MODIFICATION OF THE GREAT LAKES BY EARTH 



MOVEMENT. 1 



By G. K. Gilbert, 



United States Geological Surrey. 



The history of the Great Lakes practically begins with the inelting 

 of the Pleistocene ice sheet. They may have existed before the in- 

 vasion of the ice, but if so their drainage system is unknown. The ice 

 came from the north and northeast, and spreading over the whole Lau- 

 rentian basin invaded the drainage districts of the Mississippi, Ohio, 

 Susquehanna, and Hudson. During its wandering there was a long 

 period when the waters were ponded between the ice front and the up- 

 lands south of the Laureutian basin, forming a series of glacial lakes 

 whose outlets were southward through various low passes. A great 

 stream from the Erie Basin crossed the divide at Fort Wayne to the 

 Wabash River. A river of the magnitude of the Niagara afterwards 

 flowed from the Michigan Basin across the divide at Chicago to the 

 Illinois River; and still later the chief outlet was from the Ontario 

 Basin across the divide at Rome to the Mohawk Valley. 



The positions of the glacial lakes are also marked by shore lines, 

 consisting of terraces, cliffs, and ridges, the strands and spits formed 

 by their waves. Several of these shore lines have been traced for 

 hundreds of miles, and wherever they are thoroughly studied it is 

 found that'they no longer lie level, but have gentle slopes toward the 

 south and southwest. Formed at the edges of water surfaces, they 

 must originally have been level, and their present lack of horizontality 

 is due to unequal uplift of the land. The region has been tilted toward 

 the south-southwest. The different shore lines are not strictly parallel, 

 and their gradients vary from place to place, ranging from a few inches 

 to 3 or 4 feet to the mile. 



The epoch of glacial lakes, or lakes partly bounded by ice, ended 

 with the disappearance of the ice field, and there remained only lakes 

 of the modern type, wholly surrounded by land. These were formed 

 one at a time, and the first to appear was in the Erie Basin. It was 



"Reprinted from the National Geographic Magazine, Vol. VIII, No. 9, September, 



1897. A more extended paper of similar scope entitled " Recent earth movement in 



the Great Lakes region," was printed in the Eighteenth Annual Report of the 



United States Geological Survey, Part II, pp. 595-647. 



<U9 



