Xt)iONioy£\ FALLS . ,__r-'M 



Y^ ON ON D\aG A L-^T^ 



Fig. 1. Key Map of the Region around Rochester, (former cit}^ limits in 

 dot and dash), showing the four sections (numbered) of the Irondequoit 

 Valley separated by dash hues, and the relation of the area of our maps 

 (shaded quadrangle) to the Genesee River and the belts of rock outcrop. 

 The strata (dotted boundaries) dip gently toward the south (base of 

 map) passing under the Onondaga limestone escarpment, north of which 

 the belt of weak Salina shales is deeply drift buried, especially by huge 

 kames between Mendon and Henrietta and from Victor to Fairport. 

 Somewhere beneath the former area lies the preglacial rock channel of 

 the Genesee, as suggested by the broken arrows. 



part in determining the final topographic expression of the middle 

 and lower valley, whereas the ice-made features control in the 

 upper valley. 



Always, dtunng its waning over western Xew York, the reced- 

 ing margin of the ice-sheet abutting against the north-sloping land 



