YELLOW MEDICINE, LYON AND LINCOLN COUNTIES. 601 



Glacial drift. Coteau des Prairies.] 



the deep glacial furrows. On the other rock-outcrops seen in this district 

 the glacial striae have been effaced by weathering. . 



Till, or the unstratified boulder-clay deposited by the ice of the glacial 

 period, forms a thick sheet, probably averaging a hundred feet in depth, 

 upon the surface of all this district, the underlying rocks being seen only 

 in the deeply eroded valley of the Minnesota river and in the few other 

 outcrops which have been described. 



Though no exposures of strata older than the drift have been found 

 upon the Coteau des Prairies in this district and northwestward, the un- 

 derlying formations are believed to rise here much higher than on either 

 side, in the basins of the Minnesota, Big Sioux and James rivers. The al- 

 titude of the Coteau is doubtless thus caused by -the greater hight of the 

 formations, probably Cretaceous, upon which these drift deposits lie, rather 

 than by extraordinary thickness of the drift beyond that which it com- 

 monly has throughout southwestern Minnesota. 



The depth that is added to the general drift-sheet by the accumulations of the terminal 

 moraines does not appear to average more than 50 to 75 feet. Upon the Coteau des Prairies the 

 knolls and hillocks of the morainic belts rise 20 to 50 and rarely 75 or 100 feet above the adjoin- 

 ing hollows; and the thickness which they add to the drift-sheet appears to be from 50 to 150 

 feet. That the prominence of this highland is not due to these morainic accumulations is shown 

 in Dakota at Goodwin and farther north, by the greater elevation that is reached within a dis- 

 tance of two to five miles by the smooth sheet of till at their west side, which there forms 

 the water-shed, and beyond descends to the Big Sioux river. 



FIG. 48. SECTION ACROSS THE COTEAU DES PRAIRIES IN YELLOW MEDICINE COUNTY, MINNESOTA, 

 AND DEUEL AND CODINGTON COUNTIES, DAKOTA. 



In crossing the Coteau from northeast to southwest, there is generally a very gradual, smooth 

 slope, rising 100 to 200 feet in a few miles; then comes a steeper ascent, which amounts to 300 

 feet or more within a width of two or three miles, coinciding through the greater part of its extent 

 across southwestern Minnesota with the tract of knolly and hilly drift that forms the inner belt 

 of the moraine. The average hight beyond, sometimes after a slight descent, continues to rise, 

 but only slowly, amounting to 100 or 150 feet in crossing the smoother, undulating or rolling area, 

 ten to fifteen miles wide, between this and the outer morainic range, which next rises 100 to 200 

 or 300 feet within two or three miles, and forms the crest of this highland along nearly its whole 

 extent. West of this moraine in Lincoln county the surface soon drops 50 to 100 feet, this descent 

 being greatest at the south and diminishing northward; and thence a smooth slope of till falls 

 southwesterly some 200 feet within ten miles. Farther to the north, from lake Hendricks nearly 

 to Goodwin, a gently undulating expanse of till, slightly lower than this western belt of drift- hills, 

 extends from them westward approximately level for a width of several miles, beyond which a 



