446 H. L. FAIRCHILD — GLACIAL GENESEE LAKES. 



upper falls. At that point the limestone escarpment probably produced 

 at first only rapids in the stream which then for toward two miles flowed 

 in a deepening gorge of Niagara shale until it reached the level of the 

 Iroquois waters. The excavation of the lower ])art of the canyon did not 

 begin until the Iroquois lake was drained to the Ontario level. 



The studies of Mr Gilbert liave sliown that there were considerable 

 changes in the level of lake Iroquois produced by the differential eleva- 

 tion or warping of the land of the Ontario basin, and it is regarded as prob- 

 able that the whole region during tlie Warren and Iroquois episodes was 

 at a much lower altitude as regards sealevel than it is now. The unequal 

 elevation has caused tlie old Warren and Iroquois beaches to be thrown 

 out of their horizontality and to liave now a progressive elevation north- 

 eastward. 



With the removal of the glacial ice-dam from the vallc}^ of the Saint 

 Lawrence, lake Iroquois was drained by the lower outlet and the last of 

 the series of glacial Genesee lakes came to a reluctant end. 



TE Xr II STAGE: LAKE ONTARIO {XON GLACIAL.) 



Outlet l)y the Saint Lawrence river. 



It will be understood that this is not a stage oi glacial waters, ))ut it is 

 included here as completing the postglacial lacustrine history of the 

 Genesee valley. 



SUMMARY. 



The first stage in the glacial drainage of the valley was from the head- 

 waters to both the Sus(|uehanna and the Ohio-Mississippi, with altitudes 

 of water surfaces over 2,200 feet. 



The second, third and fourth stages drninod to the Ohio-Mississippi, 

 with altitudes respectively 2,068, 1,600 and 1,496 feet. 



The fifth and sixth stages drained to the Susquehanna, with altitudes 

 of l,?y20 and 1,210 feet. 



The seventh and eighth stages drained to the Illinois-Mississipi:)i, with 

 altitudes from 1,200 down to 880 ± feet. 



The ninth stage drained to the Hudson, with an altitude of 435 to 440 

 feet. 



The tenth stage is the non-glacial Saint Lawrence drainage,with present 

 altitude of 247 feet. 



Contemporary local glacial Lakes. 

 in genesee hydrographic area. 



Conditions affecting formation of local lakes. — Local or restricted glacial 

 lakes could only form in such creek valleys as incline northward or have 



