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 melting 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 wandering there was a long period when the waters were ponded between the ice front and the up- lands south of the Laurentian 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.
