452 E. K. SOPER 



water was also observed in driving the tunnels through the sand 

 rock on the University campus for the new heating plant. In the 

 course of this work a fissure in the sandstone was encountered 

 through which a large stream of water issued and temporarily- 

 flooded the tunnel. With this indisputable evidence of the occur- 

 rence of cavities in the rock below the surface, it is entirely possible 

 that certain of the depressions in the limestone surface, as shown 

 by excavations through the drift, may be due to the slumping of 

 the limestone over similar cavities. 



IV. RELATIONS OF THE BURIED ROCK SURFACE TO THE EXISTING 



DRIFT SURFACE 



The major features of the present topography of the surface of 

 the ground throughout the city of Minneapolis are in general inde- 

 pendent of the underlying rock, and this is what one might expect 

 in a region so heavily glaciated. There are, however, certain rela- 

 tions between the present surface and the buried topography which 

 are suggestive and which, when understood, can be used to advan- 

 tage in interpreting the character of the buried surface. The 

 localities where there is an evident relation between the two sur- 

 faces are along the buried river channels. These old stream courses, 

 which have been filled with debris to a depth of 200 to 250 feet, may 

 still be roughly traced across the city by a series of depressions or 

 low areas in the drift, some of which are now the sites of lakes. 

 These depressions do not form a continuous and unbroken course 

 in the present drift, for occasionally the independence of the glacial 

 topography is manifested by abrupt morainal hills extending across 

 the chain of depressed areas. An inspection of the accompanying 

 map will show that nearly every natural lake within the city lies in 

 the course of some buried stream. Not only are these pre-glacial 

 channels indicated by a chain of lakes and shallow basins, but the 

 general depression of the drift over the old stream beds has, at 

 several places, resulted in determining the direction of the present 

 surface drainage. Bassett's Creek and Minnehaha Creek both flow, 

 for at least parts of their courses, directly over much larger drift- 

 filled channels (see Fig. i). Even the present channel of the Missis- 

 sippi River, north of the mouth of Basset's Creek, to the city limits 



