194 GEOLOGY 



point may be destroyed, and since the meanders are continually 

 migrating, terraces are continually disappearing. The same 

 would be true of the second terrace, if a second is developed. 

 Again, tributary streams cut through the terraces of their mains. 

 New gullies develop on the faces of the terraces, and their heads 

 work back across them, dissecting them still further. At the same 

 time, sheet erosion and other phases of slope wash tend to drive the 

 scarps of the terraces back 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 present 

 extent of river terraces once developed, 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. 



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. 



Laboratory work. The study of topographic and geologic maps, :md 

 of photographs, slides, etc., illustrating river work should be taken up in 

 connection with this chapter. Numerous sheets of the topographic maps 

 and several of the folios published by the United States Geological Survey 

 afford available illustrations of river work. Plates XXIII I A XX IX of 

 Professional Paper 60 of the United States Geological Survey also afford 

 good illustrations. This work should not be left until the chapter has been 

 completed, but may well be sub-divided. Illustrations of stream erosion under 

 varying conditions and at various stages of development; the topographic 

 effect of unequal hardness; structural adjustment and piracy; effects of <!< 

 of level; cycles of erosion; alluviation, etc., may best be taken up in connection 

 with the discussion of these several topics. 



