6 INTRODUCTION 



sequent melting of large irregular blocks of ice. The Oconomowoc, 

 Lauderdale and Waupaca lake groups are of this type. A few of 

 the larger and deeper lakes, as Green and Mendota, are situated in 

 valleys eroded by preglacial streams that were possibly modified by 

 glacial action and then closed by the building of a morainal dam which 

 impounded the waters of the present lake. 



The northeastern lake group lies chiefly in Vilas, Oneida and Iron 

 counties. There are hundreds of lakes in this area and their closeness 

 of position may readily be inferred from the fact that while the largest 

 of them, Trout Lake, covers only 6.5 square miles the 346 lakes and 

 ponds in Vilas county occupy 140 square miles or over 15 percent of 

 the area of a county nearly as large as Rhode Island. In few parts of 

 the world are there more lakes to the square mile. Parts of the State 

 of Minnesota, the Province of Ontario, and Finland furnish the 

 only parallel. These lakes are small, irregularly shaped, and connected 

 by streams with most irregular courses. They are all glacial in origin 

 and formed either by shallow depressions of the ground moraine, hol- 

 lows in outwash plains, or damming of recessional moraines. The en- 

 tire region is so deeply buried by the glacial drift that no lake is due 

 to a damming of a preglacial valley. This entire region also contains 

 numerous swamps (muskegs) characteristics of a poorly drained area. 



The northwestern lake area lies in Barron, Polk, Burnett, Washburn, 

 and Sawyer counties. In general it is similar to the northeastern lake 

 area in structure and origin but the lakes do not occupy so great a 

 portion of the land's surface. It has been suggested that some of the 

 lakes in this region were formed before the last glacial invasion or Late 

 Wisconsin glacial epoch. Here we find a few lakes formed by a dam- 

 ming of preglacial valleys; Bone, Beaverdam, and Wapagossett lakes 

 for example. 



The mean average temperature of Wisconsin varies from 6.6 C. in 

 the southern portion to 3.3 C. in the northern. The state has a mean 

 summer temperature similar to that of France, Germany, or south- 

 eastern England (14.5 to 18) ; the average winter temperature is com- 

 parable to that of northern Sweden or central Russia (-5.4 to -8.8). 

 All lakes in the state are frozen over during the winter, ice forming 

 during November and December and lasting until March or April in 

 the southern ones, while on some of the northern lakes ice appears in 

 October and remains until the latter part of May. The midsummer 

 surface temperature of the northern lakes is 18-24 C. and 20-26 

 in the southern lakes. 



The altitude of the lakes in the northeastern part of the state varies 

 from 1550-1700 feet above sea level; in the northwestern area 1050- 

 1350 feet, and from 750 to 900 feet in the southeastern. These differ- 

 ences in elevation are too slight to be of any biological significance. 



