416 GEOLOGY. 



appreciable advances, as for example that brought out by the demon- 

 stration of Taylor that the Belmore beach of southwestern Michigan 

 was formed by such an advance later than the Arkona beaches that 

 stand below it. Phenomena connected with the moraines themselves 

 imply advances in other cases. It cannot therefore be assumed con- 

 sistently that the retreat of the ice from its maximum Late Wisconsin 

 advance to its position at the time the Niagara gorge began to be 

 cut, was a rapid, uninterrupted one. Rather must it be assumed 

 that the agencies that made for advance closely matched, and occa- 

 sionally over-matched, the agencies that made for retreat. 



Before attempting to place a value upon the period so represented, 

 the time at which the gorge below St. Anthony Falls began to be cut 

 may well be considered a.so. From the normal methods of the glacial 

 streams of retiring ice-sheets, it is to be presumed that for a time sub- 

 sequent to the retreat of the ice-edge from the present location of 

 St. Anthony Falls, at Minneapolis, the outwash trains of the region 

 were being deposited, for the waters issuing from the edge of the ice, 

 so long as it lay on the southern slope, must apparently be presumed 

 to have been overburdened with glacial detritus which they were 

 throwing down along the courses of their channels to the southward. 

 Degradation may have taken place locally in the interest of a read- 

 justed gradient, but the general phenomenon must apparently have 

 been aggradation. This should have continued until the ice passed 

 beyond the northerly water-shed, or until the glacial waters, through 

 the agency of large lakes, were freed of their detritus. In direct sup- 

 port of this conception is the abundant evidence that the Mississippi 

 trench, as far down as the mouth of the Chippewa river, was filled 

 with glacial detritus to heights ranging from 100 to 120 feet or more 

 above the present river surface. Below the mouth of the Chippewa, 

 the glacial filling appears to have declined gradually to heights of 

 80, 70, 60, and 50 feet above the river, the last in the latitude of central 

 Illinois. Beyond this, satisfactory tracing of the terrace remnants has 

 not yet been made, but in the Mississippi valley below, there is a per- 

 sistent series of terraces ranging from 40 to 60 feet above the present 

 river, which have been tentatively regarded as the probable southern 

 representatives of this stage of aggradation. As- far down as Natchez, 

 these terraces are fully 50 feet in height, which seems to imply that 

 the glacial filling reached a graded condition about the middle latitude of 



