236 ROCHESTER ACADEMY OF SCIENCE. [ Juiie 9, 



The President gave a lecture, illustrated with maps, drawings 

 and lantern views, entitled : 



THE GEOLOGY OF IRONDEOUOIT BAY. 

 By Herman L. Fairchild. 



[Abstract.] 



This body of water lies three miles northeast of the city of Roch- 

 ester and four miles east of the lowest section of the Genesee river. 

 The geography of the bay and its surroundings is shown in the 

 accompanying map, plate 3. 



The bay of Irondequoit is now practically a distinct lake, about 

 four miles long and three-fourths mile wide, since the wave and cur- 

 rent action along the Ontario shore, aided by the filling for the Rome, 

 Watertown and Ogdensburg Railroad, has closed the original open 

 mouth of the bay by a heavy embankment or bar, leaving only a 

 narrow passage for escape of the surplus Irondequoit water. 



The Irondequoit valley is about two miles wide at the top of the 

 cross-section, and as a traceable valley extends at least 15 miles south- 

 ward from Lake Ontario to near the village of Fishers. 



The depth of the valley to the rock is unknown, but water of the 

 bay is 78 feet deep. The large dimensions indicate a preglacial valley 

 of a large stream, which before the glacial period flowed north to join 

 an ancient Ontarian river. It is believed that there exists no other 

 equally large trench in the rock strata between Niagara river and 

 Sodus bay. The inference follows that this depression is a deserted 

 section of the ancient preglacial valley of the Genesee river, which 

 from its present mouth at Charlotte to several miles southwest of 

 Rochester is in a new or postglacial canyon. Apparently a section of 

 the ancient valley, having an easterly trend, somewhat oblique to the 

 •ice movement, has been buried by drift and wholly obscured, like 

 another section of the same river valley between Portageville and 

 Nunda.* 



In the same manner that the river was forced by drift obstruction 

 into a new rock channel below Portageville, so in the Rochester 

 section the river has been thrown out of its old valley somewhere in 

 the vicinity of Avon, The Irondequoit valley is the only wide break in 

 the rock strata forming the south side of the Ontario basin between 



*See Bull. Cieol. Soc. Am., \'ol. 7, pp. 438-442. 



