286 G. K. GILBERT — OLD TRACKS OF ERIAN DRAINAGE. 



the drainage lines end at the Iroquois shore, but others terminate, so far 

 as traced, at higher levels, indicating that before the establishment of 

 lake Iroquois there was in the eastern part of its basin a higher base- 

 level, probably determined by an ice-dam in the lower Mohawk valley. 

 Near Syracuse certain of the channels are clogged by glacial drift in such 

 way as to demonstrate a readvance of the ice after their formation b} 7 " 

 running water. 



As the positions of details of the channels were determined by the rela- 

 tion of the ice-front to the configuration of the northward sloping land, 

 their mapping aids in determining the trend of the ice-margin, and this 

 trend is shown to have been approximately east and west in the Rochester- 

 Syracuse region. 



West of Clarendon the valley between the outcrops of the Niagara and 

 Corniferous limetones appears to have held a shallow lake just after the 

 retreat of the ice from the Niagara escarpment, and this lake initially 

 discharged over the escarpment at five points — Clarendon, Shelby, Gas- 

 port, Lockport, and Niagara. The controlling sill on the line of Niagara 

 river was at the Johnson ridge. Most of these overflows were of brief 

 duration, but that at Lockport continued for a considerable period, com- 

 peting with the Niagara for establishment as the permanent outlet of 

 lake Erie. 



