Glacial and Modi£ed Drift 30 r 



most northern sources of the Mississippi, did iKJt probably 

 occupy more than ten or fifteen centuries. If the recession of 

 a Mississippi cataract from the site of Fort Snelling north- 

 ward, now called the Falls of St. Anthony, began only about 

 8,000 years ago, as shown by Winchell, the latest melting of 

 the icefields on our northern boundary took place within some 

 one thousand or fifteen hundred years afterward, that is, be- 

 tween 7,000 and 6,000 years ago. 



Far greater age, however, must be attributed to the glacial 

 drift of a tract 40 to 50 miles wide in southeastern Minnesota, 

 lying next east of the outer moraines, which run southerly 

 from St. Paul through the west part of Dakota county, and 

 through Rice, Steele, and Freeborn counties,, into Iowa. The 

 tract of more ancient drift comprises much or all of Dakota, 

 Goodhue, AVabasha, Dodge, Olmsted, Mower, and Fillmore 

 counties, lying between the Altamont or first moraine and a 

 large driftless area, which includes a width of 20 to 40 miles 

 in the southeast edge of this state from lake Pepin southward, 

 between the attenuated margin of the glacial drift and the Mis- 

 sissippi river. The same remarkable driftless area reaches 

 thence nearly 100 miles east in Wisconsin, but has its greatest 

 extent of about 150 miles from north to south, continuing, 

 mainly east of the Mississippi, to the northwest corner of 

 Illinois. 



In the series of stages or epochs of the glacial period, char- 

 acterized by alternating growth and wane of the continental 

 ice-sheet, with advance, retreat and re-advance of its borders, 

 ascertained by Chamberlin, Salisbury, Leverett, Calvin, and 

 others, our tract of the old drift outside the moraines in the 

 southeast part of Minnesota belongs probably to the Kansan 

 stage of glaciation, when the ice-sheet attained its greatest ex- 

 tension in the center of the continent, probably fifteen or 

 twenty times as long ago as the final departure of the ice from 

 this state. 



The great age of this drift is indicated in Dakota and 

 Goodhue counties, bordering the Mississippi from St. Paul to 

 lake Pepin, by occasional columnar or towerlike remnants of 

 the St. Peter sandstone, of which the most noteworthy are 

 Castle rock,, about a mile east of the railway station of that 

 name and Chimney rock, in the east edge of the northeast 

 quarter of section 31, Marshan, about eight miles south of 



