Fig. 27.—Sketch map showing a very early stage in the history of the Great Lakes when two relatively small lakes in front of the ice wall separately drained into the Mississippi River. (Drawn by the author from map by Taylor & Leverett.)
Fig. 28.—Lake Whittlesey stage of the Great Lakes history when the ice had retreated far enough to allow the eastern and western ice margin waters to join with a single outlet past Chicago. (Drawn by the author from a map by Taylor & Leverett.)
Fig. 29.—The Algonquin-Iroquois stage of the Great Lakes when their whole area was ice-free, and all their waters drained through the Mohawk-Hudson Valleys of New York into the Atlantic Ocean. (After Taylor, published by New York State Museum.)
We shall now very briefly trace out the principal stages in the history of the Great Lakes during the final retreat of the vast ice sheet. This may best be done by the aid of maps which need only brief explanation. When the ice sheet had retreated far enough northward to uncover the very southern end of the Lake Michigan basin and a little beyond, a small glacial lake (Lake Chicago) developed against the ice wall. Its outlet was through the Illinois River and thence into the Mississippi. At the same time a larger glacial lake, held up by the ice wall, developed over the western part of the Erie basin and beyond. Its outlet was through the Wabash River. With further retreat of the ice a large lake (Whittlesey) covering considerably more than the area of Lake Erie developed, with outlet westward across Michigan into the enlarged Lake Chicago which continued to drain into the Illinois River. During a still later stage of ice withdrawal the remarkable set of three glacial lakes existed—Lakes Duluth, Chicago, and Lundy. Each of these large lakes had its own outlet. Lake Duluth covered about half of the Lake Superior basin and drained through the St. Croix River into the Mississippi. Lake Chicago expanded to cover nearly all of the Michigan basin and continued to drain through the Illinois River. Lake Lundy covered not only more than the area of the Erie basin, but also considerable territory north of Detroit, and drained eastward alongside the ice lobe of the Ontario basin through the Mohawk and Hudson valleys of New York, and into the Atlantic Ocean. Just after the ice completely withdrew from the area now occupied by the Great Lakes, but still blocked the St. Lawrence Valley, the vast body of water called Lake Algonquin more than covered the sites of the present Superior, Michigan, and Huron. At this time the land was distinctly lower toward the northeast than at present, causing the outlets to the west to be abandoned. The great Lake Algonquin poured its waters eastward through the Trent River channel of Ontario, Canada, into glacial Lake Iroquois, which was the great ancestor of Lake Ontario. Lake Iroquois, in turn, had its outlet eastward through the Mohawk and Hudson Valleys of New York. For part of the time at least, Lake Erie maintained a separate existence discharging into Lake Iroquois near Buffalo. During the Algonquin-Iroquois stage the combined area of all the lakes was notably greater than the present area of the Great Lakes. The volume of water discharged by the lakes through the Mohawk Valley of New York was doubtless greater than that which now goes over Niagara Falls. Gradually, as the St. Lawrence ice lobe waned, the outlet waters of the lakes began to move alongside the ice through the St. Lawrence Valley. Finally the ice withdrew far enough to free the St. Lawrence Valley and the waters of the Great Lakes region dropped to a still lower level, bringing about the Nipissing Great Lakes stage not greatly different from the present. East and northeast of the Lakes the land was low enough to allow tidewater (the so-called Champlain Sea) to extend through the Hudson, Champlain, and St. Lawrence Valleys, and possibly into the Ontario basin, as proved by the occurrence of marine beaches and fossils. The waters in the Erie and Ontario basins covered about the present areas, while the Nipissing Lakes, which covered a little more than the present areas of the three upper Great Lakes, had their outlet through the Ottawa River channel into tidewater (Champlain Sea). Postglacial warping of the land has brought the whole region to the present condition.