210 GEOLOGY. 



toward the bluff beyond. By the time a second series of terraces is 

 well developed, no more than meagre remnants of the first may remain. 



From the foregoing considerations it is clear that the extent to 

 which river terraces once developed, now remain, is dependent in part 

 on the length of time which has elapsed since the river sank its channel 

 below them. Other things being equal, the greater their age the more 

 meagre their remnants. 



Terraces developed from river plains formed chiefly by alluvia- 

 tion stand a better chance of long life than most other alluvial ter- 

 races. This is because of the configuration of the original valley, the 

 aggradation of which gave origin to the plain. The principle involved 

 is illustrated by Fig. 197. In developing the second flood-plain the 



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Fig. 197. — Diagram to show why certain terraces are longer lived than others. 



river encounters the rock wall of the valley. This greatly retards 

 lateral erosion, and the terrace above, defended^ by the rock, is likely 

 to be long-Hved. 



Alluvial terraces, like rock shelves, are popularly thought to mark 

 ^'old levels of the river. '^ In one sense this is true, but not in the sense 

 in which the expression is commonly used. Every level, from the 

 crest of the bounding bluffs to the bottom of a valley, is a level at which 

 water ran for a longer or shorter time; but the terrace does not mean 

 that the river was once so much larger than now as to fill the valley 

 from its present channel to the level of the terraces. 



Termini of terraces. — From the mode of development of terraces 

 it will be seen that, traced up-stream, each terrace should theoretic- 

 ally grade into a flood-plain at its upper end (Fig. 194), and that the 

 upper end of the second (from the top) terrace, where there are two, 

 would not be so far up-stream as the upper end of the first (highest). 

 This is represented diagrammatically in Fig. 198. 



^ This point has recently been emphasized by Davis, loc. cit., pp. 282-346. 



