FORMER HIGHER LEVEL OF THE GREAT LAKES. 247 



escarpment and the present margin of the lake is a series 

 of parallel terraces, each showing the altitude at which the 

 receding waters have made a pause. These terraces along 

 the southern shore of Lake Erie range from ninety to. one 

 hundred and twenty feet above the present level of the 

 water. In Huron and Sandusky Counties the subsidence 

 of the escarpment permits them to diverge a greater dis- 

 tance from the lake. Curving northward into Michigan, 

 they pass through Monroe, Wayne, and Oakland Counties, 

 and continue in that direction. They are recognized again 

 on the shores of Lake Michigan. 



At Mackinac Island are deeply engraved records of a 

 higher level of the waters. The island itself is but a ves- 

 tige of an ancient formation which once filled the straits, 

 and joined the highlands on the west and south. It tow- 

 ers, a monument of the destructive agencies of geogony, 

 three hundred and eighteen feet above the lake. The prin- 

 cipal plateau of the island is one hundred and fifty feet 

 above the lake. In the midst of this rises "Sugar Loaf," one 



hundred and thirty- 

 four feet higher (Fig. 

 85). This is a rem- 

 nant of the rocks form- 

 ing the upper plateau 

 which once extended 

 over the whole island. 

 It is a conical mass of 

 brecciated limestone, 

 all whose sides bear 

 the marks of the long- 

 continued action of 

 the waves. On one 

 side an ancient grotto 



Fig. 85. View of Sugar Loaf, Mackinac Island, has tllUS been exca- 



