PLATE XVI. 



BLUE EARTH COUNTY, 1884. WAKKEN UPHAM. 



A large part of this county, probably five-sixths of it, was originally prairie, and 

 very flat, due, apparently, to the prevalence of a glacial lake during the later part of 

 the ice retreat, by which the drift sheet was smoothed and washed oft', filling the 

 depressions with a lacustrine clay. Yet this county contains a large supply of timber. 

 This prevails in the northeastern quarter of the county, where it is well known as 

 the "big woods," and along the numerous streams that converge in the Undine region 

 to unite with the Minnesota river at Mankato, of which the Blue Earth is the main 

 trunk, the spreading forks being known from the west as the Minneopa, Watonwan, 

 Perch, Willow, Blue Earth, Maple, Big Cobb, Little Cobb, Le Sueur and losco. This 

 general and remarkable descent toward Mankato serves to render that locality a 

 favorable one for artesian wells, of which there are several. 



Since the spreading of the drift and the lacustrine clay the surface drainage has 

 served to reopen some of the pre-glacial (and pre-Cretaceous) gorges, and hence the 

 present streams, beginning on the surface of the drift deposits and running on it with- 

 out much excavation for many miles, at length find some of these old rock-cut valleys, 

 and pursue these gorges till they reach the Minnesota river, which itself runs in a 

 similar pre-glacial gorge from Minneopa northward. Southward from Mankato are 

 to be found numerous such gorges, partially uncovered, not now occupied by streams, 

 which once were portions of the drainage system of the region. The county has 

 many lakes, situated on the upland drift surface. There are two small rolling tracts 

 approaching the characters of a moraine, but which may be due to locally copious 

 waters acting on the drift in the process of deposition, due to the disturbing effect of 

 prominent irregularities in the underlying rocky surface. These are in the north- 

 west part of Sterling and the southeast part of Pleasant Mound. There is also a 

 wide, more undulating belt extending northwestward from lake Jackson through 

 Vernon Center and Garden City. 



The valley of the Minnesota is from 200 to 225 feet below the general surface, 

 and that of the Blue Earth and its tributaries is about the same toward the north, 

 but toward the south it is much less. 



The rocks of the county belong to the Upper Cambrian and to the Cretaceous. 

 The former furnish good building stone and quicklime and cement at Mankato, while 

 the latter contain much kaolinic clay, a residual product of rock decay in pre-Creta- 

 ceous time. Below the Upper Cambrian the deep wells at Mankato entered a great 

 thickness of red sandstone and unctuous red shales, belonging to the later Kewee- 

 nawan or early Potsdam age, although without trap. This red formation, which here 



