GLACIAL GEOLOGY OF THE SCHENECTADY QUADRANGLE 33 



the barrier of till, or till and rock, in the Aqueduct region. The 

 dammed up waters, everywhere pressing against the slopes of the 

 basin, gradually cut through these deposits and coursing northward 

 flowed over the area in the vicinity of East Line, discharging into 

 Lake Albany in that quarter. 



These two exits of the Mohawk flood waters from the basin near 

 Schenectady to Lake Albany continued for a considerable period. 

 During this period the waters of Lake Albany further subsided. 

 The three watercourses radiating from the East Line locality were 

 established and the erosion of the Lake Albany sediments now rep- 

 resented by their channels, in their broader aspects, including the 

 Round lake depression, took place. 



At the same time the spillway in the Aqueduct region had deep- 

 ened in that portion of its bed which was least resistant to erosion 

 and had thus gradually acquired the character of a gorge. Its 

 capacity, however, had not been increased to the extent that it 

 afforded an exit for the entire volume of waters that the Mohawk 

 poured into the basin, so that the excess of waters, above the 

 capacity of the nascent gorge, continued to be diverted into the 

 Ballston channel. 



While the condition just named must have been for a long time 

 a factor in the maintenance of the two contemporaneous outlets, it 

 does not appear that it was a condition necessary to this end. For 

 if we suppose that the rate of subsidence of Lake Albany was not 

 greater than the rate of lowering of the channel of the two streams 

 by erosion then the two outlets must have persisted. Even when 

 the Aqueduct gorge, in the process of its gradual enlargement, 

 acquired a capacity sufficient to contain the entire volume of 

 Mohawk waters, the Ballston channel remained open as long as its 

 bed was maintained at the same level as that of the Aqueduct chan- 

 nel. As long as, due to the subsidence of Lake Albany, an impetus 

 was given to the flow of the waters in the two channels and as long 

 as this flow kept the beds of the two channels eroded to the same 

 level, both outlets must have persisted. 



At length the time came when the Ballston channel currents were 

 unable to maintain this equality of erosive effects. The greater 

 length of bed to be deepened as compared with its rival gave the 

 advantage to the latter. As soon as a slight difference in depth of 

 bed was established in favor of the Aqueduct passage, the waters 

 of the Ballston channel began to be drawn off. The divide in the 

 channel emerged as land surface and the present system of drain- 

 age was initiated. 



