THE FLOODS OF THE GREAT LAKES. 43 



stretching southward and westward over a large part of the 

 state of Illinois. 



What hemmed in this broad expanse of water on the 

 south ? This interesting question has not been completely 

 answered. We know that in southern Illinois are the remains 

 of an ancient barrier which crossed the Mississippi, and was 

 worn down for the passage of the great river. The barrier is 

 a prolongation of the Ozark range, from Missouri. The gap 

 cut through is at Grand Tower. Perhaps here was the bar- 

 rier which held the waters back at the west, until the Missis- 

 sippi gradually sawed the notch which drained the inland sea. 

 At the east, however, we know no barrier adequate to hold 

 the lakes at the level of the 220 feet terrace of Lake Erie. 

 The high flood of the lakes must have been 182 feet higher 

 than the escarpment or wall of rock back of Lewiston, through 

 which the Niagara river has cut its gorge. Undoubtedly, this 

 escarpment, which runs east nearly parallel with the shore of 

 Lake Ontario, was formerly much higher than at present ; but 

 we have no evidence that it stood 190 feet higher than in 

 our time. 



The Lewiston escarpment is at present 38 feet above Lake 

 Erie, and could have dammed the lake to that height, at any 

 time before the Niagara gorge was begun. The water, setting 

 back to the site of Chicago, would have buried it 22 feet 

 deep. Even this would overflow the present southern barrier 

 of Lake Michigan, and inundate the prairie region of Illinois. 

 Thus, the existence of a terrace, but 38 feet above Lake Erie 

 would indicate that the great lakes once flooded the greater 

 part of the state of Illinois. Now, if we examine the nature 

 of the prairie deposit, it presents every indication of forma- 

 tion in the bottom of a lake. Here is the stratified arrange- 

 ment ; here are the clay and marls, and here are even the 

 shells of the molluscs which dwelt in the water. These facts 

 must be borne in mind. 



The high water of the Great Lakes has occurred since the 

 Drift was deposited since the latest semi-stratified Drift was 

 laid down. The lake terraces rest on the Drift. All the 



