ROCHESTER AND ONTARIO BEACH QUADRANGLES 7 



mite forms the upper member and its outcropping edge is almost 

 directly above the Medina, as is shown in the nearly vertical section 

 exposed along the Niagara river at Lewiston. In the vicinity of 

 Rochester, however, the northern edge of the Lockport dolomite has 

 weathered back from the Medina for a distance which averages 

 more than a mile. 



The outcrops of the geologic formations on this map extend in a 

 nearly east and west direction. Owing to the depth of the gorge, 

 the upstream deflection of the outcrops are well shown in the Genesee 

 river. Most interesting, however, is the southern V-shaped deflec- 

 tion of the rock outcrops, into which the southern end of Irondequoit 

 bay extends. On the basis of the rock geology alone, we have here 

 ample evidence of a valley far more ancient than that of the present 

 Genesee, which cuts through the same series of rocks but 5 miles 

 farther west. 



A noteworthy feature and one which in no small degree has given 

 to the vicinity of Rochester its geological prominence, is the fact 

 that at two places and at but short distances apart, the rocks of the 

 Niagaran group have been cut through by the Genesee river and the 

 southerly extending streams tributary to Irondequoit bay, thus 

 exposing to view and making possible the fine sections specially well 

 shown along the Genesee, 



The formations which are represented on the map all have a south- 

 east dip of about 80 feet per mile and belong to the Ontario or Upper 

 Siluric system. Of the members comprising this system only the 

 basal portion of the Medina is not represented in the region border- 

 ing on Lake Ontario. The southern portion of the map includes the 

 members of the Salina stage. The Cobleskill dolomite is found a 

 short distance south of the limits of the map. The Rondout water- 

 lime and the Manlius limestone, the highest members of the Siluric 

 system, are absent from this section of the State, thus making the 

 Cobleskill the only formation of the Siluric, as known in western 

 New York, that will not be represented on the map. 



