Fig. 351.—Three successive diagrams to set forth in order the late glacial lake history of the Scottish glens.
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.
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.
Fig. 353.—Map of Lake Agassiz (after Upham).