COTTONWOOD AND JACKSON COUNTIES. 5Q7 



Moraines. Interglacial drainage.] 



five miles eastward, descending with an imperceptible slope to the Blue Earth river, and beyond 

 this rising in the same manner to the belts of drift hills at the sources of the Le Sueur and Cannon 

 rivers, well named by Nicollet " the N. E. prong of the Coteau des Prairies," since they are of 

 the same age with the moraines of these counties and a curved continuation from them (see page 

 406). The eastern two-thirds of Lakeside and Carson, and all of Mountain Lake township, in- 

 cluded in the vast area of intra-morainic till, are slightly undulating and differ only five to 

 ten feet in broad swells and depressions from being a perfect plain. This expanse, stretching on 

 all sides to the horizon, would be commonly called level, but the survey of the Saint Paul & Sioux 

 City railroad shows that its descent eastward is uniformly about twenty feet per mile through 

 these townships, or some 200 feet in the ten miles from the railroad summit a mile west of Bing- 

 ham Lake to the east line of this county. If the same slope were continued westward it would 

 pass over the summit of the Blue mounds; hence they cannot be seen east of Bingham Lake. 



Mountain lake, which has given its name to a railroad station and township, is so called be- 

 cause it contains an island that rises about 35 or 40 feet in steep bluffs, attaining the same hight 

 with the bluffs that surround the lake, even with the average surface of its vicinity. The prob- 

 able origin of this depression and of its steep enclosing bluffs, has been pointed out in treating of 

 the chains of lakes in Martin county, the most western of which appears to have its beginning in 

 this lake. 



West of the second moraine, the eastern shore of Heron lake mainly rises in gradual slopes of 

 till, reaching the summits of the morainic belt at a distance of three or four miles; the south end 

 of this lake, lying within the edge of the moraine, is enclosed by banks about forty feet high; but 

 on the west and southwest is a very flat expanse of till, 10 to 20 feet above the lake, only undu- 

 lating five to ten feet in slopes a mile long, stretching with slowly increasing hight as far as the 

 view extends westward. On the Sioux City railroad in the ten miles southwest from Heron Lake 

 to Hersey, the ascent is 68 feet; in eight miles on its branch from Heron Lake northwest to Dun- 

 dee, 26 feet; and on the Southern Minnesota railroad in seven miles northwest from its intersec- 

 tion with the Sioux City line to De Forest, is 32 feet. Trains approaching De Forest from the 

 southeast come into sight near the south end of Heron lake, and are visible during forty minutes 

 before their arrival. This smooth plain of till continues south through Rust and Ewington town- 

 ships, having the same slight ascent to the west, and crossed from north to south or southeast by 

 occasional water-courses and sloughs ten to twenty feet below the general level. 



Interglacial drainage. Heron lake lies in the continuation of the southeast course of the 

 upper Des Moines river below lake Shetek. There seem to be good reasons for believing that 

 lake Shetek, this part of the Des Moines, Heron lake, and Spirit and Okoboji lakes in Iowa, re- 

 semble the chains of lakes of Martin county, in occupying portions of what was originally a con- 

 tinuous valley excavated by interglacial drainage in the thick till of the earlier and severer gla- 

 cial epoch, before the time of the last ice-sheet by which the terminal moraines in this and ad- 

 joining states were formed. It is probable that the Des Moines river then continued southeast 

 where Heron lake is now, and oi^vard in the same course through Hunter, where the rolling 

 and hilly drift of the second terminal moraine now forms a watershed a hundred feet above 

 Heron lake; thence southward at the east side of Minneota to Spirit lake and the Okoboji lakes; 

 then, from West Okoboji lake south along the course of the Little Sioux river, which now re- 

 ceives the outflow of these lakes, to its bend three miles east of Spencer; and thence eastward 

 about twenty miles, by Trumbull, Palo Alto and Lost Island lakes, re-entering the present val- 

 ley of the Des Moines river at Emmettsburg. Hights along this distance are approximately as 

 follows: lake Shetek, about 1,475 feet above the sea; the Des Moines river at its point nearest to 

 Heron lake, about 1,375; Heron lake, 1,403; railroad summit between Heron lake and Jackson, 

 1,517; Spirit lake, about 1,400; the Okoboji lakes, about four feet lower than Spirit lake; Little 

 Sioux river at Spencer, about 1,300; lakes and lowest part of the divide between Spencer and 

 Emmettsburg, about 1,350; and the Des Moines river at Emmettsburg, about 1,125. The re- 

 markable depth of the south part of West Okoboji lake, exceeding one hundred feet, is thus very 

 probably in an unfilled portion of an interglacial valley, elsewhere choked up with the drift of 

 the later ice-sheet by which the morainic hills and swells, partly rough and partly smooth, ad- 

 joining this lake and covering most of northern Dickinson county, in Iowa, were accumulated. 



'At Emmettsburg this interglacial Des Moines river was joined by a large tributary from the 

 north, formed by the union of the streams whose courses are marked by the chains of lakes in 



