CRITERIA REQUISITE TO A GLACIAL AGE 8 1 



It should be noted in passing that these are not true alluvial 

 deposits in the typical cases, though they may grade into allu- 

 vial deposits and be interstratified with them. They are not 

 habitually well assorted and stratified. They are not formed 

 by persistent streams, but are the products of periodic surface 

 wash and are heterogeneous, unassorted, and, in a sense, struc- 

 tureless, and hence again their liability to be mistaken for one 

 or another of the unassorted drifts. 



Among the favoring conditions for such recomposition 

 deposits, there is one which is systematic and recurrent, and is 

 peculiarly adapted to occasion the burial of relics, since it causes 

 a succession of alternate erosions and depositions at localities 

 specially liable to be frequented by primitive man. It is the effect 

 of the meandering of a principal river on erosion and deposition 

 in the mouths of its tributaries. 



The meandering of a principal river as a cause of alternate erosio?i 

 and deposition in the mouths of its tributary valleys. — A meandering 

 river with a deep, readily shifted bottom-filling of the glacial 

 type imposes upon its tributary valleys alternate stages of exca- 

 vation and filling. These result (i) from the action of the 

 aggressive bends of the river loops against the mouths of the 

 tributaries, and (2) the replacement of these, after a time, by 

 the flood-plain peninsulas that lie within the loops, more spe- 

 cifically, it is the alternate cutting of the stream itself, working 

 hard against and under the mouth of the tributary valley, fol- 

 lowed by the building up of the river's higher flood-plain across 

 the mouth of the valley. The first causes the waters of the 

 adjusted tributary to erode ; the second, to make deposits in the 

 mouth of the tributary ; for in the first stage the axis of the 

 tributary opens out on the river itself, which may be twenty or 

 thirty feet or more lower than the upper flood-plain, and hence 

 the tributary then has its lowest and best opportunity to dis- 

 charge its waters and their detrital burden. Besides this, the 

 river itself, while in this aggressive attitude, sweeps into the 

 mouth of the tributary in its flood stages and aids in its excava- 

 tion, and the rushing by of the river's strong current drags out 

 by friction, on the principle of draft, the waters of the tribu- 



