PLEISTOCENE HISTORY OF THE GENESEE VALLEY 83 



waters it is not possible to correlate with precision the production of 

 the cataracts and the limiting lake waters. The upper cataract 

 correlates with the Mount Morris-Geneseo lake, though the begin- 

 ning of the canyon was during the life of the Dansville lake. The 

 middle cataract, with a crest altitude of 1005 feet, seems to belong 

 with the Lake Hall stage. The lower cataract correlates with Lake 

 Vanuxem and later stages. The initiation of the Portage canyon 

 was an interrupted process, covering a varied lake history and a 

 long period of time. 



The Mount Morris or High Banks canyon is in shales of such 

 weakness and uniformity that probably no large cataract was ever 

 produced. The river rapidly removed all obstructions to its flow 

 and acquired a uniform or graded slope. The initiation of the gorge 

 was probably during- the closing phase of Lake Hall, and the gorge 

 may have been entirely cut during the life of the first Lake Va- 

 nuxem and the Avon lake. The gorge was cut from the top down- 

 ward, and probably the erosion kept pace with the fall of the waters 

 in the Dansville valley, li the strata were as hard as the Portage 

 rocks probably cataracts would occur here also. 



The history of the Rochester canyon is similar to that of the 

 Portage. The upper cataract was established during the life of 

 Lake Iroquois, the plane of which was nearly 200 feet above that 

 of Lake Ontario. With the falling away of the Iroquois waters the 

 lower and northern part of the gorge, with the middle and lower 

 cataracts, came into existence. 



Deformation of the lake planes 



In making close correlation of the lake phenomena and drainage 

 it is necessary to take into the account the warping or tilting of 

 the land which has occurred since the glacial time, and the conse- 

 quent rise to northward which has been given to all the planes of the 

 ancient lakes. The amount of deformation of the Portage-Mount 

 Morris district is not known with precision but probably it is not 

 far from 2 feet per mile. 



The difference in the height- of the correlating features according 

 to latitude has not been regarded in the above description, except 

 in a few cases. The figures used in the description are the altitudes 

 which the features have as they stand today. In order to find the 

 correlating planes for localities of different latitude 2 feet per 

 mile should be added for northing or substracting for southing.^ 



^ Some discussion of the matter of deform5tion of the lake planeb is given in N. Y 

 State Mu5. BuL io6, p. 76-79. 



