592 THE GEOLOGY OF MINNESOTA. 



[Topography. 



an alluvial bottomland, mostly within reach of the highest floods, and 

 steep enclosing bluffs, from whose top a vast prairie of undulating till or 

 glacial drift stretches away on each side in an apparently almost level ex- 

 panse as far as the view extends. 



In these twenty-five miles the Minnesota valley grows deeper from northwest to southeast, 

 its depth being approximately 100 feet at Montevideo; 150 feet at Granite Falls; 165 feet at Min- 

 nesota Falls; and 175 feet at the mouth of tho Yellow Medicine river. This tributary has cut a 

 valley equal in depth, along the lower part of its course, to that of the Minnesota, and from a 

 half mile to one mile in width, leaving between these valleys in the north part of Sioux Agency 

 township a promontory or plateau, some three miles long and less than a mile wide, upon which 

 the ruins of the buildings of the Upper Sioux Agency remain. At Sorlien's mill, in the south 

 edge of section 35, Minnesota Falls, about four and a half miles above its mouth, the valley of 

 the Yellow Medicine river is 75 to 100 feet deep. Westward in this county and the northwest 

 part of Lyon county, its depth is diminished from 75 to 40 or 50 feet; and at the foot of the 

 Coteau, where its head-streams are crossed by the railroad, they flow in channels only 20 to 30 

 feet below the average surface. The bluffs of both the Minnesota and Yellow Medicine valleys 

 are cut by mf.ny short ravines, occupied by springs and rivulets, and all their tributary creeks 

 have formed channels that increase in depth as they approach these great valleys. The branches 

 of the Lac qui Parle river, from the foot of the Coteau to the north line of Yellow Medicine 

 county, flow 30 to 40 feet below the general level; and valleys of about the same depth have been 

 eroded by the Redwood river in the vicinity of Marshall and thence to the east line of Lyon 

 county; by its tributary, Three Mile creek, from Grandview to its mouth; and by the most north- 

 ern and main stream of the Cottonwood river. 



Excepting the valleys which have been thus cut by streams, a gently undulating or in some 

 portions moderately rolling sheet of till covers the northeastern half of this district, reaching 

 from the Minnesota river twenty-five to thirty miles southwest to the Cottonwood river and Mar- 

 shall lake in eastern Lyon county, and to the Winona & St. Peter railroad in its course thence 

 through Marshall. Minneota and Canby, and nearly to the state line. This expanse has a grad- 

 ual ascent southwestward of six to ten feet per mile, so that the railroad between Marshall and 

 Canby is from 150 to 250 feet above the general level of the country adjoining the Minnesota 

 valley. 



The most rolling portions of this area are in two belts. One of these lies in eastern Yellow 

 Medicine county, within four to eight miles west from the Minnesota valley, including Stony 

 Run, the northeast part of Hazel Run, Minnesota Falls, eastern Wood Lake, Sioux Agency and 

 Echo. In going westward from Granite Falls, the first one and a half miles are on the bottom- 

 land, which here averages 20 feet above the river and has many scattered knobs and small ridges 

 of gneiss, 10 to 40 feet higher; then the southwestern bluff of the valley is ascended, to the gen- 

 eral surface of the prairie, some 150 feet above the river. This is moderately undulating for 

 about a half mile, at which distance from the Minnesota valley it is marked by a depression, from 

 an eighth to a quarter of a mile wide, and 20 to 30 feet deep, bordered by gently sloping sides, 

 and reaching several miles from northwest to southeast, parallel with the Minnesota valley. The 

 entire extent of this hollow was not traced. It is believed to have been excavated by drainage 

 during the final melting of the last ice-sheet. Similar old water-courses, now dry, extend in the 

 same direction through Omro and Wergeland in the west part of this county. These are de- 

 scribed, and the mode of their formation explained, in treating of the glacial drift. West of this 

 channel the surface is quite rolling for two or three miles, in smoothed swells and mounds, 20 to 

 30 or sometimes 40 feet high, mostly trending from northwest to southeast. Thence the contour 

 becomes gradually more even, so that at six or seven miles from Granite Falls, and onward, it is 

 nearly flat and very smooth, undulating only 10 to 20 feet in long slopes. Southeastward from 

 the Yellow Medicine river, by the east side of Wood and Sand lakes to the lake of the Woods, 

 some portions of the surface are more prominently rolling than west of Granite Falls, having 

 here and there massive swells, 25 to 50 feet above the average of this region, which both in its 

 hilly and in its nearly level areas is till. 



