Niagara Falls 



1903 If all these changes should take place, however, there can be 



Upham no doubt that the harbors and waterways, including canals, which 



now receive the vast and growing commerce of Buffalo, Cleve- 

 land, Toledo, Detroit, Chicago, Milwaukee, Duluth and 

 Superior, and many other cities and towns on these lakes, will 

 still be maintained in full utility. For the largest city of this 

 area, Chicago, although it is mainly built on land only a little 

 above the level of Lake Michigan, we may be confident that no 

 inundation will ensue. A drainage canal leading to the Des 

 Plaines and Illinois rivers has been cut down below the lake 

 level, and it needs only enlargement to carry the whole outflow 

 of these lakes, and to preserve the water level at Chicago 

 unchanged by the land movement. 



The present very slow tilting of this region is a continuation 

 of a great and far extended differential uplift which has taken 

 place during late glacial and postglacial time. The vast country 

 that had been ice-covered and depressed under the weight of the 

 thick continental ice-sheet was gradually uplifted, and to a 

 greater height at the north than at the south, during the removal 

 of the ice burden. While lakes Agassiz and Warren still existed, 

 the northern parts of this area were raised, in comparison with 

 their southern outlets, 300 to 400 feet or more. It is also found 

 by the present inclinations and relationship of the successively 

 formed shorelines of these and the other associated glacial lakes, 

 that this movement proceeded as a permanent wave of land eleva- 

 tion from the periphery of the old ice-sheet inward to its central 

 area. 



Both North America and Europe have experienced great 

 differential movements during and since the Ice age. From their 

 high preglacial elevation, the ice-enveloped lands sank beneath 

 the weight added by the snows of many thousand years; and 

 the warmer climate thereby produced on the boundaries of the 

 ice-sheets caused them to melt away, their latest remnants being 

 on the central areas where the ice accumulation was thickest. 

 The sea then overspread the borders of the depressed lands. In 



654 



