OF WISCO N S I N AND M I N N E S U T A. 



143 



Drift of Wisconsin. — In the preceding pages I have had frequent occasion to 

 mention the position and composition of the drift deposits of the interior. At pre- 

 sent only a few general remarks on this subject remain to be made. 



The sand which constitutes the most bulky part of the drift of the interior of 

 Wisconsin, north of the forty-third degree of latitude, has evidently been derived 

 from the denudation of beds pertaining to F. 1. Most of the ground elevated above 

 the swamps and the overflow of the rivers, south of the great water-shed, is com- 

 posed of this material. Where it rests on the igneous rocks, it supports the pine 

 forests that constitute one of the principal sources of wealth in the Northwest. 

 The trees seem, however, not to attain any great size, except where the siliceous 

 earth is enriched and improved by an admixture of saline matter from some other 

 formation. The source whence these fertilizing elements are derived is usually, if 

 not always, the subordinate hypogene rocks. 



Blocks of these rocks, of sizes varying from a few inches to six or eight feet in 

 diameter, form the next most conspicuous materials of the drift. Among these, 

 trappean rocks are much the most common. The position in which they are now 

 most frequently observed is in the bottoms and lining the shores of rivers, lakes, 

 ponds, and swamps. Many of them have no doubt found their way into their pre- 

 sent position from more elevated situations, by the undermining and transporting 

 power of water, and perhaps ice. Most of them are confined to belts of country 

 adjoining to rapids where trap rock is either found in place, or where there is reason 

 to believe that it is only hidden by a superficial covering of the detached blocks in 

 question. 



They originated at the time of the upheaval of the trap, and at a comparatively 

 recent period. There are facts ascertained which render it probable that a large area 

 of the Northwest Territory has been raised during very modern periods, even since 

 the present Fauna inhabited its rivers and lakes. Below Parkhurst, on the west 

 bank of the Mississippi, I have observed, over a considerable tract, multitudes of 

 Unios, besides a variety of other fresh-water mollusca, of the same species as those 

 now inhabiting the Mississippi and its tributaries, elevated far beyond the reach 

 of the highest freshets ; and I am informed that the same deposit can be found 

 in some places, as much as a hundred feet, or more, above high-water mark. It is 

 well known to those who have travelled much in the swampy and undine regions 

 of the Mississippi Valley, that there is a gradual drainage of its waters taking place, 

 even at this time ; so that land which was formerly covered with water is now 

 completely dry ; and shell marls found through portions of the prairie country show 

 that many of these plains are but drained lakes, or expansions of the great water- 

 courses. 



Finally, the fine siliceous and loamy marls, widely distributed in the Valley of 

 the Mississippi, at an elevation of a hundred to two hundred feet above the present 

 rivers, containing Cyclostoma, Physa, Succinea, Helices, Helicina, and Planorbis, with 

 occasionally TJnio, Paludina, and Melanea, and considered to be of the age of the 

 Loess of the Rhine, in Germany, afford evidence of a modern rise of the lands of 

 the interior of the Northwest. 



