COS GEOLOGY OF CENTRAL WISCONSIN. 



CHAPTEE V. 

 THE QUATERNARY DEPOSITS. 



THE GLACIAL DRIFT. 1 



The first and most striking fact that presents itself to the investi- 

 gator of the drift phenomena of Wisconsin is the existence of an 

 extensive driftless region, the remainder of the state at the same 

 time displaying an altogether extraordinary development of the drift 

 materials. In the driftless region, which occupies 12,000 square 

 miles of the southwestern part of Wisconsin, or nearly one-fourth 

 the entire area of the state, the drift is not merely insignificant, but 

 absolutely wanting. Except in the valleys of the largest streams, 

 like the Wisconsin and Mississippi, not a single erratic bowlder, nor 

 even a rounded stone, is to be seen throughout the district; whilst the 

 exception named is not really an exception, the small gravel deposits 

 that occur on these streams having evidently been brought by the 

 rivers themselves, during their former greatly expanded condition, 

 from those portions of their courses that lie within the drift-bearing 

 regions. 



The outline of the driftless area is for the most part quite 

 sharply defined, both by a more or less sudden cessation of the drift 

 materials, and by a change in the topography as the line is crossed 

 from one side to the other. This is more especially true of the east- 

 ern boundary, close to which are often found heavy morainic heaps, 

 with numerous bowlders of a large size, and on the different sides of 

 which the topographical effects of purely subaerial erosion with- 

 out drift, and those of partial glacial erosion with drift, are strongly 

 contrasted. The northern boundary, on the contrary, is largely in a 

 level country, the drift materials increasing quite gradually in quan- 

 tity as it is left behind to the southward. 



1 It is not possible to give, in the space available, more than a brief summary of the 

 large amount of material with regard to the glacial drift that has been obtained during 

 the survey of the Central Wisconsin district, with the main conclusions reached. 

 These, it is believed, when taken together with the results of the work of 6ther mem- 

 bers of the corps, will be found of considerable interest. 



