230 



GEOLOGY OF EASTERN WISCONSIN. 



cannot be supposed to accurately represent the details of the oi-iginal 

 advance of the glacier, but rather the movements at the time of its 

 retreat; but there is suflacient evidence to warrant the general state- 

 luent that the original advance was along the two great valleys men- 

 tioned. Then came the melting away and the consequent retreat of 

 the ice mass, leaving strewn over the surface the Bowlder Clay, and 

 by a great halt and advance in the midst of its retreat, forming the 

 great morainic Kettle Kange. Following the retreat of the ice sheet, 

 there was an advance of the lake, giving rise to Beach Deposit A, and 

 the oif-shore lacustrine formation, the Lower Eed Clay. When this 

 advance had reached its maximum extent, the waters were withdrawn, 

 apparently with much rapidity, since we find little or no evidence of 

 beach action on the surface of the red clay. As to how far the lake 

 retreated there is no evidence, but returning, it again advanced upon 

 the laud, but to a less extent than before, and in that advance, pro- 

 duced Beach Formation B, and the Upper Bed Clay, as the coordin- 

 ate oS shore deposit, after which it again retired, leaving little or no 

 evidence of its retreat upon the surface. A third time it advanced 

 but at a lower elevation. It cut deeply into the previous deposits, 

 whether of drift or rock, forming the extensive terraces that charac- 

 terize the shores of Lake Michigan and Green Bay. After this, for a 

 third time the lake retired, and is now advancing at a still lower level. 

 These movements will perhaps be more vividly appreliended by con- 

 sulting the accompanying diagram. 



Fia. 16. 



BiAGKAM Illustrating the Advance and Retreat of the Lake Shore, and the Oscillations 

 OF tub Lake Level, since the Glacial Epoch. 



The Moraine and the Bowlder Clay were formed by the Retreating Glacier. The white bands through 

 >, hich the arrows pass represent Beach Deposits A and B, and the lined bands, the Upper and 

 Lowur Red Clays. 



It has already been stated that some of these advances of the lake 

 were due to relative depression of the land. It becomes evidently a 

 question of the most vital importance to ascertain whether the pres- 

 sent advance is due to simple erosion, or to a subsidence of the land, 

 and we are thus led to the subject of Lake Encroachment. 



Encroachment of Lalce Michigan. A considerable portion of the 

 shore of Lake Michigan is formed by high, steep banks of clay, sand 

 and gravel. These are being continually undermined, thrown down, 

 and borne away by the restless activity of the waves. The rate at 



