PRE-GLACIAL RIVER VALLEYS OF MINNEAPOLIS 459 



the otherwise almost flat surface are at Powderhorn Lake, where a 

 small buried valley heads, and at Lowry Hill and the region around 

 Loring Park, where a belt of low morainal hills extends from the 

 west across the edge of the area and breaks the monotony of the 

 surface. 



In some portions of the city, especially in Southeast Minneapolis, 

 in the vicinity of the University and northeastward to the city 

 limits, there is a bed of peat several feet thick which occurs only a 

 few feet below the surface. This peat bed rests upon drift largely 

 composed of sand, clay, or gravel, or a mixture of the three, and is 

 covered by recent deposits of sand or soil which are of post-glacial 

 or glacial origin. 



In East Minneapolis, near Tower Hill in the Prospect Park dis- 

 trict, deposits of loess, a formation of loamy material of aeolian 

 origin, occur at the surface. Therefore, it is apparent that while 

 the retreating ice left a mantle of drift over the entire region, the 

 surface of this drift has been slightly modified by post-glacial 

 deposition. These deposits of sand, silt, and other material were 

 chiefly laid down as flood-plain deposits in the streams and marshes 

 fed by waters from the melting ice. A small amount of post- 

 glacial erosion has also operated to modify the original drift surface, 

 but this has been unimportant except along the channel of the river 

 and its tributaries. 



The origin, composition, and character of different drift deposits 

 and their modification by post-glacial agencies have all been 

 described in detail in reports by N. H. Winchell,^ Warren Upham,' 

 F. W. Sardeson,^ and F. F. Grout and E. K. Soper.'* 



VII. INFLUENCE OF BURIED RIVER CHANNELS UPON BUILDING 

 CONSTRUCTION 



In those portions of the city which are underlain by the buried 

 valleys and in other parts of the city where the drift is deep, it is 

 impracticable to carry excavations to bedrock for building founda- 

 tions and cellars. For the ordinary small store or residence, or for 



^ Final Report Minn. Geol. and Nat. Hist. Survey. ^ Ibid. 



3 "Geology of the Twin Cities," Minneapolis and St. Paul Folio, U.S. Geol. 

 Survey. Now in press (1914). 



^ Grout and Soper, "The Clays and Shales of Minnesota," Bull. No. 11, Minne- 

 sota Geological Survey, chapter on Hennepin County (1914). 



