78 STUDIES FOR STUDENTS 



and in general its coarsest, deposits are likely to be made on 

 their borders where a low ridge or natural levee is built up. This 

 takes place while the stream is still overflowing its bluffs at high 

 stages. But these may remain on the bluff border after the 

 stream has cut its valley so deep as to cease this overflow. This, 

 of course, depends upon the chance whether the river encroaches 

 upon its bluffs at the given place or not. Such deposits and 

 their contents are to be referred normally to an advanced stage 

 of degradation of the river, and hence presumably some consid- 

 erable time after the glacial aggradation of the valley and the 

 sequent adjustments had ceased. The strong presumption is 

 against the reference of these deposits or their contents to a gla- 

 cial age. 



The second class are formed by wind action conditioned by 

 the bluff itself, and hence are subsequent to the formation of the 

 bluff. The sloping face of the bluff directs upward the wind 

 that blows against it from the valley, and this upward current 

 lifts the horizontal current at the level of the top of the bluff 

 and causes an eddy of relatively quiet air just back of the bluff 

 edge, in which sand, and such pebbles as the intensified wind 

 at the bluff edge may be able to roll back, to lodge and form a 

 low flat ridge. Mr. Knapp has observed many of these in New 

 Jersey and assigned to them their true cause, as it would seem. 

 Their relationship to the bluff edge and to the strong winds 

 leaves no serious ground to doubt their origin. As these are 

 later than the bluff and the bluff is later than the gravels in 

 which it is cut, and as these gravels may even be later than the 

 close of the ice invasion, there is no ground for referring relics 

 found in such deposits to the ice age. 



VI. DERIVATIVE FORMATIONS. 



These have been touched upon at several points already, but 

 as they are the special danger-ground of the unwary and are 

 liable to deceive the very elect, some further special notice 

 seems required. 



Inverted deposits. — The reversal of the order of deposition 

 in the process of bliff formation is a special instance. A river 



