GEOLOGICAL HISTORY OF THE EARTH 



49 



government put in a solid wall of masonry and an apron 

 to preserve the falls, not in their picturesque form, but 

 so as to conserve the water power. (See E in Figure 13.) 



Figure 13. The Mississippi river. A, Falls of St. Anthony. B, dam below the 

 falls. C, the Minnesota river at the confluence of the two rivers. D, apron to prevent 

 the falls from further receding. E, retaining wall above the falls. K, the present city 

 of Minneapolis. M, the group of great flouring mills. O, river below dam. Z, drift 

 above the limestone. X, limestone. Y, sandstone. 



In 1896 a dam was built in the rapids a short distance 

 below the falls, for added power to be obtained. (See 

 B, Figure 13.) As the Mississippi river descends quite 



rapidly between the 

 great falls and the con- 

 fluence of the two 

 rivers, there is room 

 between the high bluffs 

 for other dams which 

 are being erected. 



In Figure 12 is a 

 diagram showing the 

 recession of the Falls 

 of St. Anthony very 

 soon after the recession 

 began. In Figure 13 

 is a diagram showing 

 the falls at the present 

 time, also the mills for 

 which they furnish 

 power. Minnehaha 



Figure 14. A diagrammatic map showing how the 

 water in flood times, during the glacial period, flowed 

 Hiound the higher land at X, X, following the 

 flood course of F, F, F, and entered through a 

 branch stream into the river at K below the 

 present gorge from G to G. No doubt there was 

 once a falls that gradually receded from G to G. 

 as the water pouring over the limestone layer of 

 rock disintegrated the soft sandstone underneath 

 it and undermined the thin limestone. At the 

 confluence of the two flood streams a great gravel 

 bed was formed at O from the materials washed 

 out through the floodway at F-F. Another gravel 

 bed waa formed, possibly at a later date, at P. 



