PRE-GLACIAL RIVER VALLEYS OF MINNEAPOLIS 445 



of the drift at all points within the city of Minneapolis; and (3) to 

 trace the courses of the glacial and pre-glacial river channels, now 

 filled with glacial debris, throughout the city. The results of these 

 investigations are shown on the topographic map on which surface 

 and solid rock is contoured and in the series of geologic structure 

 sections which accompany this paper and to which frequent refer- 

 ence will be made in the following discussion. 



The results show the following essential facts. In pre-glacial 

 time the region around Minneapolis was dissected by a large river 

 (referred to tentatively as the pre-glacial Mississippi) and its tribu- 

 taries, which cut deep valleys into the rock. At a later time, dur- 

 ing the Glacial period, the surface of the rock included in the areas 

 between these old stream valleys was also deeply eroded by ice 

 gouging and planation. While the effect of this glacial erosion was 

 to produce a generally flat rock surface above the valleys, there 

 were numerous irregularities of surface developed, many of which 

 have never before been accurately delineated. The ancient valleys 

 became filled with glacial debris which was brought down by the 

 ice and left behind as a thick mantle covering the entire region when 

 the ice sheet receded to the north. Not only were the old valleys 

 choked up with glacial drift, but the entire rock surface was buried. 

 The depth of this drift has been determined at as many points in the 

 city as possible and has been found to vary from o to 250 feet. It 

 is thickest in the old buried valleys and thinnest where post-glacial 

 erosion has been active. The depth, width, and courses of the old 

 buried valleys have been determined and are shown in contour on 

 the accompanying map which gives a picture of the buried rock sur- 

 face throughout the city. In constructing the map (Fig. i), the 

 surface topography has been superimposed upon this buried topog- 

 raphy so that the depth to bedrock at any point may be readily deter- 

 mined from the map by taking the difference between the elevations 

 shown by the surface contours (solid lines) and the bedrock contours 

 (broken lines). The large depression which is the site of Powder- 

 horn Lake was found to be due to the head of a small buried valley 

 in this locality. Five secondary valleys which were tributary to the 

 main buried river valley have been traced. These are shown on the 

 map. Finally, it has been shown that the position of these buried 



