GLACIAL GEOLOGY OF SOUTHERN MICHIGAN. 103 



feet high and a few yards wide, but their continuity and regularity give 

 them a striking appearance which led to their recognition at a very 

 early day. Parts of the shores of the Great Lakes are also marked by 

 high sand ridges or dunes formed by the wind from material gathered 

 on the beach. Some of these in western Michigan reach a height of 

 more than 200 feet. 



PHYSICAL FEATURES OP THE SOUTHERN PENINSULA. 



When one turns to the southern peninsula of Michigan it is surpris- 

 ing to find how largely its physical features are the result of glacia- 

 tion. Indeed the rock formations are of so little consequence in giving 

 this peninsula its topography and scenery that they might almost be 

 ignored. Every lake within the State and the Great Lakes on its bord- 

 ers owe their existence to glaciation, while, the hills, dales, basins, and 

 plains are nearly all glacial products. The only places where the rock 

 formations markedly influence the topography are in a strip leading from 

 Huron to Hillsdale county, and a narrow strip next to Lake Huron in 

 Alpena, Presque Isle, and eastern Cheboygan counties. Throughout 

 much of the peninsula the rock surface lies below the base of the drift 

 ridges and even below the bottom of the valleys. 



The most prominent parts of the peninsula are (1)^ an elevated tract 

 lying between Saginaw Bay and the south end of Lake Huron, and ex- 

 tending southwestward into Indiana, which is called the Thumb of 

 Michigan; (2) an elevated tract northwest of Saginaw Bay extending 

 westward to headlands that border Lake Michigan between Manistee 

 and the Straits of Mackinac; and (3) a series of large morainic ridges 

 that run southward through the western part of the State a few miles 

 inland from the shore of Lake Michigan. There is also a narrow strip 

 of prominent dunes on the immediate shore of Lake Michigan. Plains 

 are found on the border of the peninsula along the entire shore of Lake 

 Huron, Lake St. Clair, and the west end of Lake Erie, and on the shore 

 of Lake Michigan from near Manistee southward. As indicated above 

 the shore of Lake Michigan from Manistee to the Straits of Mackinac 

 carries a series of headlands and these are interrupted only by valley- 

 like lowlands occupied usually by bays and lakes. The most extensive 

 plain in the peninsula is one spreading from Saginaw Bay southwest- 

 ward nearly to Grand Eapids. This is traversed by a few low and com- 

 paratively smooth morainic ridges, and it carries also several gravel 

 ridges of the esker type. 



The highest point in the southern peninsula is found about ten miles 

 south of Cadillac, in northern Osceola county, and reaches an altitude, 

 by aneroid measurement, 1,700 feet above sea level or 1,120 feet above 

 Lakes Michigan and Huron. This point, as well as the entire elevated 

 tract of the northern part of the peninsula, is known to have an enor- 

 mous thickness of drift. The wells have revealed no place in which the 

 rock formations rise more than 300 feet above lake level, and through- 

 out much of the region they are less than 200 feet above it. On the shore 

 of Lake Michigan, from Frankfort past Manistee to Ludington, the 

 borings penetrate nearly or quite to sea level before reaching rock, but 

 this very low rock surface probably does not extend far inland. While 

 no wells yet made have penetrated more than 650 feet of drift, it is 

 probable that in places where prominent drift ridges cross the line of 



