84 NEW YORK STATE MUSEUM 



For close correlation of the lake-level features some allowance 

 should be made for the depth of water in the outlet channel, and for 

 the down cutting of the outlet during the life of the lake. 



Detrital filling of the valleys 



All the ancient valleys have been so filled with glacial drift and 

 lake sediments that in the absence of borings the grade slopes of 

 the streams can not be determined. The localities of diverted drain- 

 age have been blocked by morainal drift, while the more open 

 stretches of the valleys are, more or less filled with stream detritus 

 and lake silts, which may overlie glacial deposits. 



The old valley above Portage has been considerably filled with 

 glacial rubbish and later by lake sediments during the lacustrine 

 stages 4 and 5. 



The St Helena valley was largely filled with delta stuff during the 

 stages 6 to 8, and during the life of the St Helena-Gibsonville mor- 

 ainal lake. With the cutting down of the " High Banks " it has been 

 reexcavated. Any flooding during the stages 11 to 13 that over- 

 topped the outlet stream must have produced deposition with sub- 

 sequent reexcavation. 



The Nunda valley was subject to lake deposition during the stages 

 6 to 9, and stages it and 12. 



The old and broad valley from Dansville to Avon has evidently 

 been subject to lake filling during all the stages from 6 to 13, and 

 stream aggradation is still active. 



