642 THE GEOLOGY OF MINNESOTA. 



[Glacial lake. Modified drift 



A glacial fake covered much of Le Sueur county during the recession 

 of the ice, which was a barrier upon the lower part of the Minnesota basin. 

 The outflow from this lake in its latest stage appears to have been by the 

 route of lakes Tetonka and Sakata and the Cannon river.* 



Terraces of modified drift. The valley of the Minnesota river in this 

 county, and in Nicollet county opposite, contains high terraces or plains 

 of modified drift, elevated about 150 feet above the rive/ and bordered by 

 bluffs of till that rise 50 to 75 feet higher. These terraces are evidently 

 remnants of an ancient flood-plain of the Minnesota river, which was de- 

 posited after the valley had been excavated through the sheet of till. Their 

 frequent occurrence along a distance of a hundred miles, from New Ulm 

 to the mouth of the river, shows that this valley was here filled with de- 

 posits 75 to 150 feet thick, sloping about two feet per mile, through which 

 the channel has been cut anew. Extensive portions, however, of the bluffs 

 on each side in this part of the valley are destitute of any traces of modi- 

 fled drift, which has been wholly removed by the river, besides perhaps 

 some encroachment upon the bluff's of till. 



Remnants of the valley drift in Le Sueur county are a terrace east and south of Kasota, and 

 the Le Sueur prairie. The first reaches from the S. E. J of section 28 to the S. E. i of section 8, 

 Kasota, being three and a half miles long. Its width is from one-third to two-thirds of a mile. 

 In hight it is approximately 75 feet below the plain of till which begins at the lop of the bluff on 

 its east side, 75 feet above the railroad and plain or terrace of Shakopee limestone on its west side, 

 and 150 feet above the river. A mile from its north end it is crossed by Shanaska creek, in the 

 S. E. } of section 33. Thence through one and a half miles southwest to the south side of section 5, 

 the outer part of this bank of modified drift for an average width of a quarter of a mile is 20 to 25 

 feet below its main portion, thus constituting a lower terrace, 50 feet above the limestone plain. 

 These terraces, below the soil, are composed in part of laminated clay or silt, but are mostly fine 

 gravel and sand, extending in at least one place to a depth of 60 feet, as shown by a well. They 

 have no boulders, excepting upon the slope at their west side and in its short ravines, where ordi- 

 nary granitic and gneissic boulders up to five feet in diameter are often quite abundant. These 

 probably belong to a ridge of till underlying the west border of the stratified drift. 



Le Sueur prairie extends five miles, from the middle of sections 25 and 26, Ottawa, north to 

 the middle of section 36, Le Sueur. In width it varies from two to four miles, the greatest width 

 being from the east part of section 17, Sharon, to the west side of section 14, Ottawa. This area 

 is mainly a plain about 150 feet above the river, descending ten or fifteen feet northward in its 

 length of five miles. Its material is sand and fine gravel, with thin layers of clay, extending from 

 50 to 100 feet in depth, as shown by wells, which find till below. A few miles southwest, on the 

 opposite side of the river, a similar extensive plain of modified drift, at about the same hight, lies 

 northwest of Saint Peter, and is known as the " Sand prairie". Both these tracts are properly 

 included within the Minnesota valley, for its bluffs of till rise to the general level, 50 to 75 feet 

 above them, at the border of each on the side away from the river. The south branch of Le Sueur 

 creek flows through the east part of Le Sueur prairie in Sharon, from the church in the S. E. \ of 

 section 19 to Glen mills, beyond which to its mouth it forms the northern boundary of this area of 

 modified drift. A mile farther west this plain is crossed from south to north by a channel which 

 the Minnesota river excavated during the process of removing its plain of valley drift, the rem- 



*For the history of this glacial lake, see pages 461, 606 and 622. 



