Glacial and Modified Drift 303 



"Mississippi valley was partly filled with modified drift, or the 

 stratified gravel, sand, and fine silt, which form terraces or 

 plains on either side of the river. An abundant supply of wa- 

 ter from the dissolving ice and from rains caused the river to 

 be in a continual flood stage during the summers; and much of 

 the drift was then carried into the valleys of the great river and 

 its tributaries, filling them from side to side up to the levels of 

 their highest gravel and sand terraces. In proportion as the 

 ice-sheet withdrew front this area, the tribute of drift borne 

 into the valleys was greatly diminished, while yet the Minne- 

 sota and St. Croix rivers, and the Mississippi below their 

 mouths, were flooded through every summer by the outflow 

 of lake Agassiz and of the Western Superior glacial lake, both 

 held by the barrier of the retreating ice-sheet farther north. 

 Then the valley floodplains so lately formed were deeply chan- 

 neled until the Mississippi flowed at levels 50 to 100 feet lower 

 than now along some parts of its course in and adjoining Min- 

 nesota. 



After the great discharge of the glacial lakes ceased, the 

 ability of these rivers to erode their valleys was less, and in 

 consequence the alluvium of tributaries has in some cases 

 partly refilled the main valley. Thus the silt brought to the 

 Mississippi by the Chippewa river during the Postglacial per- 

 iod has been spread as a barrier at its mouth and southward, 

 forming lake Pepin, 25 miles long and i to 3 miles wide, which 

 has a maximum depth of 56 feet in its southern part. The 

 Mississippi valley for many miles below the lake has been re- 

 filled with the Chippewa alluvium nearly to that depth. In the 

 same way lake St. Croix, 25 feet deep, has been formed in the 

 St. Croix valley just above its mouth, by the barrier of the 

 Mississippi alluvium ; and likewise the shallower Lac qui 

 Parle, on the Minnesota river, owes its existence to refilling 

 of the Minnesota valley by the silt of the Lac qui Parle river. 



In the city of St. Paul a very coarse valley deposit, allied 

 with the modified drift, but consisting mainly of small and 

 large fragments and blocks of the underlying Trenton lime- 

 stone, is spread here and there on the level Trenton terrace 

 plain^ about 90 to 120 feet above the Mississippi and also about 

 100 feet below the tops of the valley bluffs. The coarse lime- 

 stone debris, occasionally holding blocks of all sizes up to 20 

 •'^r 30 feet in length or diameter, is seen in man\- places, but 



