422 TnE GEOLOGY OF MINNESOTA. 



[Geological structure. 



mon elder, high-bush cranberry, and species of honeysuckle and cornel. 



GEOLOGICAL STRUCTURE. 



In the valleys of the Minnesota river, and of the Blue Earth, Waton- 

 wan, Le Sueur. Maple and Big Cobb rivers, are numerous exposures of the 

 middle members of the Lower Magnesian or Calciferous series, these being in 

 ascending order the St. Lawrence limestone, the Jordan sandstone and the 

 Shakopee limestone. These formations are nearly horizontal, and they 

 probably underlie the drift or the Cretaceous throughout the whole county; 

 but, because of the great depth of the till, they outcrop only in the bot- 

 tomlands and lower half of the bluffs of these deep valleys. Under these 

 strata, the deep well at Mankato penetrates the St. Croix shales and sand- 

 stone, which are the lowest members of the Lower Magnesian or Calcifer- 

 ous series, and a great thickness of the Potsdam sandstone and shales. 

 Over the Lower Magnesian rocks, and often filling water-worn cavities in 

 them, Cretaceous beds of clay, and sometimes of sand and gravel, are found 

 at several places in the county. The various geological formations to be 

 described in the order of their age, from the oldest to the newest, are: 

 1. Potsdam sandstone and shales; 2. St. Croix sandstone and shales; 3. St. 

 Lawrence limestone; 4. Jordan sandstone ; 5. Shakopee limestone ; 6. Creta- 

 ceous beds; 7. Glacial and modified drift. 



Potsdam sandstone and shales. One of the deepest drillings ever made in 

 the United States or the world, is that done a few years ago at Mankato, 

 in the hope of obtaining an artesian well. This was in the southeast edge 

 of the city, at the top of a portion of the bluffs which is commonly called 

 " Bunker hill ". Its elevation above low water of the Minnesota river is 

 about 225 feet, making its hight above the sea approximately 975 feet. The 

 depth of this drilling is 2204 feet, of which the greater part, reaching from 

 about 900 feet to the bottom, is in red sandstone and shales that are believed 

 to belong to the later part of the Potsdam period, being intermediate in age 

 between the St. Croix group and the Cupriferous or Keweenawan series, 

 which Prof. Winchell and the writer refer to the earlier part of this Pots- 

 dam period. 



No exact record can be found to show the character of all the strata passed through and the 

 depths at which each began and ended; but two sets of specimens of the rock encountered at suc- 

 cessive depths are preserved, one by Mr. W. Hodapp, druggist, showing the material at eighteen 



