98 LAKES OF .NORTH AMERICA. 



ancient river valley leading south from the southern end of Lake Mich- 

 igan, which is reported to be scored with glacial grooves, and obstructed 

 by glacial deposits.^ As will be noticed below, this same channel was also 

 an outlet for the waters of the Lake Michigan basin in post-glacial times. 



When the glaciers of the Glacial epoch were at their maximum, the 

 drainage from the ice found a free escape southward, as is abundantly 

 testified by immense deposits of gravel that were dropped by the over- 

 loaded glacial streams, as well as by numerous water-worn channels which 

 are too large for the streams now occupying them and are without water- 

 sheds commensurate with their size. 



As the ice sheet retreated, there came a time when its southern margin 

 was north of the drainage divide, passing in an irregular east and west 

 direction through Central New York and Central Ohio, and now parting 

 the waters flowing south from those that find their way northward to the 

 Laurentian lakes. When this occurred, lakes were formed between the 

 margin of the ice and the high land to the south. These earlier lakes 

 stood at various levels and discharged southward across the lowest dejjres- 

 sions in their shores. Stream channels were excavated by the outflowing 

 waters and became deeply filled with gravel and sand, but in many instances 

 are still clearly traceable. One of these ancient channels starts near Fort 

 Wayne, Indiana, leads soutliAvest and afforded an escape for the waters that 

 accumulated in the western portion of the Erie basin. A similar outlet 

 at the south end of the Lake Michigan basin has already been referred to. 

 Other points of discharge have been reported at other localities on the 

 southern margin of the Laurentian basin. 



As the ice occupying the Erie-Ontario basin Avithdrew northward, 

 the lakes about its margin expanded and became united one with another. 

 When the ice barrier between the two basins was broken the higher lake 

 discharged into the lower one, and its former outlet leading south was 

 abandoned. 



When a single water body occupied the Erie-Ontario basin, the site of 

 Niagara river was deeply submerged. When the water fell to the level 

 of the Mohawk outlet, the two basins became divided and Niagara river was 

 born. The river from the upper basin discharged across the lowest sag- 

 in its rim and cut back a deep gorge, until an old channel excavated in pre- 

 glacial or possibly inter-glacial times, was discovered and the work of 

 extending it renewed. When the falls shall have receded so as to drain 



1 Farther evidence seems to be needed, however, before the presence of a pre-glacial 

 channel leading south from Lake Michigan, can be considered as definitely determined. 



