A RECONSTRUCTION OF WATER PLANES OF THE 

 EXTINCT GLACIAL LAKES IN THE LAKE 

 MICHIGAN BASIN^ 



JAMES WALTER GOLDTHWATT 

 Dartmouth College 



CONTENTS 

 The Raised Beaches. 

 Method of Investigation. 

 The Shore Lines of Lake Chicago. 

 The Algonquin Beach. 

 Construction of a Profile of Water Planes. 

 Significance of the Fan-like Profile. 



THE RAISED BE.\CHES 



Around the borders of Lake Michigan are many fragments of 

 abandoned .shore lines, which stand at different heights above the 

 present lake. They mark a series of stages of extinct lakes of late 

 glacial times, known as Lake Chicago, Lake Algonquin, and the 

 Nipissing Great Lakes. Near the .south end of the lake, where 

 ero.sion has been .slight and the accumulation of .shore drift has been 

 going on .since the earlie.st times, there is a record of nearly all the 

 stages through which the lake has passed. On both the east and west 

 .sides of the lake, however, in Michigan and Wisconsin, where the 

 cutting back of cliffs at the present level of Lake Michigan has been 

 vigorous, the old .shore lines have been partly or wholly destroyed for 

 .stretches of five to twenty-five miles. Even where the present lake 

 has not cut away the record, it is usually incomplete, because the 

 higher beaches have been destroyed by cliff reces.sion during the 

 lower of the extinct stages. It follows that the old .shore lines pre- 

 served at one locality do not necessarily correspond with those at a 

 neighboring locality, either in number or in order. The matter 

 of correlation is not a very simple one; the highest beach at a given 

 locality may not correspond with the highest at a neighboring locality, 

 even though it may be less than a mile away. Moreover, while the 



I With the permission of the Director of the U. S. Geological Survey. 



459 



