ABSTRACTS OF PAPERS 69 



greater clearness than before the larger facts of the lake history as recorded 

 in the northern lake basins. 



The small ancient island which forms the hump on the back of the "great 

 turtle" is surrounded on three sides by a compact and strongly developed 

 series of gravel ridges — the upper group of the Algonquin beaches. On the east 

 side the waves of the same time cut heavily into the ancient island, leaving a 

 cliff of limestone over 100 feet high. The rock fragments torn from this side 

 supplied the main bulk of the material for the beach ridges on the other sides. 

 The lower limit of this group is just as sharply defined as the upper limit. 

 The vertical interval covered by the group is about 35 feet, measured from 

 the highest to the lowest beach crest, or nearly 50 feet to the base of the lowest. 

 This highest beach is 809 feet above sealevel, or about 229 feet above Lake 

 Huron. There is a little gravel about 3 feet higher, but it has not the form 

 of a wave-made beach. 



Below the base of the upper group a space covering a vertical interval of 

 about 110 feet has only a few beaches, nearly all weak and fragmentary. 

 These are the Battlefield and Fort Brady beaches of the late stages of Lake 

 Algonquin. The upper part of this interval is heavily wave-washed and is 

 mainly bare rock, or carries only very thin soil. This wave work was accom- 

 plished during the making of the upper group of Algonquin beaches. 



Below the Fort Brady group of the Algonquin comes the Nipissing and lower 

 beaches. These are strongly developed, and where beach building was favored 

 they cover the entire slope from the Nipissing down. At Mackinac Island the 

 vertical interval which they cover is about 50 feet; thus at Mackinac the 

 record of the lake history is expressed in two strong groups of closely set 

 beaches, with a relatively wide, nearly barren interval between — two zones of 

 prolonged wave action separated by an interval showing very little wave 

 action. 



The two strong beach belts correspond to times of relatively permanent or 

 slowly changing levels of the lake waters — to the second and third or two 

 main stages of Lake Algonquin and to the transitional or two-outlet stage of 

 the Nipissing Great Lakes, with a considerable part of the post-Nipissing or 

 present stage. The barren zone between these belts corresponds to the time 

 of the relatively rapid uplifting of the land in the closing stages of Lake 

 Algonquin. These stages of relative stability and of rapid uplifting were 

 worked out more fully in earlier studies covering much of the lake region, and 

 the details on Mackinac Island agree with those results. The island is small, 

 hut it is relatively high, and stands in the midst of the northern waters like 

 a monument bearing the record oC the lake stages and uplifts that affected 

 that region. 



It has been held by some that strong beach ridges, like those of the Upper 

 Algonquin group, mark as many pauses in the uplifting movement of the land; 

 but this idea is apparently disproved by both of the heavy beach series on 

 Mackinac Island. On the south sirle of the ancient island the Upper Algonquin 

 group is represented by eleven or twelve beach ridges — by six strong ones and 

 five or six weak ones — whereas on the west side about forty well defined ridges 

 cover the same vertical interval. The forty ridges are spread over a wider 

 liorizoiifal area, but their averag(> stroncth of development is loss, as would 

 be expected in a less exi>osed jmsition. wliere wave action was weaker. 



VI— Buu.. Geol. See. A.M., Vor. L'C. 1014 



