PRE-GLACIAL RIVER VALLEYS OF MINNEAPOLIS 449 



This erosion was not confined to the pre-glacial streams but 

 was, in a smaller degree, effective over the entire area. The 

 Decorah shale, which is a persistent formation in Goodhue, Olm- 

 sted, Rice and other counties in southern Minnesota, has been 

 almost completely eroded from the area included in the map. Only 

 small isolated areas remain, capping some of the higher points in 

 the south central and southeastern parts of the city and especially 

 along the banks of the present channel of the Mississippi from the 

 University campus southward to Minnehaha Creek. 



Not only has pre-glacial and glacial erosion removed most of the 

 shale from the area, but the limestone is also more or less eroded at 

 all points within the city and completely removed from much of 

 the northern part of Minneapolis, especially the northwestern por- 

 tion along the area drained by Shingle Creek. Nevertheless, the 

 Platteville limestone is the principal rock formation immediately 

 below the drift and covers by far the greater j)art of the area within 

 MinneapoHs. This limestone is absent only along the stream 

 channels, in the northwestern part of the area, and in a few scat- 

 tered areas in north Minneapolis where erosion has reached the 

 underlying sandstone. The distribution and relations of the forma- 

 tions within the city of Minneapolis may be seen on the map and 

 sections accompanying this paper. 



III. THE SURFACE OF THE BURIED ROCK 



It will be seen from an inspection of the sections (Figs. 2 and 3) 

 that the rock surface beneath the drift is generally nearly flat. 

 The only notable features of relief are found al(jng the old river 

 channels which have been eroded to an average depth of 200 to 

 250 feet below the rock surface of the intervening upland plain. 

 There are, however, numerous minor irregularities of surface due 

 to the unequal erosion by the ice. The dip of the rocks is so flat 

 as not to be noticeable to the eye except along the north edge of 

 the city, where there is a low dip. Owing to the scarcity of out- 

 crops this dip is only evident when the elevations of the top of the 

 rock are compared to those in the central and southern part of the 

 city. The top of the limestone along the south edge of Minneapolis 

 stands at an elevation of about 790 to 800 feet above sea-level, 



