HOW THE GREAT LAKES WERE BUILT. 171 



their basins (see Fig. 14), but later even the Iroquois Gulf was 

 contracted so as not to occupy even the head of the present On- 

 tario basin (see Fig. 14). 



The great deformation of the whole region since the close of 

 the Iroquois episode has from that day to this been slowly raising 

 the northeastern rims of the lake basins so as to cause them to 

 flood more and more the lowlands and valleys at their southwest- 

 ern extremities, and even to raise the waters so high as to cover 

 some of the deserted shores in those directions. At the same 

 time the waters are leaving their old margins at their northeast- 

 ern ends, as shown on the map (Fig. 14). 



The changes have not been quite simultaneous in the different 

 basins, as the heights of the lake barriers and the rate of terres- 

 trial movements have not been uniform. Thus, in terms of Niag- 

 ara Falls, it is estimated that the Iroquois Gulf sank below the 

 Iroquois plane about fourteen thousand years ago ; but that the 



Fm. 15. — Section showing the Tilting of the Iroquois BEArii South of Lake Ontario 

 AND THE St. Lawrence River as far as the Northeast Corner of the Adirondacks. 



waters of Lake Huron, which had been emptying by way of the 

 Nipissing Strait for twenty-four thousand years, were turned into 

 Lake Erie only eight thousand years ago. Again, after the 

 waters of the Ontario basin had sunk much below the present 

 western margin of the lakes, they were rising again to near their 

 present height only some three thousand years ago. 



Of the absolute amount of rise of the continent we do not 

 know, for the axis of uplift has not been ascertained, but it is evi- 

 dently in the interior of the continent. The differential rate of 

 elevation varies, being about a foot and a quarter a century in the 

 Niagara district, two feet northeast of Lake Huron, and nearly 

 four feet north of the Adirondacks. 



The Future Drainage of the Upper Lakes into the Mis- 

 sissippi River. — With the land rising as at present, it will be 

 only a matter of time until the northeastern rim of Lake Erie 

 will be so high that the drainage must turn into Lake Huron, and 

 thence by way of Lake Michigan and the Chicago Canal into the 

 Mississippi, and Niagara Falls will then end their life history. 

 Some fifteen hundred years ago there was a barrier about a mile 

 north of the present site of the falls that had risen so high in the 

 general regional uplift as to actually cause some of the waters of 

 the upper lakes to overflow where the Chicago Canal is now being 

 built ; but, owing to the peculiar buried valley just behind this 

 ridge crossing Niagara River, when the falls had passed the bar- 

 rier, before the change of outlet of the upper lakes from the Ni- 



