l88 ROCHESTER ACADEMY OF SCIENCE. [Jan. 9, 



edge of Pittsford the Irondequoit, where it is crossed by the Erie 

 canal and for three miles southward, is about loo feet lower than the 

 Genesee near the south line of the city of Rochester, above the State 

 dam. Especially thick accumulations of the glacial drift in Mendon 

 caused the Genesee after the Ice age to take its new course through 

 Rochester ; and its rock gorge, extending from the center of this city 

 to its mouth at Charlotte, has been eroded during the Postglacial or 

 Recent epoch, of which, like the gorges below the falls of Niagara 

 and of St. Anthony, it affords a means of measurement, if the extent of 

 recession of the Genesee falls during the present century can be 

 determined. 



Southward and eastward, elevations of equal height with the 

 Pinnacle hills are first found at the distance of 7 to 10 miles, being 

 prominent drift accumulations later to be described in this paper, 

 lying in the southwest part of Pittsford and northwestern Mendon, 

 and between Victor and Fairport. 



The relation of the Pinnacle hills to the adjoining region will 

 be further exhibited by the following list of altitudes, which are 

 mostly derived from maps in the office of Mr. J. Y. McClintock, city 

 engineer of Rochester, others being from the United States Lake 

 Survey. They all are referred to mean tide sea level. 



Altitudes in Rochester and its J^icinity. 



Feet above 

 the sea. 



Lake Ontario, low and high water, 245-249 ; ordinary stage.. 247 

 Iroquois beach, between Irondequoit bay and the Genesee 



river (Gilbert) 436 



Erie canal, coping and tow-path of viaduct crossing the Gen- 

 esee river in Rochester, 510; water 508 



New York Central railroad track at Rochester station 516 



Wide Waters of the Erie canal on southeastern line of Roch- 

 ester 500 



Railroad at Brighton station 4*^° 



Canal at Brighton 480 



Summit of Arbutus avenue one-fourth mile south of Brighton, 



crossing the east end of the Pinnacle hills esker 536 



Top of esker about 50 rods west of last 570 



Top of esker one-third mile farther west 652 



Depression close southwest of last 590 



Highest point of Cobb's hill, one-third mile farther W. S. W.. 663 



Top of esker about 40 rods westward 608 



