440 THE GEOLOGY OF MINNESOTA. 



[Glacial drift. 



-rested upon the land and moved slowly forward because of the pressure of 

 its own weight, covering the northern half of North America, as now the 

 Antarctic continent and the interior of Greenland are buried beneath ice 

 thousands of feet deep. In Blue Earth county and generally through the 

 greater part of Minnesota, the material of the drift is principally the un- 

 modified deposit of the ice-sheet, composed of clay, sand and boulders, 

 mixed indiscriminately in an unstratified mass. Very finely pulverized 

 rock, forming a stiff, compact, unctuous clay, is its principal ingredient, 

 whether at great depths or at the surface. This formation is denominated 

 till, boulder-clay, or hardpan. Layers of stratified gravel and sand are 

 enclosed in this deposit, and are the source of the sudden inflow and rise of 

 water frequently found in digging wells. 



In this county and upon the western two-thirds of this state, the till has 

 a dark bluish color, except in its upper portion, which is yellowish to a 

 depth that varies from five to fifty feet, but is most commonly between 

 fifteen and thirty feet. This difference in color is due to the influence of 

 air and water upon the iron contained in this deposit, changing it in the 

 upper part of the till from protoxide combinations to hydrous sesquioxide. 

 Another important difference in the till is that its upper portion is com- 

 monly softer and easily dug, while below there is a sudden change to a hard 

 and compact deposit, which must be picked and is far more expensive in 

 excavating. There is frequently a thin layer of sand or gravel between 

 these kinds of till, which have their division line at a depth that varies from 

 five to thirty or very rarely forty feet. Owing to the more compact and 

 impervious character of the lower till, the change to a yellow color is usu- 

 ally limited to the upper till. The probable cause of this difference in 

 hardness was the pressure ot the vast weight of the ice-sheet upon the 

 lower and older till, while the upper till was contained in the ice and 

 dropped loosely at its melting. 



Again, in numerous places the upper till as here described is directly 

 underlain by a softer till, moist and sticky, and dark bluish in color. This 

 is usually of considerable thickness, or between twenty and fifty feet. It 

 often encloses or is underlain by beds of water-bearing sand; but occasion- 

 ally it has been penetrated and is found to lie directly upon a bed of very 

 compact till, such as usually comes next below the upper till. In some 



