production of plant and animal life, as might be expected, is found 

 to vary when the biotas of the respective types of lakes are com- 

 pared. The chief types are: 1) hard water drainage lakes, stream 

 or spring-fed, with an outlet, at least during part of the year; 2) 

 hard water seepage lakes (rare), high in calcium, magnesium, and 

 half-bound carbon dioxide, landlocked; 3) soft water drainage 

 lakes (uncommon in Wisconsin and Michigan), low in calcium and 

 half -bound carbon dioxide, with inlet and outlet; 4) soft water 

 seepage lakes (common, particularly in northern parts of the area, 

 in the northern part of the Lower Peninsula, the Upper Peninsula 

 of Michigan, and in upper Wisconsin), low in calcium, magnesium, 

 and half-bound carbon dioxide, fed by seepage or drainage from 

 bogs, without outlet. 



To these four classes, two other types of lakes should be added: 

 5) acid bog lakes, mostly seepage, low in calcium; 6) alkaline 

 bog lakes, mostly drainage, relatively high in calcium. 



In general, the lake types are determined by differences in their 

 geological history, differences principally related to glaciation. The 

 most recent glaciation. Late Wisconsin, obviously had the greatest 

 influence on the present physiography of the region. Although 

 most lakes had their birth during and following the closing years of 

 this period, it appears likely that a few of the deeper lakes. Lake 

 Geneva and Green Lake in Wisconsin, for example, may antedate the 

 Late Wisconsin. There are at least four types of lake formation in 

 the Great Lakes region: 1) depressions formed by the melting away 

 of great blocks of glacier fragments and the subsequent sloughing 

 off of glacial drift so that mounds of debris were left about a kettle- 

 hole, which is usually soft and is frequently the acid bog type; 2) 

 lake basins formed by the damming of preglacial valleys; 3 ) basins 

 created when terminal moraines were formed in parallel ridges and 

 the intervening valleys dammed subsequently by deposits at either 

 end; and 4) depressions formed in the ground moraine. (See 

 Juday, 1914. ) 



The lakes which were left with an outlet became immediately a 

 part of a drainage system Other drainage systems were evolved by 

 subsequent wearing away of impounding glacial deposits and 

 through variations in water level. Thus some lakes were included 

 in a drainage system, but others were left perpetually land-locked 

 and doomed consequently to extinction. Fundamental differences 

 between the drainage and seepage types of lakes, which are so con- 

 spicuous today, are related, therefore, to the mode of the lakes' for- 

 mation in the remote past. 



[14] 



