To derive the principles underlying this history, it is at once seen that _all changes are initiated by the retirement of the ice front to such a point that it unblocks for the waters of a lake an outlet that is lower than the one in service at the time_. This is the principle which explains nearly all episodes of glacial lake history. Thus, when the ice front had retired so as to open direct connections between Glen Roy and Glen Glaster, the col at the head of Glen Roy was abandoned as an outlet, and the waters fell to the level fixed for Glen Glaster. A still further retirement at last opened direct connection between Glen Glaster and Glen Spean, so that the lake common to Glens Glaster and Roy fell to the level of the col which was the outlet of the Spean valley at the time. This stage continued until the ice front had retired so far that the waters drained naturally down the river Spean to Loch Lochy and thence to the ocean. [Illustration: FIG. 352.—Harvesting time on the fertile floor of the glacial Lake Agassiz (after Howell).] Only in their far grander scale and in the lesser relief of the land over which they formed, do the complex histories of the great ice-blocked lakes of North America differ from these little valley lakes whose beaches may be visited and the relationships worked out, thanks to Jamieson, in a single day’s strolling. [Illustration: FIG. 353.—Map of Lake Agassiz (after Upham).] =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). [Illustration: 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).]