660 THE ICE AGE IN NORTH AMERICA. 



originally upon an ancient surface that has been since covered 

 deeply by the modified drift which forms the terrace. It will 

 be remembered that the quartz chips and implements discovered 

 by Professor Winchell in this vicinity are contained in the up- 

 per stratum of the terrace-plain ; but the notch quartzes do 

 not occur at the terrace-top, and can not have been derived 

 from it, but are confined strictly to a single stratum of the 

 lower gravels closely overlying the till. Hence the two sets of 

 objects can not be synchronous, though they may have been 

 produced by the same race at different stages of its existence. 

 The notch quartzes must, of course, be older than those de- 

 scribed by Professor Winchell, by at least the lapse of time 

 required for the deposition of the twelve or fifteen feet of modi- 

 fied drift forming the upper part of the terrace-plain, above 

 the quartz- bearing stratum." 



This description by Miss Babbitt shows that these imple- 

 ments and fragments of chipped quartz occurred in a well-de- 

 fined thin layer in the modified drift forming the glacial flood- 

 plain of the Mississippi River, as shown in the section which I 

 have drawn (see the following figure). I have examined the 



W - Stratum containing Terraceo fM>iifU : a. Drift, E- 



- R' VER SotomU*&CAUwr 





Fig. 180.— Section across the Mississippi Valley at Little Falls, Minnesota, showing the 

 stratum in which chipped quartz fragments were found by Miss F. E. Babbitt, as de- 

 scribed in the text. (Upham.) 



terraces and plains of this valley drift from St. Paul and Min- 

 neapolis to Brainerd, some twenty-five miles north of Little 

 Falls, and find them similar in material and origin with the 

 modified drift terraces in the valleys of the Merrimack, Con- 

 necticut, and other rivers in New England. These water- 

 courses extending southward from the region that was covered 

 by the ice-sheet became the avenues of drainage from it during 

 its retreat. A part of the drift which had been contained in 

 the lower portion of the ice was then washed away by the 

 streams formed on the ice in its rapid melting and was depos- 

 ited as modified drift, forming layers of gravel, sand, and fine 

 silt, in the valleys along which the floods supplied by this 

 melting descended toward the ocean. Along the Mississippi 



