120 WORK OF RUNNING WATER 



der on its second and later flood-plains, as on its first and highest one. 

 Wherever the meanders on its second flood-plain undercut the first 

 terrace, the terrace at that point is subject to destruction, and since 

 the meanders are continually migrating, terraces are continually 

 disappearing. Again, tributary streams cut through the terraces 

 of their mains, and new gullies develop in 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 set of terraces is well developed, no 

 more than meagre remnants of the first may remain. 1 



Other river terraces. There are valley terraces which do not 

 represent necessary stages in a valley's history, (i) Some are due 

 to inequalities of hardness (Fig. 80). (2) Again, if an alluvial flood- 

 plain has been built as the result of an excessive supply of sedi- 

 ment (p. 112), the exhaustion or withdrawal of the excessive sup- 

 ply would leave the stream relatively clear, and free to erode where 

 it had been depositing. It would forthwith set to work to carry 

 away the material which it had temporarily unloaded on the plain. 

 The valley plains built up in many valleys in the northern part 

 of our continent during the glacial period, when drainage from 

 the ice flowed through them, have been partially destroyed since 

 and their remnants are terraces. (3) A notable increase in the 

 volume of a stream, without corresponding increase in load, as 

 when one stream captures another, may occasion the develop- 

 ment of terraces by allowing the enlarged stream to deepen its chan- 

 nel. (4) The uplift of a region in which there are well developed 

 river flats, would rejuvenate the streams, and parts of their old 

 flood-plains would be left as terraces. Other occasional causes which 

 need not be mentioned here, develop terraces from flood plains. 



In conclusion, it is to be emphasized that many river terraces, 

 mostly very low, are normal features of valley development, coming 

 into existence at definite stages in a valley's history. They are 

 generally composed, in large part, of river alluvium. Others result 

 from more or less accidental causes, working singly or in conjunc- 

 tion, and to this class belong many of the more conspicuous terraces 

 developed from flood-plains. 



Laboratory work. See excercises III-IX, in laboratory manual Interpretation 

 of Topographic Maps; also Professional Paper 60. U. S. G. S. Pis. XXIII-LXXXIX. 



1 For a fuller statement of the manner in which alluvial terraces are devel- 

 oped, see the authors' Geologic Processes. 



