210 Taylor — Highest Shore Line on Mackinac Island. 



Art. XXY. — The Highest Old Shore Line on Mackinac 

 Island ;* by F. B. Taylor. 



Evidence of post-glacial submergence lias long been known 

 to exist on Mackinac Island, which lies in the strait between 

 Lake Huron and Lake Michigan. Its amount has been vari- 

 ously stated by different writers, some putting it at 250 feet, 

 while others claim that evidences on the higher parts of the 

 adjacent mainland indicate an amount of submergence that 

 would rise far above the summit of- the island which attains 

 an altitude of about 300 feet above the present level of Lake 

 Huron. The observations of which I give an account here 

 were made during the autumns of 1890 and 1891. 



With the exception of the rocky faces of the great limestone 

 cliffs which rise on its eastern and western sides, all the lower 

 levels of Mackinac Island show plain evidence of post-glacial 

 submergence. The modern beach is composed almost entirely 

 of limestone pebbles which are generally well rounded. The 

 proportion of crystalline drift material from the north is not 

 great and is confined mainly to bowlders of considerable size. 

 The narrow strip of land upon which the village is built, and 

 which rises to an altitude of 40 to 50 feet at the base of the 

 cliff, is composed entirely of the same characteristic beach 

 material ; as may be seen in any of the little gardens of the 

 villagers and in the deep cut back of the Astor House and the 

 old court house. But on ascending to the higher levels of the 

 island the evidences of post-glacial submergence are even more 

 marked. At an altitude of about 170 feet there is a heavy, 

 well developed beach ridge.. This ridge is the lowest of a 

 series of four or live like ridges which rise by successive steps 

 to an altitude of about 205 feet above the strait or about 

 787 feet above sea level. These old beaches may be seen to 

 best advantage on the short target range back of Fort Mackinac. 

 The ground is there cleared of trees and is covered with a 

 short, thick turf, so that the whole series of beach ridges,, 

 with their intervening troughs and minor ridges, is admirably 

 exposed to view. At this place the ridges are parallel and 

 comparatively narrow, though strongly developed, and they are 

 also more compactly arranged and more convenient of access 

 than in any other part of the island. 



The short target range crosses the beaches nearly at right 

 angles and the width of the series is here a little less than a 

 quarter of a mile. But on the southwestern side of the island 



* An abstract of this paper was read before the Geological Section of the 

 A. A. A. S., at Washington. 



