National Geographic : 1897 Sep
THE National Geographic Magazine VOL. VIII SEPTEMBER, 1897 No. 9 MODIFICATION OF THE GREAT LAKES BY EARTH MOVEMENT* BY G. K. GILBERT, U. S. Geological Survey The history of the Great Lakes practically begins with the melt ing of the Pleistocene ice-sheet. They may 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 spreading over the whole Laurentian basin invaded the drainage 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 crost the divide at Fort Wayne to the Wabash river. A river of the mag nitude of the Niagara afterward 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 markt 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 *Publisht by permission of the Director of the United States Geological Survey. A more extended paper, of similar scope, entitled " Recent earth movement in the Great Lakes region," will appear in the Eighteenth Annual Report of the Survey.