PLATE XXXVI. 



SlBLEY AND NlCOLLET COUNTIES, 1888. WARREN UPHAM. 



Excepting the Minnesota valley, and the short valleys of its tributaries, these 

 counties present a nearly flat or gently undulating surface composed of till, whose 

 low divides and swells rise slowly from ten to twenty or rarely thirty feet above the 

 intervening depressions, in which last are frequently marshes or shallow lakes. 



Timber covers the northeastern one-third of Sibley county and small scattered 

 tracts about lakes, and along the Minnesota river in Nicollet county; but in general 

 these counties are characterized by prairie. 



The Minnesota river lies from 175 to 180 feet below the bluffs at Fort Ridgely; 

 about 200 feet at Mankato; 230 at St. Peter, and 210 to 225 at Henderson and north- 

 ward. 



The soil is fertile and black to the depth of one and one-half to two feet, being 

 about the same in the timbered tracts as in the prairie. 



The underlying rocks outcrop only in the lower flats of the Minnesota valley. 

 The Archean is seen in the town of Kidgely, consisting partly of porphyritic granite 

 with abundant coarse, gray crystals of orthoclase and partly of a flesh-colored feld- 

 spathic granite not noticeably porphyritic, with patches made up almost entirely of 

 black mica; also a rough- weathering belt of mica schist extends for several rods 

 north and south, dipping toward the west from 40 to 60. 



At four miles below Fort Ridgely is the place known to the early voyagers and 

 Indians as Little Rock, consisting of reddish granite and gneiss. This rock is visible 

 about a mile, from northwest to southeast, rising in knobs from forty to sixty feet 

 above the flood plain. 



There is also a small knob of red granite in the S. W. \ sec. 27, Courtland, oppo- 

 site New Ulm, which is overlain almost directly by a coarse, reddish conglomerate, 

 the basal beds of the Potsdam. 



The age of the Animikie intervened between the granite and the Potsdam, but 

 no trace of its strata are found in situ in this part of the state. Its debris, however, 

 has been detected in this conglomerate.* Mingled with abundance of quartz and 

 quartzyte and some of gneiss are many felsyte and taconyte pebbles. This conglom- 

 erate is closely followed conformably by the red quartzyte and red shale exposed a 

 little further east, and extending a mile and a half toward the east and southeast, 

 on the north side of the Minnesota river. This quartzyte rises from 100 to 125 feet 

 above the river, with an average dip toward the north-northeast, the whole thick- 



* American Geologist, vol. xvii, pp. 155-1B2, September, 1895. 



