THE 



National Geographic Magazine 



Vol.. VIII SEPTEMBER, 1S!»7 No. 9 



MODIFICATION OF THE GREAT LAKES BY EARTH 



Movement* 



By G. K. Gilbkht, 

 U. S. Geological Surrey 



The history of the Great Tiiikes practically hegiiis with the melt- 

 ing of the Pleistocene ice-sheet. They niay have existed before 

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

 The ice came from the north and northeast, and sproadinij over 

 the whole Laurentian basin invaded the drainaji;e districts of the 

 Mississippi, Ohio, Susquehanna, and Hudson. During its wan- 

 ing there was a long period when the waters were ponded between 

 the ice front and the uplands south of the Laurentian basin, form- 

 ing a series of glacial lakes whose outlets were southward through 

 various low passes. A great stream from the Erie basin erost the 

 divide at Fort Wayne to the Wabash river. A river of the mag- 

 nitude of the Niagara afterward flowed from the Michigan basin 

 acro.ss 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 markt by shore-lines, 

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

 formed by their waves. Several of these shore-lines have Ix-en 

 traced for hundreds of miles, and wherever they are tlmrougbly 

 studied it is found that they no longer lie level l)ut have gentle 

 slopes toward the south and southwest. Formed at the edges of 



*Publisht by permission of the Director of the United States Geologioul Survey. 

 A more extended paper, of similar scope, entitled " Keoent enrtli moTement in lh« 

 Great Lakes region," will appear in the Eighteenth Annual Report of the Surrey. 

 16 



