430 THE GEOLOGY OF MINNESOTA. 



[Shakopce limestone. 



This is approximately the thickness of this formation exposed to view in its outcrops 

 through its whole extent of sixty miles along the Minnesota river. The quarries at the north end 

 of Front street in Mankato exhibit the first nine numbers of the foregoing section, with a very 

 slight dip northeast. The terrace, 75 feet above the river, one to two miles wide and ten miles 

 long, made by the Shakopee limestone, underlain by the Jordan sandstone, extending from Man- 

 kato north through Lime and Kasota to St. Peter, has been described in speaking of the surface 

 features of this county. 



Opposite to Mankato this limestone and the underlying sandstone form the lower half of the 

 river-bluff in Belgrade. Nicollet county. A mile west of Mankato, the Shakopee limestone makes 

 the small plateau called Sibley mound, which lies at the east side of the Blue Earth river close to 

 its mouth; and the similar plateau just opposite, on the west side of this river, to which the name 

 L'Huillier mound has been given, consists of the same limestone with a considerable thickness of 

 Jordan sandstone at the base. These mounds together reach about a third of a mile from east to 

 west. The hight of the former is approximately 50 feet, and of the latter 75 feet, above the bot- 

 tomland, which is five to ten feet above the Minnesota and Blue Earth rivers. Channels cut here 

 by these streams, perhaps since the ice age, bave separated these mounds from the Belgrade bluffs 

 and from each other.* 



Professor Wincbell reports the following 



Section of L'Huillier mound. 



1. Pebbles and soil at the brink of the bluff 2 ft. 



2. Dislodged, broken layers of Shakopee limestone 35 ft. 



3. Crust of iron and manganese 2-4 in. 



4. Green clay, or shale, becoming white toward the top and on the outer surface; 



evenly laminated, the laminae passing up into the white color. This is uncon- 

 formably overlain by masses of dislodged Shakopee limestone, the under sur- 

 face of which is crusted and rounded by water action. It also ascends 

 between openings in these masses 3 ft. 



5. Perpendicular cliff of Jordan sandstone, showing irregular seams and laminae of 



green shale, also small balls and bunches of curious shapes, sometimes con- 

 forming to the general sedimentation, and somewhat also to the false bedding, 

 so called. These thin deposits of green clay are fourteen feet below the gen- 

 eral bed of green clay (No. 4) above 10-1 5 ft. 



6. Talus, covering the Jordan sandstone, and reaching to the alluvial flood-plain. . 25 ft. 

 The same strata outcrop in many places through a distance of six miles west-southwest from 



Mankato, occurring in the bluffs of the old channel of the Le Sueur river between three-fourths of 

 a mile and one and a half miles north of Indian lake, in the bluffs of Blue Eirtli river a half mile 

 farther west, in the terrace at South Bend, as before mentioned, thinly covered by modified drift, 

 and again in the bluffs of the Blue Earth river a mile south of South Bend and Miuneopa. The 

 top of the Shakopee limastone in these exposures has a hight 75 to 100 feet above the Minnesota 

 river, or about 825 to 850 above the sea; and the glacial drift, lying on this limestone and forming 

 the higher part of the bluffs, has its top 2i)0 to 225 feet above the river, at which elevation its 

 slightly undulating expanse forms table-lands on each side of the valleys and thencs reaches with 

 imperceptibly ascending slopes to the east, south and west, beyond the boundaries of the county. 



Like this sheet of drift, the underlying rocks appear to have a nearly level but slightly sloping 

 top, which may have been the surface of this region before the ice age, but more probably was 

 planed and brought to its comparative uniformity in hight by glacial erosion. In Blue Earth 

 county the rock-surface, uncovered along the Minnesota valley, makes the terrace of Jord..n sand, 

 stone in Judson and thence toMinneopa falls, and its continuation capped by Shakopee limestone 

 at South Band; is exposed, overlain by drift, in the bluffs of the Blue Earth and Le Sueur rivers, 

 and of the Minnesota river in Belgrade; forms the L'Huillier and Sibley mounds; and, below Man- 

 kato, reaches in a broad terrace to Saint Peter. The Minnesota river, after cutting through the 

 overlying 125 to 150 feet of till, found here an old valley which had been channeled in these rocks 

 by pre-glacial streams. 



"The east mound derives its name from the encampment near it of the troops under the command of Gen. H. H. 

 Sihley, on tlieir return fro n auppresitng tua Indian outbreak in 1852. L'Huillier was tuu assayer who examined LeSueur's 

 copper ore, aud from whom his fort was named (see page 17). 



