100 ~ THE MICHIGAN ACADEMY OF SCIENCE. 



REVIEW OF THE GLACIAL GEOLOGY OF THE SOUTHERN 



PENINSULA OF MICHIGAN.* 



PRANK LEVERETT. 

 CONTENTS. 



Introductory statement. 



Physical features of the southern peninsula. 



Possible extension of the Keewatin ice field over Michigan. 



Evidences in Michigan of successive advances of Labrador ice field. 



Lobation of the ice margin. 



Structure of the drift in Michigan. 



Glacial lakes. 



Origin of the Great Lakes. 



Bibliography. 



INTRODUCTORY STxVTEMENT. 



The northern states are found to be covered with a mass of stony and 

 earthy material Avhich has l)een brought in from regions farther north, 

 and cominonlv called the drift. It is of verv irregular thickness 

 from 1,UUU feet or more doAvn to a mere skim coating scarcely sufficient 

 to conceal the surface of the underlying rock. It fills up to a great 

 degree the inequalities of the rock surface, and in places has been built 

 high above the tops of the rock ridges. Wells often show its great 

 thickness where valleys and ravines fail to cut through to the under- 

 lying rock. The roadside and stream exposures furnish numerous 

 opportunities for studying it, and show the great variations in com- 

 position and structure. Rocks of many kinds are to be found, and 

 these present various sizes and shapes. The water worn, well rounded 

 pebbles constitute but a small part of the mass, and are often inter- 

 mingled with angular rocks both large and small. In places definite 

 beds of sand or gravel occur in which the rock fragments are water 

 worn. These beds alternate with or grade into masses of earth and 

 rocks in which no bedding can be discerned. The unassorted, unstrati- 

 fied material is called bowlder clay or till, the latter name being a 

 Scotch term Avhich has come into general use. 



In addition to the variations just noted the drift is found ,to be sep- 

 arable into formations of widely difl:"erent age, one formation being 

 superimposed on the weathered and eroded surface of another or separ- 

 ated from it by a bed of peat or a well defined soil. The degree of weath- 

 ering of the buried land surface is generally greater than the weathering 

 found on the surface of the overlying sheet of drift, from Avhich it is 

 inferred that the time which elapsed between the two drift depositions 

 is greater than has elai)sed since the later drift was laid down. 



The drift was known to be transported material many years before 



♦Published by permission of Director U. S. Geological Survey. 



