MUST.] RECESSION OK THE FALLS OF ST. ANTHONY. 311 



the Minnesota at some place above Fort Snelling. It is probable that 

 this old channel of the Mississippi was then occupied by waters th.it 

 drained the northern pari of the State, and had existed through many 

 ages,datingbacfctopre-Cretaceoustimes. [t seems to have remained open 



to the advent of the ice of the last glacial period, when niorainic accumu- 

 lations so choked it that the water of the river was driven out and com- 

 pelled to seek another passage to the Minnesota. When this last event 

 took place the Falls of St. Anthony probably began at Fort Snelling, 

 the water being precipitated over the rock bluff of the pre-existing old 

 gorge, unless, as was very likely the case, the whole valley was too 

 deeply buried under water. Whether this was at the beginning or at 

 the acme of cold, or at the recession of the ice. is a question to be con- 

 sidered, but at this time the only point that is claimed is that it was 

 not earlier than the beginning of the last glacial epoch, and was very 

 probably near the (dose of this epoch, for the following reason. It has 

 been before stated that the .Minnesota was the outlet of the glacial lake 

 Agassi/, and as such must have carried away enormous quantities of 

 water resulting from the rapid melting of the ice sheet toward the north. 

 There is good reason to believe that this swelled the liver to such a 

 height that the level of its waters rose as high as, if not higher than, 

 the limestone ledge at Port Snelling. If such was the case there could 

 have been no fall at that place. When Lake Agassi/, began to be 

 drained to the north, the Minnesota's volume of water was much 

 decreased and the water level fell below the limestone ledge. Then 

 the waters of the Mississippi began to be precipitated over the face of 

 this ledge, and then, and not until then, the recession of the Falls began. 

 If this reasoning is correct, the date of the beginning of the recession 

 of the Falls could not have been until near the (dose of the glacial epoch. 

 From calculations based on the recession from L680 to 1856 Prof. 

 Wiuchell obtained from three sources the rates of 4.79, 6.73, and 5.08 

 feet (1.46, 2,05, and 1.51 m.) per year, and the periods necessary for the 

 recession of the Falls from Fort Snelling (a distance of a trifle over 8 

 miles) (13 km.) 8,<S1!>, 6,276 and 8,315 years. The average of these 

 results is 7,803 years. 



