CHAPTER IV. 



FORMATIONS OF THE INTERIOR OF WISCONSIN AND MINNESOTA. 



SECTION I. 



THEIR LITHOLOGICAL CHARACTER. 



Extensive deposits of drift prevail throughout the interior of the Chippewa Laud 

 District. These fill up the inequalities of the surface, and give, for long distances, 

 a greater degree of flatness and uniformity of contour than one would expect to 

 encounter in a country near the sources of so many large streams. 



Between the western tributaries of the Chippewa River and the heads of the 

 eastern branches of the St. Croix and Rum River, this drift seems to rest chiefly on 

 the northern extension of the Lower Protozoic Sandstones of Wisconsin, heretofore 

 treated of in Chapter I. ; which formation appears to be invaded only at a few 

 points by intrusive rocks of igneous origin. The drift of this part of Wisconsin, 

 which, in a great measure, conceals these underlying formations, is chiefly of a 

 light, sandy, and gravelly nature ; supporting locally, multitudes of boulders, man}" 

 of which do not appear to be far removed from the parent rock. Where valleys 

 have been excavated by streams, these boulders, undermined and rolling from the 

 higher grounds, have accumulated on the banks and in the beds of the rivers, 

 causing frequent obstructions in the channel, or covering it as with an artificial 

 pavement. 



Northeast of the Chippewa, towards the Michigan boundary, the drift reposes 

 chiefly upon Metamorphic Schists and Granitic Rocks ; and the same is true of the 

 extreme northern portion of the District, and of a belt of country, some forty or 

 fifty miles in width, ranging north-northeast and south-southwest, from Millelacs, 

 through the Rapids of the Mississippi and St. Peter's Rivers, between longitude 

 94° and 95°. In these latter regions, besides the coarse boulder drift, sand, and 

 gravel, a deposit of finer materials, more marly and argillaceous, and of an ash-gray 

 colour, prevails over considerable areas. 



