THE PLEISTOCENE OR GLACIAL PERIOD. 417 



Illinois, and thence to the Gulf took on a gradient comparable to that 

 of the existing flood-plain of the Mississippi. 



When therefore the glacial aggradation ceased, it was first necessary 

 to clear out the Mississippi trench and lower the river before effective 

 cutting of the gorge below St. Anthony Falls could begin. The waters 

 of Lake Agassiz appear to have been an effective factor in this clearing 

 out, for, on account of the extent of the lake, the detritus of the 

 streams emptying into it from the ice was effectually deposited, 

 and the waters issuing from the lake were clear and capable of taking 

 up and rolling on the gravel and sand that filled the great trench. It 

 would appear from the configuration of the Minnesota valley, that by 

 the time Lake Agassiz ceased to discharge through the Minnesota 

 River, the filling of that river and of the upper Mississippi had been 

 cleared away to such a depth as to give the upper Mississippi an effective 

 fall for cutting the gorge below St. Anthony Falls. Perhaps the cutting 

 might have been gradually initiated somewhat before, but the time- 

 rate of the recent falls could not be properly applied to it until after 

 the full height of the fall was attained. The position of the ice-border 

 at the stage at which Lake Agassiz ceased to flow through the Minne- 

 sota river is not yet known, but it had retreated far enough to permit 

 the lake waters to escape by some northerly route. Lender any proba- 

 ble hypothesis, this implies a retreat of the ice-edge some 700 to 800 

 miles from its extreme extension at Des Moines, a distance appre- 

 ciably greater than that requisite for initiating the Niagara gorge- 

 cutting. 



Glacialists vary much in their estimates of the average rate of 

 retreat of the ice-border under such conditions. This retreat is of 

 course not measured by the rate of melting of the ice alone, but by 

 the difference between the rate of melting and the rate of advance of 

 the ice, and it is not to be forgotten that the evidence indicates that 

 the latter was at times superior to the former. If, however, to de- 

 velop a definite conception, and to aid every one in forming his own 

 judgment as to the probabilities of the case, we assume that there 

 were 200 days of effective melting in each year (which each will in- 

 crease or diminish according to his judgment), and if we allow that the. 

 melting was sufficiently superior to the onward movement of the ice 

 to cause the ice-edge to retreat one foot per day (which each again 

 will modify to meet his judgment), and if no advance was made during 



