The origin and history of the Great Lakes constitutes one of the most interesting and remarkable chapters in the recent geological history of North America. Most of the salient points have been well worked out and they may be very briefly summarized, as follows: Before the Ice Age the Great Lakes did not exist, because the region, prior to that time, had been land subjected to erosion for millions of years--a time altogether too long for any lake to survive. Their sites were occupied by broad, low, stream-cut valleys which were quite certainly locally somewhat deepened by ice erosion during the Ice Age. Ice erosion is, however, altogether insufficient to account for the great closed basins. The two most important factors entering into the formation of the basins of the Great Lakes were doubtless the great glacial (morainic) accumulations acting as dams along the south side, and the tilting of the land downward on the north side of the region. In support of this explanation it has been established that the great dumping ground of ice-transported materials from the north was in general along the southern side of the Great Lakes and southward. It has also been well established that, late in the Ice Age, the land on the southern side of the Great Lakes region was lower than at present, as proved by the tilted character of beaches of the well-known extinct glacial lakes which were the ancestors of the present lakes. Such a down-warp of the land must have helped to form the closed basins by tending to stop the southward and southwestward drainage of the region. [Illustration: 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.)] [Illustration: 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.)] [Illustration: 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.)]