GEOLOGICAL HISTORY OF THE EARTH 47 



extends from St. Paul north to Lake Itasca, and is 

 broader than the watershed of the Minnesota ; and hence 

 the Minnesota changed from much the larger river to 

 the smaller one. 



The recession of the Falls of St. Anthony. The waters 

 of the Mississippi now had to fall over a precipice, as 

 shown at D, Figure 12, to get down to the deep bed of 

 the Minnesota River, where it had previously flowed 

 directly out into the waters which rilled the original 



Figure 10. The area inclosed by the line thus ........ is the area formerly 

 drained by the Minnesota river. The area at present drained by the same river is 



surrounded by the solid line, thus . The area then, as now, drained by the 



Mississippi river above where the two rivers flow together la inclosed by a broken 

 line, thus -------- 



banks of the Minnesota, as shown in B, Figure n. 

 At this time and in this way the Falls of St. Anthony 

 were formed. The water fell over a ledge of limestone 

 rock which is underlaid with a very thick stratum of 

 loosely cemented sandstone, that is easily worn away 

 by the falling water. The waterfall thus gradually 

 undermined the overlying limestone, which broke off 

 in large masses and was washed away. Thus the falls 

 had receded northwestward about six miles to within a 



