GLACIAL BAMS, LAKES, AND WATERFALLS. 357 



condensed that we can do no better than reproduce the most 

 of it : 



Previous to the Glacial period the elevation of this por- 

 tion of the continent was considerably greater than now, and 

 it was drained by a river system which flowed at a much lower 

 level than at present. At that time our chain of lakes — 

 Ontario, Erie, and Huron — apparently formed portions of the 

 valley of a river which subsequently became the St. Lawrence, 

 but which then flowed between the Adirondacks and Appa- 

 lachians, in the line of the deeply buried channel of the Mo- 

 hawk, passing through the trough of the Hudson and empt} r - 

 ing into the ocean, eighty miles southeast of New York. Lake 

 Michigan was apparently then a part of a river-course which 

 drained Lake Superior and emptied into the Mississippi, the 

 Straits of Mackinaw being not yet opened. 



With the approach of the cold period, local glaciers formed 

 on the Laurentian Mountains, and, as they increased in size, 

 gradually crept down on to and began to excavate the plateau 

 which bordered them on the west and south. The excavation 

 of our lake-basins was begun, and perhaps in large part effect- 

 ed, in this epoch. 



As the cold increased and reached its maximum degree, a 

 great ice-sheet was formed by the enormously increased and 

 partially coalescing local glaciers of the former epoch. This 

 many-lobed ice-sheet, or compound glacier, moved radiatingly 

 from the south, southwest, and western slopes of the Canadian 

 highlands, its Ohio lobe reaching as far south as Cincinnati. 

 The effect of this glacier upon Lake Erie and Lake Ontario 

 would be to broaden their basins by impinging against and 

 grinding away with inconceivable power their southern mar- 

 gins. To the action of this agent we must ascribe the peculiar 

 outline of the profile sections drawn from the Laurentian 

 Hills across the basin of Lake Ontario to the Alleghanies, and 

 across that of Lake Erie to the highlands of Ohio, viz., a long, 

 gradual slope from the north to the bottom of the depression, 

 and then an abrupt ascent over the massive and immovable 

 obstacle against which the ice was banked, until, by the vis a 

 tergo, it overtopped the barrier. In New York that barrier 



