THE GLACIAL. LAKES AND LAKE li R 1 1 



flows northward over a i^ently southward -sloping bed of 

 thick hmcstonc. When tlie water of the river lalls over the 

 escarpment it drops with such force that it breaks up the 

 rocks below and thus gives the stream below the falls powerful 

 cutting tools. Under the limestone are thick beds of soft 

 shales — the same shales in which Green and Georgian bays 

 are carved. Below the shales is a thin hard limestone which 

 furnishes heavy tools when its fragments fall into the river; 

 under this is another layer of soft shales, and then a resistant 

 sandstone about at the water level below the falls. The water 

 dashing against the rocks at the base of the falls hurls the 

 fragments upward against the soft shales, smashing, biting, 

 breaking into them, and so undermining the sandstones and 

 limestones. The thick heavy upper limestone is thus under- 

 cut until it resembles an overhanging cornice; it becomes top- 

 heavy and then huge masses of it break loose and fall into the 

 maelstrom below, adding to the tools already there. This 

 process of undercutting the shales and breaking off the un- 

 supported limestone cornice has been going on ever since the 

 first water dashed against the shales in the escarpment cliff at 

 Lewiston, eighteen to twenty thousand years ago, and has been 

 the most important factor in cutting the Niagara gorge. The 

 gorge has been deepened by the river with rock-fragment 

 tools supplied from the fallen sandstone and limestone blocks, 

 but the varying widths and other characteristics of the gorge 

 are a part of the story of the lakes. 



The Second Stage of Lake Algonquin; Lake Iroquois; 



Lake Erie 



The early stage of Lake Algonquin passed when the ice 

 retreated far enough to open an outlet lower than the out- 

 lets at Port Huron and Chicago. When the level of the lakes 

 became stationary at the level of this new outlet the second 

 stage of Lake Algonquin was established, and it endured for 

 a long time. (Figure 13.) As early Lake Algonquin was 



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