RE VIE W 'S 797 



to different slopes, dependent on their relations to the glacial supply 

 of detritus thrown into them. As a rule, the gradient is much higher 

 near the ice edge than at a distance from it. These high gradients are 

 presumably the first to be reduced, while the material so derived is 

 shifted to the lower gradients which continue their aggradation until 

 the whole becomes adjusted to the new conditions. It must also be 

 considered whether the fresh drift surfaces left by the recent retreat 

 of the ice and the numerous new trenches of the young streams engaged 

 in developing the new drainage system may not have kept the Missis- 

 sippi in an aggrading, or at least static condition for a notable period 

 after the direct influence of the ice sheet was withdrawn. Any human 

 relics left on the plain during this stage should normally have been 

 subject to incorporation by scour-and-fill in the clean, fresh, stratified 

 gravel. But none are reported. 



3. After the stage of transition had passed and the Mississippi had 

 assumed the degradational phase, a period must probably be recog- 

 nized during which its shifting meanders occupied the whole of its 

 plain — except occasional protected embayments — and degraded it 

 from side to side, removing the whole surface of the previous glacio- 

 fluvial and transitional-adjustment plain. This action is dependent 

 on the balance of prevailing conditions, and these vary for different 

 rivers and different portions of the same river. To a large extent, the 

 Mississippi has continued action of this phase down to the present time 

 and has cut away the whole of the upper part of the glacio-fluvial plain. 

 It is only here and there in favored localities that any remnants that 

 can with probability be regarded as portions of the upper glacio-fluvial 

 plain can now be recognized at any great distance from the ice edge. 

 Just how long the river would continue to occupy by its shifting courses 

 the whole upper plain at Little Falls cannot be determined by any evi- 

 dence given in the paper, but the stage should be recognized in the 

 interpretation of the history of the region, and particularly in the 

 determination of the age of the surface deposits of the plain as it now 

 exists. Any human relics left on the plain in this stage would be liable 

 to be incorporated in the clean stratified gravel by scour-and-fill. But 

 none are reported. 



4. After the stages above noted had passed, the river developed a 

 more restricted track and limited its erosion essentially to this, sinking 

 its channel gradually into the broad plain and covering the remainder 

 only in flood time. It would continue to flood the upper plain until 

 the channel reached a depth greater than the height of the flood stages. 



