FIG. 363.—Map of the glacial Lake Warren, the last of the lakes in the Huron-Erie basin, which discharged through the “Grand River outlet” into the Mississippi (after Leverett).] =Lakes Arkona and Whittlesey.=—The ice front in the Huron-Erie basin now retired so far that the impounded waters, instead of following the more direct “Imlay outlet” to the Grand, passed at a lower level completely around “the thumb” of Michigan into the Saginaw basin. Meanwhile a crescent-shaped lake had developed in that basin, so that now the waters of the Maumee basin were joined to those in the Saginaw basin as a common lake, just as the lowering of the waters in Glen Roy caused a union with those of Glen Glaster in the example cited for illustration. Our records of this third North American lake stage, referred to as Lake Arkona, are however most imperfect, for the reason that it was followed by a readvance of the ice front which closed the passage around “the thumb” and raised the level of the waters until an outlet was found past the town of Ubly at a lower level than the “Imlay outlet.” When the waters of a lake are thus rising, strong beach formations result, and those of this stage, which is known as the Lake Whittlesey stage, are much the strongest that are found within the Huron-Erie basin. Traced for some three hundred miles entirely around the southern and western margins of Lake Erie, this beach is for much of the distance the famous “ridge road” (Fig. 362). =Lake Warren.=—As the ice advance which had produced Lake Whittlesey came to an end, the normal recession was resumed and a lake once more formed as a body common to the Saginaw and Erie basins. This lake, known as Lake Warren, extended a shrunk arm far eastward along the ice front into western New York, though it was still blocked from entering the great Mohawk valley (Fig. 363). [Illustration: FIG. 364.—Map of the Glacial Lake Algonquin (after Leverett).] =Lakes Iroquois and Algonquin.=—It must be evident that toward the close of the Lake Warren stage a profound change was imminent—a transfer of the glacial waters from their course to the Mississippi and the Gulf to the trench which crosses New York State and enters the Atlantic. So soon as the ice front had retired sufficiently to lay bare the bed of the Mohawk, an outlet was found by this route and its continuation down the Hudson valley to the sea. The Lake Ontario basin now became occupied by a considerably larger water body known as Lake Iroquois, and the three upper lakes, then joined as Lake Algonquin, discharged their combined waters into Lake Iroquois at first through a great channel now strongly marked across Ontario in the course of the Trent River and Lake Simcoe, the so-called “Trent outlet.” At this time a smaller Lake Erie probably occupied the basin of that lake, and later the Trent outlet was abandoned for the Port Huron outlet (Fig. 364).