\VATOS\VAX AND MARTIN' COUNTIES. 485 



Chains o] lake-..] 



Albert Lea on the east to Worthington on the west. Before the glacial 

 epoch in which the ice had its greatest extent, and probably also between 

 that time and the date of the terminal moraines that cross Wisconsin. Minne- 

 sota and Dakota, other glacial epochs spread ice-sheets upon this region ; 

 but their moraines have been leveled and covered with additional deposits 

 of till, and the interglacial soil and fossiliferous sediments of sloughs and 

 lakes have been m6stly ploughed up and mixed in the drift, while their 

 remnants have been similarly buried, by the more extended ice-sheets of 

 these subsequent epochs. Such remnants of interglacial beds, containing 

 leaves and shells, have been found in Center Creek and Silver Lake town- 

 ships in Martin comity, as stated in the notes of wells on page 487. The 

 chains of lakes in this county appear to show that interglacial rivers, be- 

 tween the time of greatest extent of the ice and the date of the last glacial 

 epoch, were here carried southward in four continent valleys to the East fork 

 of the Des Moin.es river. The present drainage of Martin county is mostly 

 transverse to this course and tributary to the Blue Earth river; but the 

 watershed and slopes that now turn it away from the Des Moines are so 

 slight that if the streams of this area had channels from north to south, 

 such as were probably eroded along the lines of these chains of lakes while 

 the margin of the ice-sheet that had reached to the farthest limit of the 

 glacial drift was receding across these counties, they would continue to flow 

 southward to the Des Moines. Probably all of this county, excepting per- 

 haps its most northeast township, was during a long interglacial epoch 

 included within the Des Moines basin, which still embraces a part of it at 

 the southwest. The last ice-sheet doubtless added considerably to the drift, 

 but did not entirely remold its topographic features; so that here even the 

 interglacial water-courses cut in the drift remain in some portions with 

 little change, still having steep bluffs and holding these series of lakes. 

 This interpretation of their meaning is strongly confirmed by features of 

 the valley of the Minnesota river, which seem to be explicable only by 

 referring them to similar causes.* 



Boulders and gravel, though always present, are nowhere abundant in the till of Watonwan 

 and Martin counties; and boulders larger than five feet in diameter are very rare. Tlie frequency 

 of limestone fragments is nearly the same as is usual through all western Minnesota. This rock 

 often makes one-third or one-half of the gravel in the till and on the beaches of lakes; but it 

 supplies a much less proportion, perhaps not exceeding one twentieth, of the boulders larger than 



Compare article on the Minnesota valley in Ihe ice ag, Prnc. of Amer. Awot-,. fur <tdv. at' Science. 1883 and Xmr 

 .low. Sci. (3), vol. xxvii. 1*81. 



