COTTON WOOD AND JACKSON COUNTIES.. 509 



Glacial drainage. Boulders.] 



taries, and contains a succession of sloughs and small reedy lakelets, connected at time of high 

 water by a stream, which is the head and most northern source of the Little Sioux river. 



Farther recession of the ice gave to the waters of Heron lake and the upper Des Moines 

 river a lower outlet by the present course northeast across the second terminal moraine at the 

 north side of the Blue mounds, and thence southeasterly along the east side of this moraine. 

 This avenue of drainage became marked by a considerable valley eroded while the ice yet lay as 

 a barrier upon the east part of Cottonwood and Jackson counties; for the top of the bluffs, and 

 the general surface of the country, bordering the Des Moines in eastern Jackson county are slightly 

 higher than the watershed between Heron lake and the Little Sioux river; and, furthermore, the 

 natural slope in eastern Cottonwood and northeastern Jackson county is eastward, so that this 

 river could not flow here to the south-southeast unless its valley had been thus formed before the 

 ice-sheet was melted at its east^side, being excavated sufficiently deep to hold the stream after- 

 ward in this course. 



An exception to the generally smooth contour of the drift-sheet north of the quartzyte ridge 

 is found in a quite roughly hilly morainic area, apparently isolated, which lies mainly in the north 

 half of Stately, the most southwest township of Brown county, and extends into Germantown to 

 the west side of section 12. Its abrupt mounds and ridges of stony till are 25 to 75 feet high, 

 having their greatest prominence in Stately along the lower part of Mound creek. This tract ap- 

 pears to belong to a third terminal moraine.* Through the middle of Germantown a notable 

 valley, having a flat bottom of stratified gravel and sand, enclosed by moderately steep slopes 

 which rise about forty feet to the undulating surface of the till on each side, was observed, ex- 

 tendii:g five or six miles in an east-southeast course from near Dry creek at the north side of 

 section 17 in this township, to Mound creek at the east side of section 30, Stately. Another val- 

 ley of similar character was noted three-fourths of a mile farther south, running parallel with the 

 last through the north part of sections 25 and 26, Germantown. These deserted water-courses 

 were probably formed during the departure of the last ice-sheet. Upon this region its border 

 doubtless retreated to the north and northeast; and while it, still lay as a barrier upon the north 

 part of Germantown and was accumulating the morainic hills that lie a few miles to the north- 

 east in Stately, the drainage from its melting was carried by these valleys southeasterly. Farther 

 northwest, the land for a considerable distance along the probable course of the ice-margin in 

 this stage of its retreat is lower than where these valleys occur, and therefore would be occupied 

 by a lake; and again southeastward, from the south part of Stately to Silver Lake in Martin 

 county, a narrow glacial lake probably extended along the border of the ice- sheet, having a hight 

 about 1200 feet above the sea, and overflowing south of Iowa lake to the East fork of the Des 

 Moines river. 



Boulders and pebbles. The boulders of the drift in these counties are 

 mainly granite and syenite, crystalline schists, quartzyte, and limestone. 

 The quartzyte ridge in northern Cottonwood county has supplied from 

 a tenth to a half of the large rock-fragments in the drift south of it. In 

 traveling from Fairmont to Worthington, boulders and pebbles of quartz- 

 yte are first seen abundantly in the vicinity of Jackson, and are plentiful 

 thence westward. At the northwest side of Spirit lake this formation has 

 supplied a ^ixth part of the larger stones and boulders, but its proportion 

 in the beach-gravel is only a fifteenth or twentieth. Of a hundred and 

 fifty small pebbles counted on a space one foot square of the beach at the 

 west side of Spirit lake, half were magnesian limestone, probably derived 

 from the formation that outcrops near Winnipeg; and the other half were 



See page 479; also tlie report of Brown and Redwood counties. 



