CHAPTER XXIII. 



POST-NIPISSING GEEAT LAKES AND SUMMARY OF GLACIAL AND NONGLACIAL 



LAKES. 



By Frank B. Taylor. 

 POST-NIPISSING GREAT LAKES. 



POST-NIPISSING BEACHES. 



The Nipissing beach of the two-outlet stage was the last shore line of the Nipissing Great 

 Lakes. It was a transition beach, and when the discharge of the upper lakes abandoned North 

 Bay entirely and had returned to Port Huron the Nipissing Great Lakes as here defined ceased 

 to exist and the post-Nipissing or present stage of the upper lakes was inaugurated. Since 

 that time there has been no change of outlet, but only a slow uplift in the north and a gradual 

 lowering by erosion of the outlet. There was probably at first a very small overflow at Chi- 

 cago, but it soon ceased as St. Clair River deepened its channel. The whole deepening since 

 the time of the Nipissing beach has lowered Lakes Michigan and Huron 11 or 12 feet, and by 

 the time the lowering had reached 3 or 4 feet the discharge at Chicago had ceased entirely. 



The existence of beaches later than the Nipissing is due to the continuance of differential 

 uplift, by which the Nipissing beach at North Bay has been raised to an altitude of 698 or 700 

 feet, while in the area of horizontality it has remained at an average altitude of about 596 feet. 

 Thus, since the upper lakes attained their present arrangement the uplift of the land at North 

 Bay has amounted to 102 to 104 feet, and it is to be remembered that while this amount of 

 uplift was going on in the north, Lakes Huron and Michigan south of the hinge line were lowered 

 by erosion 11 or 12 feet, allowing 3 feet for the height of the beach above the water. 



For some distance north of the hinge line the beaches due to this uplift are not well dis- 

 played, being compactly set over the whole slope below the Nipissing beach, and being therefore 

 difficult to recognize individually for more than a short distance. Farther north, at Mackinac 

 Island, however, the uplift has been 38 or 39 feet, and the beaches begin to be distinctly sepa- 

 rated. Still farther north they are still more distinct, especially near Sault Ste. Marie and 

 along the north channel of Lake Huron. 



At present, however, the data accumulated do not warrant any detailed description of 

 these beaches. In fact, nearly all of them are of about the same strength, and none offers any 

 certain clue by which it can be followed and identified from place to place. Nevertheless, in 

 all probability, careful study of these youngest raised beaches would yield results of great value 

 in the study of the most recent earth movements that have affected the Great Lakes region. 



Among the beaches below the Nipissing is the Algoma, which has been somewhat doubtfully 

 identified at many localities. It was first observed by the writer at Algoma Mills on the north 

 channel of Lake Huron in 1893. At that place its altitude by aneroid was made about 630 

 feet or about 50 feet above Lake Huron. This beach is probably represented at Sault Ste. 

 Marie by a beach at 620 feet, observed by Mr. Leverett. At Mackinac Island a beach which seems 

 to correspond to this has an altitude of 605 feet, and at a number of places farther south it 

 seems to be satisfactorily identified. Hobbs reports a beach corresponding to the Algoma at 

 599 feet at Clamshell Harbor, on the Garden Peninsula, northern Michigan. In the area of 

 horizontality a beach among the several light ridges below the Nipissing and having an altitude 

 of 591 to 592 feet (10 or 11 feet above the lake) is generally a little stronger than the rest and 

 is regarded as the Algoma beach. The only basis of its identification, however, is the fact that 

 it is a trifle stronger than the others above and below it. 

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