It is obvious that at the time of their formation all shore lines referable to the work of waves must have been horizontal, and hence any variations from a perfect level which they reveal to-day must indicate that a tilting movement of the ground has occurred since the waters departed from their basins. We have thus provided for us in the positions of these ancient water planes, particularly because of their wide extent, a complete record the refinement of which is not easily overstated. Interpreting this record, we find that it was the uptilt of the land to the northward which brought the glacial lake history to an end and inaugurated the present system of St. Lawrence drainage. The outlet of the Nipissing Great Lakes is to-day more than a hundred feet above the level of the outlet at Port Huron, where the upper lakes are now discharging their waters, and this difference in level can only be ascribed to an upward tilting of the land since the latest of the glacial lake stages. =The abandoned strands as they appear to-day.=—The traveler by steamer upon the upper lakes, as he comes within view of each rocky headland, may note how the profile against the horizon is notched by a series of steps or terraces (Fig. 368), and if he has followed the discussion in previous chapters, he will suspect that these terraces mark the now abandoned shore lines which have come to their present position through a series of uplifts of the ground accompanied by earthquake shocks. As his steamer skirts the shore he may chance to note a cave within the rock cliff which represents the now elevated sea-arch of an ancient shore. [Illustration: FIG. 368.—The notched rock headland of Boyer Bluff between Green Bay and Lake Michigan (after Goldthwait).] Disembarking from the steamer and traveling inland at any point where the shores are high, the traveler is certain to come upon still more convincing proofs of the ancient strands; perhaps in a storm beach of the unmistakable “shingle”, half buried though it may be under dunes of newly drifted sand, or possibly at higher levels the highway has been cut through a shingle barrier as fresh and unmistakable as though formed upon the present shore. Sometimes it is the rock cliff and terrace, at other times barrier ridges of shingle, or, again, it is the sloping cliff and terrace cut in the drift deposits; but of whatever sort, if studied with proper regard to the topography of the district, the evidence is clear and unmistakable.