408 THE GEOLOGY OF MINNESOTA. 



[Glad.l lake. 



tion of a lake that covered this district during the departure of the last ice- 

 sheet. In its recession northward the ice was a barrier which prevented 

 the water of its melting from flowing away in its present course, following 

 the northern slope of the land; so that a lake, similar in its origin to lake 

 Agassiz in the Red river valley, extended over the greater part of the basin 

 of the Blue Earth and Le Sueur rivers, its area being increased as fast as 

 the border of this ice-lobe retreated to the north, till it was so far melted 

 as to permit this glacial lake to be drained northward by the Minnesota 

 river. Its outlet, while it remained a lake, is found in Iowa, and was trib- 

 utary to the East fork of the Des Moines river, as described in the report of 

 Faribault county. 



Channels, ten to twenty-five feet in depth and five to ten rods or more in width, which may 

 have been eroded by rills and streams under nearly the present conditions of climate, but have 

 no water now running in them through the greater part of the year, cross the flat area of south- 

 western Waseca county in irregular courses. This area also contains here and there broad, 

 bowl-like depressions of similar or somewhat greater depth, often with no outlet or depression 

 continuing away on any side, and occupied by sloughs and lakes. These hollows sometimes have 

 steep sides, which have been eroded and undermined by waves ; but generally they are surrounded 

 by slopes of 10 to 15% about a third as steep as are produced by the falling down of a bluff of 

 drift that has been undermined by water. In origin they seem to be like the basins of the ordi- 

 nary small lakes that are scattered irregularly over the surface of the moderately undulating 

 drift-sheet of this state. Variations in the direction or force of the glacial currents, and conse- 

 quent irregularities in the amount of drift deposited or eroded by the ice-sheet, have commonly 

 moulded this formation in swells and hollows, the latter being often without outlet. Here the 

 surface has been smoothed by an extensive glacial lake, and the drift that would have formed 

 swells has been swept into the adjoining hollows; but it appears that occasionally the s..pply of 

 material thus carried into the depressions was insufficient to fill them, and their deep central 

 portions remain empty, constituting very remarkable features in the topography because of the 

 unusually flat tract in which they occur. These basins vary from 20 to 30 or 40 feet in depth, 

 and in extent they are from thirty rods to one or two miles long, with perhaps half or two-thirds 

 as great width, the largest area of this kind being that of Silver lake in Wilton. The shallowest 

 hollows filled by sloughs are only two to five feet lower than the surrounding land, while the 

 deepest are twenty feet below the general level. 



Streams in this part of the county, as the Little Cobb river and Bull run, have cut valleys 

 '20 to 30 feet deep. Boot creek, east of Byron, lies in a broad, shallow depression of slightly 

 undulating till, two or three miles wide and 20 or 30 fe.t below the average surface on each side. 

 The valley or channel eroded by the Le Sueur river in New Richland and southern Otisco is 20 to 

 30 feet deep ; and in the remainder of its course through this county, passing by Wilton and 

 Alma, its depth is about 40 feet. 



Elevations on the Winona & Huint Peter division of the Chicago & Northwestern railway. 

 .From John E. Blunt, engineer, Winona. 



Mile* from Feet ubove 

 Winona. the t>ea. 



Meriden (Steele county), - ... 96.85 1149 



Waseca, 102.63 1158 



Janesville, 112.91 1063 



Eagle Lake (Blue Earth county), 122.56 1012 



