The glacial Lake Agassiz.—The grandest of the temporary lakes referable to blocking by the continental glaciers of the ice age must be looked for in the largest valleys that lay within the territory invaded and which normally drain toward the retiring ice front. In North America these rivers are the Red River of the North in Minnesota, the Dakotas, and Manitoba; and the St. Lawrence River system. To the ice dam which lay across the Red River valley we owe the fertility of that vast plain of lake deposits where is to-day the most intensive wheat farming of the northwest ([Fig. 352]). Lakes Winnipeg, Winnipegoosis, and Manitoba, and the Lake of the Woods, are all that now remain of this greatest of the glacial lakes, which in honor of the distinguished founder of the glacial theory has been called Lake Agassiz ([Fig. 353]). With their natural outlet blocked by the ice in northern Manitoba and Keewatin, the waters of the Red were swollen by melting from the retiring glacier and spread over a vast area before finding a southern outlet along the course of the present Lake Traverse and the valley of the Minnesota River. Along this route there flowed a mighty flood which carved out a broad valley many times too large for the Minnesota, its present occupant, and this giant prehistoric river has been called the Warren River ([Fig. 354]).
Fig. 354.—Map of the southern end of the Lake Agassiz basin, showing the position of some of the beaches and the outlet through the former Warren River (after Upham).
Fig. 355.—Narrows of the Warren River below Big Stone Lake, where it passed between jaws of hard granite and gneiss (after Upham).
Fig. 356.—Map of the valley of the Warren River in the vicinity of Minneapolis, with the young valley of the Mississippi entering it at Fort Snelling (after Sardeson).
It is interesting to follow this ancient waterway and to discover that, like our normal, present-day streams, it was held up in narrows wherever outcroppings of harder rock had constricted its channel ([Fig. 355]). The upper end of the Warren River valley is now occupied by the long and relatively narrow Lakes Traverse and Big Stone, each the result of blocking by delta deposits where a tributary stream has emerged into the valley, but this gigantic channel continues down to and beyond Minneapolis, occupied as far as Fort Snelling by the Minnesota River—a mere pygmy compared to its predecessor. To the earnest student of glacial geology there can be few sights more impressive than are obtained by standing at Fort Snelling, just above the confluence of the Minnesota and the Mississippi rivers, and surveying first the steep and narrow valley of the Mississippi above the junction,—a stream fitted to its valley for the simple reason that it has carved it,—and then gazing up and down that broad valley in which the great Warren River once flowed majestically to the sea, now the bed of the Minnesota above the Fort and of the Mississippi below it ([Fig. 356]).