CHAPTER V. 



HELDERBERG GROUP. 



At the foot of McGulpin's Point, a little distance from the 

 former site of old Fort Mackinac, the bed of the lake is formed 

 of ledges of limestone, a series of which continues at the height 

 of lo or 15 feet above the water level around the base of the 

 promontory. Its higher levels are formed of drift rising in several 

 well-marked terraces, on one of which the lighthouse is erected. 

 The limestone is light-colored, of conchoidal fracture, rather thin 

 and unevenly bedded, abounding in hornstone concretions. It 

 contains a good many fossils characteristic of the upper Helder- 

 berg group, but few of them are well preserved. The strata 

 which form the top of Mackinac Island represent the same geo- 

 logical horizon, and if we were to admit a continuity of the strata 

 across the straits, and take those of McGulpin's Point as identical 

 with the highest beds of Mackinac, the calculated dip of the 

 strata toward the south would- amount to 30 feet per mile, 

 Mackinac Island being 300 feet above, and its distance from 

 McGulpin's Point 10 miles. 



The limestone ledges can be traced in the shoal water westward 

 to the Waugoshance Islands ; the projecting part of them is formed 

 by loose limestone pebbles and regular boulder drift, with no out- 

 crops of the underlying limestone ledges. Further west, the 

 formation extends to the Beaver Island group. The larger of the 

 islands, Hog Island, Garden Island, and Beaver Island, are all com- 

 posed of drift. Loose, angular limestone slabs, inclosing fossils of 

 the Helderberg group, are found abundantly strewn over their 

 beaches, indicating a proximity of the ledges to the surface ; the 

 ledges themselves are visible in the bottom of the lake in the cir- 

 cumference of Whiskey and Trout Islands, which likewise belong 

 to the Beaver Island group. 



On the east side of McGulpin's Point the limestone ledges 



