204 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



height of all these; for the long rapid between the upper fall and the present place 

 of the lower one will be nearly as much descent as the fall at present. 



These speculations are offered not with a view to any practical bearing, but to 

 correct an erroneous impression which arises from the first view of these falls. Since 

 there are now three falls, and since we suppose there was a period when only one 

 existed, it is natural to infer that the same cause that first produced a separation 

 would continue to operate to perpetuate the same condition. This would doubtless 

 be true so long as the nature of the strata remained the same; but it is equally 

 evident that any change in these will change all the other conditions. 



In Pis. X and XI views of the falls and gorge near Portage, and of 

 the middle and lower falls near Rochester, are presented. 



GENESEE GLACIAL LAKES. 



It was Hall's opinion that subsequent to the deposition of the drift 

 the portion of the Grenesee north from Mount Morris was occupied by a 

 lake. The lake is supposed to have been held by a "barrier on the north," 

 but the nature of the barrier is not stated.^ Later investigations point 

 strongly to the ice sheet as the barrier. 



A special study of the evidence that lake waters occupied the Genesee 

 Valley in the Glacial epoch has been made by Fairchild, whose results are 

 given in the Bulletin of the Geological Society of America.^ Attention is 

 called to the peculiarly favorable conditions for the formation of glacial 

 lakes during both the advance and the retreat of the ice sheet. The traces 

 of the former have been obliterated, for at its culmination the ice sheet 

 extended southward beyond the limits of the Genesee Basin. The paper 

 is therefore devoted chiefly to a description of the features produced by 

 the waters held in by the ice barrier during the northward retreat and 

 later by barriers of drift that were formed by the ice sheet. 



Beaches or other shore-line features are necessarily weak, as the expanse 

 of water was not sufficient to give rise to strong waves and as the waters 

 were not long stationary at any particular plane, the height varying with 

 the season and the downcutting of the outlets. Deltas formed by land 

 streams and also glacial stream deltas were well developed. Wave-built 

 and wave-cut terraces also are prominent features. It was found that 

 these deltas and terraces harmonize in level with neighboring oiitlets on 

 the borders of the drainage basin, and that in passing from the southern 



1 New York Geol. Survey, Fourth District, 1843, p. 344. 



••'Glacial Genesee lakes, by H. L. Fairchild; Bull. Geol. Soc. America, Vol. VII, 1896, pp. 423-452. 



