41S THE GEOLOGY OF MINNESOTA. 



[Valleys. Pre-glacial erosion. 



The valley of the Le Sueur river in Blue Earth county is 50 feet below the average surface at 

 Winnebago Agency, and 75 feet below the highest points; thence it rapidly deepens, and through 

 Decoria, Uapidan and South Bend, is from 1 50 to 200 feet deep. Its last three miles, in the north- 

 east part of Rapidan, and in South Bend, have numerous exposures of rock. Excepting these 

 and the other outcrops of rock before mentioned, the material through which the valleys of the 

 Blue Earth river and its tributaries are eroded, is till, which encloses only few and thin layers of 

 gravel and sand. Their bluffs rise steeply from narrow bottom lands to the nearly flat expanse of 

 the drift-sheet. The width of the valleys thus enclosed increases with their depth from an eighth 

 of a mile near their sources to a third or half a mile where they approach the Minnesota river. 



Indian lake, three miles south west of Mankato and one mile east of the junction of the Le Sueur 

 river with the Blue Earth, occupies an old valley cut by the Le Sueur river, but forsaken because 

 in their long-continued erosion the barriers bjtween these rivers was cut through. This former 

 valley is from 100 to 175 feet below the general level, and is about three miles long, extending 

 from the S. W. \ of section 35 northeast about one mile to Indian lake and thence two miles north 

 to the west part of the city of Mankato. Its highest point, about 50 feet iibove the present Le Sueur 

 river, is southwest of the lake, which outflows northward. West of this valley the remnant of the 

 drift-sheet between it and the Blue Earth river has been divided by erosion into two plateaus, and 

 the railroa & from Mankato to Wells passes between them in the N. E. } of section 26. A third 

 and smaller plateau lies a half mile southwest from this gap, at the east side of the mouth of the 

 Le Sueur. The diversified scenery here and the high and picturesque bluffs along the meandering 

 courses of all the rivers of this region are due to erosion. Along the deeper valleys this erosion 

 has usually cut through the thick sheet of drift and reaches a considerable depth into the under- 

 lying rocks. 



The valley of the Minnesota river in Blue Earth county is bounded above Mankato by bluffs 

 which are from a half mile to one miJe distant from the river. Through Mankato this distance 

 is about a mile, but below this city, in Lime township, it becomes fully two miles. The top of 

 these bluffs is from 200 to 225 feet above the river. This deep val ey has many exposures of the 

 rocks that underlie the drift. About a third part of Mankato, including Front street, is on the 

 bottomland, only 20 to 30 feet above the river, while the rest of the city occupies a gradual slope 

 that rises 40 or 50 feet to the base of the bluffs which then ascend steeply 150 feet to the general 

 level of the drift-sheet. These bluffs of boulder-clay nowhere present a smooth front like that 

 which commonly bounds terraces of modified drift; but they are seamed and gullied into deep ra- 

 vines by frequent rills and springs, many of which flow only at times of snow-melting or of large 

 rains. 



At the quarries and lime-kilns in the north part of Mankato the thickness of the limestone, 

 varying in portions to calciferous sandstone and shale, all of light buff color, is about 65 feet, and 

 this formation i underlain by white sandstone. A terrace of these strata, decreasing from two 

 miles to one mile in width, and averaging 75 feet in hight above the river, extends thence eight 

 miles north to Kasota; beyond which it continues at a less hight on the other side of the river 

 through St. Peter. From Mankato to the north line of Blue Earth county this terrace is nearly 

 two miles wide, and is bordered on the east by bluffs of till, about 150 feet high, their tops bejng 

 approximately 225 feet above the Minnesota river. 



It appears that the excavation in the old rocks along the Minnesota 

 river was principally the work of pre-glacial streams; and that the erosion 

 which has been effected here since the ice age has been mostly limited to 

 clearing away a part of the drift with which the valley was then filled. 

 The sheet of till appears to be spread with a somewhat uniform thickness, 

 averaging about 150 feet, upon the bed-rocks, and doubtless at first pre- 

 sented a nearly level but slightly undulating, unchanneled expanse, whose 

 'owest portions coincided approximately with the pre-glacial lines of drain- 



