458 E. K. SOPER 



VI. THE DRIFT 



The glacial drift which was spread over the land upon the 

 retreat of the ice, and which covers the rock surface throughout the 

 city, is variable in its composition. It ranges from typical bowlder 

 clay, which consists of a heterogeneous mixture of clay, sand, gravel, 

 and bowlders, to sorted deposits of fine sand or clayey loam. Most 

 of the drift consists of a mixture of clay, sand, and bowlders of all 

 sizes. Where the sand in the drift is free from clay and gravel, it 

 may have been deposited in two ways : either it represents a wind- 

 blown deposit and originated as sand dunes, or it is a flood-plain 

 deposit, washed up by glacial streams and sorted by water action. 

 The only place in the city where clay occurs in large quantity free 

 from pebbles or bowlders is along the banks of the Mississippi River 

 north of the mouth of Shingle Creek, These clay beds probably 

 represent lake and river deposits rather than accumulations due to 

 glacial action. They are indirectly the product of glacial action, 

 since they were deposited during the Glacial stage of the river. 



Since the surface of the drift constitutes the surface of the land, 

 except where it has been covered by post-glacial deposits of allu- 

 vium, sand, or loess of slight thickness, it controls the topography 

 of the region. Therefore, what has been said concerning the charac- 

 teristics of the topography of the city will apply also to the drift. 



The thickness of the drift is variable and ranges from o to 250 

 feet. As shown by the scarcity of outcrops (see map) the drift is 

 present nearly everywhere except along the river bluffs. The 

 localities where it is thinnest are those which border the rock out- 

 crops. In north Minneapolis there is an area bounded by Central 

 Avenue on the south and east. Twenty-third Avenue on the north, 

 and Main Street on the west, where the drift is thin, ranging from 

 o to 15 or 20 feet. Throughout the greater part of South Minne- 

 apolis, in the large area bounded by the Mississippi River on the 

 east and north, the pre-glacial Mississippi River valley on the west, 

 and Minnehaha Creek on the south, the thickness of the drift is very 

 uniform. The average depth over most of this area is from 30 to 

 50 feet, the greatest thickness being in the southern portion. Both 

 the surface of the drift and the surface of the underlying limestone 

 are generally flat over this region. The only noteworthy breaks in 



