508 THE GEOLOGY OF MINNESOTA. 



[Glacial drainage , 



Martin county, and flowing southwestward across Emmett county at right angles to the present 

 East fork of the Des Moines. Portions of its channel are preserved in Swan lake, six and a naif 

 miles long from northeast to southwest, and from one-fourth to two-thirds of a mile wide, only 

 ten to fifteen feet deep, but occupying a hollow twenty-five to fifty feet below the gently undu- 

 lating expanse of till on both sides; and in the High lakes, nearly three miles long, lying one to 

 three miles south of the soutwest end of Swan lake. This river probably coincided in its course 

 with the present Des Moines southward from the north line of Palo Alto county. Medium lake, 

 which readies four and a half miles northeast from Emmettsburg, varying from a quarter to a 

 half of a mile in width, mostly ten to fifteen feet deep, with a bottom some forty feet below the 

 average of this moderately undulating region, but at one point, a little north of its center, found 

 to be more than fifty feet deep, its surface being about thirty feet above the Des Moines river, 

 probably marks the position of another interglacial tributary of the Des Moines, joining it at 

 nearly the same place with the branch from Martin county. 



Drainage during the last glacial epoch. Very significant changes in the drainage of this region 

 have been produced by the lobe of the ice-sheet which covered these counties and a width of about 

 a hundred miles eastward during the last glacial epoch. From the south end of Heron lake to 

 Okoboji township in southern Dickinson county, Iowa, the interglacial channel of the Des Moines 

 has been principally lost by being filled with the drift of terminal moraines, accumulated at the 

 west border of the ice. The outer belt of these deposits extends in Iowa from Storm Lake in 

 Buena Vista county northward through eastern Clay county to the Okoboji lakes, and thence 

 westward to Ocheyedan mound in Osceola county. Thence passing into Minnesota, it reaches 

 northwesterly through the central part of Nobles county, western Murray county, and the most 

 northeast township of Pipestone county, forming there and farther northwest the highest part of 

 the Coteau des Prairies. The present basin of the Des Moines river from central Iowa northwest- 

 ward was entirely covered by this ice-sheet; but a small part of its interglacial valley, in southern 

 Dickinson and northern Clay county, Iowa, and most of the basin of Ocheyedan creek, here trib- 

 utary from the northwest, were outside the ice-lobe, by which they were dammed and their drain- 

 age in the old course to the east and southeast was made impossible. A lake about a hundred and 

 fifty feet deep and covering the greater part of Clay county, was thus formed at the west side of 

 the ice-lobe, until its overflow cut the deep, trough-like valley or channel in which the Little Sioux 

 river now flows along the south side of Clay county and in northeastern Cherokee county, 150 to 

 200 feet deep, and in some places only a quarter of a mile wide between the tops of its bluffs, 

 which consist wholly of glacial drift.* This outlet was so deeply excavated while the ice-sheet 

 lay as a barrier on the east that after its departure the stream continued to flow by this passage 

 to the Missouri, through a broad area of till which has its surface 100 to 150 feet higher than the 

 divide between the Little Sioux and Des Moines rivers east of Spencer. 



In northern Clay county, where the Little Sioux river takes the place of the interglacial Des 

 Moines, the broad and deep valley eroded by that stream before the last glacial epoch has become 

 nearly filled with modified drift, which forms an extensive plajp, ten miles long and two to four 

 miles wide, bordering the Little Sioux river through Summit, Kiverton and Spencer, reaching 

 west to Stony and Ocheyedan creeks. These fluvial beds of gravel and sand were deposited after 

 the excavation of the channel of the Little Sioux river, by which the lake that previously existed 

 here had been drained into the Missouri; and they are thus shown to have been supplied during 

 the latter part of this epoch, while the ice-sheet, in which they had been held, was being melted 

 away. 



The decline and departure of this ice was interrupted by a halt and probably by a re-advance, 

 forming a second or inner line of terminal moraine, which reaches through Murray, Cottonwood 

 and Jackson counties, from the east side of lake Shetek southeast to the Blue mounds west of 

 Windom, and thence south to Spirit lake, and continues southeast in Iowa within a few miles 

 west of the Des Moines river to Pilot mound and Mineral ridge. At this time the drainage from 

 the head of the Des Moines basin, in Murray county, and the waters of Heron lake and its tribu- 

 taries went southward through West Heron Lake, Bust and Sioux Valley townships, and were 

 carried by the Little Sioux to the Missouri river, instead of going southeast as now to the Missis- 

 sippi. Heron lake then stood about twenty feet higher than now, probably covering three times 

 its present area. The shallow channel of its overflow has become partly filled by the silt of tribu- 



White's Geology of Iowa, vol. ii., p. 205. 



