442 THE GEOLOGY OF MINNESOTA. 



[tilicial drift. 



the top of the river bluff, indicates the extent of a lake which covered this 

 area during the departure of the ice-sheet. In its recession from south to 

 north the ice became a barrier here, as with lake Agassiz* in the Red river 

 valley, preventing free drainage northward, and forming a lake which 

 found its outlet southward in Iowa to the East fork of the Des Moines 

 river, until the ice-sheet was melted upon the region covered by the Min- 

 nesota river from Mankato to its mouth. Besides its smooth or flat con- 

 tour, the till upon the area occupied by this lake is distinguished by slight 

 differences of its material from that of the more undulating districts sur- 

 rounding it, in having a somewhat scantier intermixture of boulders and 

 gravel, and occasionally in its imperfect stratification. Yet even where it 

 shows distinct lamination, it usually is more like till than like ordinary 

 modified drift, and contains stones and gravel through its entire mass. 

 Rarely may be seen small areas of true laminated clay destitute of gravel. 

 In the report of Faribault county, the outlet, boundaries, area and depth 



of this lake are treated of more fully. 



Near Mankato Junction on the Winona & St. Peter division of the Chicago & Northwest- 

 ern railway, in section 32, Lime, a cut eighty feet deep (figures 27 and 28) as made in till at the 

 edge of the valley-bluff. The upper forty feet here is yellowish, and the lower forty feet dark 

 bluish. Their line of contact forms a narrow shelf or bench in the cut. six to eight feet wide, 

 apparently due to the greater hardness of the lower till; but their outlines and position make it 

 probable that here their differences both in color and hardness have resulted from weathering. 

 At the southeast end of this cut the yellow till for an extent of two or three rods and a hight of 

 thirty feet is intersected by many nearly vertical banded veins which form an intricate network 

 (figure 29, representing a space ten feet square) upon the steeply sloping face of the excavation. 

 These veins or seams (figure 30) are two or three inches wide, and consist of films of ferric oxide, 

 parted by lamina? of clay, often including near the middle a white or gray calcareous band from 

 an eighth to a third of an inch wide. They appear to be veins of segregation, of somewhat similar 

 origin with the tubular irony concretions which are often met in stratified clay and sand, and 

 more rarely in till. Nowhere else have such vertical veins been found during all my exploration 

 of the glacial drift. 



FIG. 27. FIG. 28. FIG 29. FIG. 30. 



ILLUSTRATIONS OF THE GLACIAL DRIFT, SEC. 32, LI.MK. 



Wells in Blue Earth county. ^ , 



The material and general character of the drift are illustrated by the following rec- 

 ords of wells, including examples in most of the townships of this county. For the better ex- 

 hibition of the succession of glacial deposits, this list is principally selected from the deeper wells 

 of the county. Commonly an ample supply of excellent water, hard because of the presence of 

 dissolved carbonate of lime, but not alkaline, is obtained from fifteen to forty feet below the sur- 



Compare the eighth, tenth and eleventh annual reports. 



