CHAPTER XXIV. 



POSTGLACIAL DEVELOPMENT OF CONNECTING RIVERS OF THE GREAT LAKES. 



By Frank B. Taylor. 



RELATIONS OF RIVERS. 



At its maximum extent the last ice sheet covered not only all the Great Lake basins, but 

 also the sites of all the rivers which now connect the lakes with each other and with the sea. 

 Special interest attaches to the development of these connecting rivers, not only because of 

 the intimate relation between their histories and lake history in general, but also because of 

 the processes and stages of development which they show and the remarkable youth or new- 

 ness which characterizes their beds. 



The Great Lake basins are certainly very old and were probably more or less connected 

 by channels in preglacial times. But the vicissitudes of several successive glacial invasions, 

 the lake and river developments of the intervening warm periods, and the differential uplifts 

 which have affected the land have probably obliterated and reestablished the main connecting 

 channels several times over. What is said here relates only to the reestablishment since the 

 obliteration caused by the last ice sheet. 



Of all the different facts that show the relative recentness of the postglacial changes which 

 have brought the Great Lakes to their present state none perhaps are so clear and convincing 

 as those pertaining to the rivers which connect the basins. Nipigon River is sometimes called 

 the upper St. Lawrence from the fact that it is much the largest tributary of Lake Superior. 

 In the same sense all the connecting rivers — St. Marys, St. Clan, Detroit, and Niagara — are 

 parts of the St. Lawrence system. The St. Lawrence proper, connecting Lake Ontario with 

 the sea, should be included in any comprehensive view of the development of this river system, 

 for it has had the same general history as the other parts and displays the same striking evidences 

 of newness. 



However, only St. Clair and Detroit rivers lie within the region discussed, and only these 

 will be considered in detail. The St. Clair and the Detroit are the oldest of the system; then 

 come the Niagara, the Nipigon, the St. Lawrence, and the St. Marys. The relative ages of the 

 Nipigon and the St. Lawrence are not certainly made out, but their beginnings were probably 

 not far apart in time. The Niagara was distinctly earlier than either of these and the St. Marys 

 was distinctly later. The different lake stages and the separation of the different links of this 

 river system by the intervening lake bodies set uncommonly sharp limits to the beginning 

 and duration of the different links. 



EARLY INVESTIGATIONS. 



The literature bearing particularly on St. Clair and Detroit rivers and Lake St. Clair, 

 which alone he within the field covered by this monograph, is not extensive. Much has been 

 Avritten about Niagara River, especially about Niagara Falls and the gorge, but this is outside 

 of the range of the present inquiry. 1 



In one or two of his earlier papers Spencer speaks of the waters which preceded the Great 

 Lakes as "lakes," and he found the outlet of one stage to be "southeast of Georgian Bay by 

 way of the Trent Valley to Lake Ontario." At that time he attributed the change of outlet 

 to the later uplifting of the Trent region, whereby the overflow passed southward to Lake 



1 The development and history of Niagara River and the Falls are discussed in Niagara folio (No. 190), Geol. Atlas U. S., U. S. Geol. Survey, 1913. 

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