THE GEOLOGY OF MINNESOTA. 



[Glacial lake. River Warren. 



its southwestern border appears to have extended across Big Stone county, but traces of these 

 moraines have not been sufficiently looked for there. They are probably indistinguishably blend- 

 ed in the area of rolling till, 50 to 75 feet higher than the average of this county, that was noted 

 a few miles east of Brown's Valley (page 616), and the somewhat rolling, lake-sprinkled surface 

 that stretches thence southeastward. 



Glacial lake in the basin of the Blue Earth and Minnesota rivers. When the ice-sheet, dis- 

 solved by a warmer climate, was retreating northeastward across Lac qui Paiie county, the 

 waters of its melting were carried to the southeast along the margin of the ice, which was a bar- 

 rier preventing their flow in the direction of the present drainage. After the ice had receded 

 from the Antelope moraine, a glacial lake (page 461) with its surface 11 50 to 1200 feet above the sea, 

 probably increasing somewhat in elevation from southeast to northwest, was formed in the Minne- 

 sota basin along the front of the ice and reached from Faribault and Blue Earth counties to Big 

 Stone lake. Its overflow was by Union slough in Iowa, until the continued retreatof the ice-sheet 

 permitted a lower outlet to the Cannon river, at first about 1 ,075 and afterward 1,025 feet above the 

 sea. By this submergence the drift in Lac qui Paiie county and upon a large part of the Minnesota 

 basin farther southeast was spread more evenly, and many of its hollows that would have held 

 small lakes were filled. This modification in contour doubtless is accompanied by a partial strati- 

 fication, especially on low areas; but nearly everywhere the drift in this county and throughout 

 this basin is a clay containing gravel and occasional boulders, seldom showing such assorting ac- 

 tion as to transform it from till to modified drift. During the somewhat later recession of the ice 

 across Big Stone county, free drainage could take place from its border, and the drift presents a 

 more undulating and rolling surface, dotted by many little lakes. 



River Warren. The excavation of the remarkable valley, or channel, 

 occupied by lakes Traverse and Big Stone and the Minnesota river, was 

 first explained in 1868 by Gen. G. K. Warren,* who attributed it to the out- 

 flow from an ancient lake, since named lake Agassiz,f that filled the basin 

 of the Red river and lake Winnipeg. The bights of lakes Traverse and 

 Big Stone are respectively 970 and 962 feet above the sea, and the lowest 

 point of the divide between them is only three feet above lake Traverse. 

 These lakes are from one to one and a half miles wide, mainly occupying 

 the entire width of this trough-like valley. Lake Traverse is fifteen miles 

 long; it is mostly less than ten feet deep, and its greatest depth probably 

 does not reach twenty feet. Big Stone lake extends in a somewhat crooked 

 course from northwest to southeast twenty-six miles; its greatest depth is 

 reported to be from fifteen to thirty feet. The portion of the channel be- 

 tween these lakes is widely known as Brown's valley. As we stand upon 

 the bluffs here, looking down on these long and narrow lakes and the valley 

 which extends across the five miles between them, where the basins of 

 Hudson bay and the gulf of Mexico are now divided, we have nearly the 

 picture that was presented when the melting ice-sheet of British Am erica 



*"On certain physical features of the upper Mississippi river," American Naturalist, vol. ii, pp. 497 502, Nov., 1868; 

 Annual report of the chief of engineers, U. 8. army, for 1868, pp. 307 314; "An essay concerning important physical features 

 exhibited in the valley of the Minnesota river, ana upon their signification," with maps, Report of chiff of engineers, 1874; 

 "Valley of the Minnesota river and of the Mississippi river to the Ohio. Its origin considered depth of the bed-rock," 

 with maps, Report of chief of engineers, 1878. (General Warren died August 8, 1882.) 



fCoinpare the eighth and eleventh annual reports. 



