Page:Popular Science Monthly Volume 49.djvu/181

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167
HOW THE GREAT LAKES WERE BUILT.

all the way from the head of Lake Michigan. This gentle deformation of the surface of the country when carried over such long distances gives rise to the great physical reliefs of the mountain regions of the north and east, which were much lower before the lake epoch than now, as is apparent if the tilting be straightened out, as may be seen in the sections given. Furthermore, the character of the river courses at the surface of the country north of the lake regions indicates that even in the ice age the relatively high reliefs north of the lakes did not obtain.

The Gulf Epoch.—In the re-elevation of the lake district after the post-glacial submergence, when the continent was high enough to partially inclose a large gulf, already referred to, to which the name of Warren Water has been given, or more correctly Gulf, there were several water connections through the valleys to the south and west of it. Its last stage as one body of water is marked by the Forest beach, as shown on the map (Fig. 13). Upon the further rise of the land the surface of Warren Gulf fell below the level of the Forest beach for a depth of a hundred and fifty feet. The movement was gradual, without striking

PSM V49 D181 Section of michigan highlands to lake nipissing.jpg

Fig. 12.—Section from the Michigan Highlands back of Alpina to the Laurentian Highlands beyond Lake Nipissing. Length of section, three hundred and thirty miles. The tilted beach represents the deformation of the Algonquin beach.

interruptions, for there is no intervening strand, as the water did not remain at one level long enough to leave beaches. Warren water was now broken up into two great gulfs; the one called the Algonquin, occupying the basins of Lakes Superior, Michigan, and Huron, and opening to the northeast, through the strait to the Ottawa Valley, as is shown on the map (Fig. 13); the other was the Lundy Gulf, occupying most of Erie basin, and extending over the Ontario Valley at a great height. In the region of Nipissing Strait the two gulfs united.

From the deserted shores of these waters, which are now tilted up so much to the northeast, free communication between the lake region and Hudson Bay is indicated, for the Laurentian highlands are now rarely more than fifteen hundred feet above the sea and commonly less. Of course, there was free communication to the Atlantic Ocean by way of the St. Lawrence and also southward of the Adirondacks. These various characteristics and changing conditions would require a volume to tell all that we know about them, and this has been partly done in Duration of