THE HISTORY OF THE NIAGARA RIVER 235 



glacier was supplied by snow falling upon regions far to the north. To 

 a certain extent the glacier discharged to the ocean lil^e a river, break- 

 ing up into icebergs and floating away; but its chief discharge was 

 upon the land, through melting. The climate at its southern margin 

 was relatively warm, and into this warm climate the sheet of ice steadily 

 pushed and was as steadily dissolved. 



Whatever stones and earth were picked up or torn uj) by the ice 

 moved with it to its southern margin and fell to the ground as the ice 

 melted. If the position of the ice margin had been perfectly uniform 

 its continuously deposited load might have built a .single high wall; 

 but as the seasons were cold or warm, wet or dry, t])e ice margin 

 advanced and retreated with endless variation, and this led to the 

 deposition of irregular congeries of hills, constituting what is known 

 as the " drift deposit." Eventually the warm climate of the south pre- 

 vailed over the invader born of a cold climate, compelling it to retreat. 

 The motion of the ice current was not reversed, but the front of the 

 glacier was melted more rapidly than it could be renewed, aud thus its 

 area was gradually restricted. During the whole ])eriod of retrei ch- 

 ment the deposition of drift proceeded at the margin of the ice, so 

 that the entire area that it formerly occui)ied is now diversified by 

 irregular sheets and heapings of earth and stone. 



The ancient configuration of the country was more or less modified 

 by the erosive action of the ice, and it was further modified by the 

 deposits of drift. The destructive and constructive agencies together 

 gave to the laud an entirely new system of hills and valleys. When 

 the ice was gone the rain that fell on the land could no longer follow 

 the old lines of drainage. Some of the old valleys had i)erhai)s been 

 obliterated; others had been changed so that their descent was in a 

 difterent direction, and all were obstructed here and there by the heaps 

 of drifts. The waters were held upon the surface in innumerable lakes, 

 each overflowing at the lowest side of its basin, and thus giving birth 

 to a stream that descended to some other lake. Olten the new lines 

 of descent — the new water courses — crossed regions that before had had 

 no streams, and then they were compelled to dig their own channels. 

 Thus it was that the whole water system of a vast region was refash- 

 ioned, and thus it has come to pass that the streams of this region are 

 young. 



Like every other stream of the district of the Great Lakes, the 

 Niagara was born during the melting of the ice, and so we may begin 

 our chronicle with the very beginning of the rivt r. 



If you will again call to mind the features of a general map of the 

 United States aud Canada, and consider the direction in which the 

 streams flow, you will perceive that there is a continuous upland, a sort 

 of main divide, separating the basin of the Great Lakes from the basin 

 of the Mississippi.* It is not a mountain range. In great part it is a 



* A part of its course appears as a broken line on the maps in Figs. 3 and 4. 



