212 Taylor — Highest Shore Line on Mackinac Island. 



probably not far from true. But after determining the fact 

 that the highest old shore line on Mackinac Island has an alti- 

 tude of about 205 feet above Lake Huron, another broader 

 and more difficult question arose. Did post-glacial submerg- 

 ence exceed this amount? Suppose that it did, as has been 

 implied and suggested by those who point to shore lines at 

 higher levels on the adjacent mainland. What evidence can 

 be brought to bear upon this question? 



A careful examination of the ancient little island of Mack- 

 inac, as described above, and of the surrounding mainland 

 reveals among others, the following facts : The whole surface 

 of the ancient island, except where the rocks were laid bare 

 along the edges by the formation of cliffs, is covered with 

 glacial drift. This drift is typical in its composition and topo- 

 graphical features. Its surface has that irregular rolling ap- 

 pearance which is characteristic of a drift-covered, broken 

 country. Along the edges near the cliffs the drift has been 

 notched by the little rain rills which carry away the surface 

 water. Otherwise, the surface shows no evidence of postglacial 

 modification. 



The composition of the drift is well shown in the excava- 

 tions of old Fort Holmes on the highest point of the island. 

 The parapet of the fort was built of material dug partly out of 

 the interior of the enclosure and partly out of an outer ditch. 

 This ditch is live or six feet deep and seems to have been ex- 

 cavated entirely out of bowlder clay, which is well exposed in 

 several foot paths which cross the earthworks. The clay is 

 very tough and of a reddish color, and when wet is quite 

 slippery under foot. But when it is dried in the sun, it 

 forms a very hard surface. It contains a fair sprinkling of 

 bowlders of moderate size, but appears at the fort to include 

 comparatively little sand and gravel. The clay and coarser 

 constituents form a heterogeneous mixture in characteristic 

 drift fashion, and a fair proportion of the bowlders show strias. 

 Although the composition of the drift varies somewhat in 

 different places, it everywhere, presents such characters as 

 clearly preclude the supposition that it has been washed or 

 worked over by the action of waves since its deposition by the 

 ice sheet. 



The whole surface of the ancient island is well sprinkled 

 with bowlders and a large number of them are erratics of 

 northern origin. With hardly an exception their exposed sur- 

 faces are strongly weathered. I found only two small surface 

 bowlders with stride still preserved, and both looked so fresh 

 that they probably came from some of the excavations near 

 by. I examined hundreds of bowlders below the 205 foot 

 level, but found none with glacial scratches. Exposures in 



