PRE-GLACIAL RIVER VALLEYS OF MINNEAPOLIS 453 



and beyond, is superimposed upon the buried gorge of the pre- 

 glacial river. Over the greater portion of the city, however, there 

 is no suggestion in the surface topography of the irregularities in 

 the buried rock surface. The topography of the existing surface is 

 typically glacial in its character, and, although the maximum relief 

 is not as great as in the case of the underlying rock surface, yet the 

 general relief is seen to be stronger than that of the underlying rock 

 surface, if we do not consider the old buried gorges. The lowest 

 point on the surface within the city is at the bottom of the Missis- 

 sippi River just below the mouth of Minnehaha Creek, in the 

 extreme southeastern corner of the city, where the elevation is about 

 690 feet above sea-level. The highest points are along the ridge of 

 morainal hills in the northeastern part of the city from Columbia 

 Heights to a point about one-fourth mile southeast of Hillside 

 Cemetery, where there are several points which attain an altitude 

 of 965 feet, or thereabouts. This gives a maximum surface relief 

 of about 275 feet. The lowest point accurately known on the sur- 

 face of the bedrock was determined from the record of a well at the 

 Bath House on the north shore of Lake Calhoun where the surface 

 of the rock was encountered at an elevation of 619 feet. The highest 

 point attained by the rock is in the Columbia Heights district where 

 the top of the shale stands at about 930 feet. This gives a maxi- 

 mum relief of 311 feet for the rock surface, which is 36 feet greater 

 than the relief of the existing drift surface. 



There are probably two factors which are mainly responsible 

 for the superimposed drainage system of this region and the general 

 occurrence of low areas along the courses of the buried river valleys. 

 Perhaps the most important one is the great depth to which these 

 old gorges had been eroded. During the process of filling these 

 valleys with glacial debris it may be that the amount of material 

 available was not always quite equal to the amount required to fill 

 them level with the tops of the banks at all points. This would 

 result in slight depressions at intervals along the courses of the 

 former streams. Another important factor is probably the settling 

 of this newly deposited and loosely consolidated material. This 

 settling would result from the saturation of the mass by ground 

 water or from the gradual packing of the filling lender its own 



