THE WORK OF RUNNING WATER. 209 



barrier which dams a stream, a flood-plain is hkely to be developed. 

 When the barrier is removed the stream will cut more or less deeply 

 into the plain above, leaving terraces. (5) The recession of a falls 

 through a flood-plain converts such parts of it as remain, into terraces. 



In conclusion, it may be stated that many river terraces, mostly 

 very low, are normal features of valley development, coming into exist- 

 ence at definite stages in a valley's history. They are generally com- 

 posed, in large part, of river alluvium. Others result from more or less 

 accidental causes, working singly or in conjunction, and to this class 

 belong all of the more conspicuous terraces developed from flood-plains. 

 The structure of a terrace often affords some clue to its origin (Fig. 196). 



ym?/M7T?7,^r,=,,rrf77m^^ 



Fig. 196. — Terraces partly of rock and partly of alluvium. Such terraces indicate 

 successive uplifts, or some other change which had a similar effect on the stream 

 which made the valley. 



Discontinuity of terraces. — When a stream sinks its channel into 

 its flood-plain, it does not follow that a terrace remains on each side. 

 Where the stream's deepened channel is in the middle of its flood- 

 plain, there is, temporarily, a terrace on either side; but wherever 

 the deepened channel is at one margin of its flood-plain, a terrace remains 

 on the other side only. Even where continuous at the outset, terraces 

 soon become discontinuous, for all processes of subaerial erosion con- 

 spire to destroy them. A stream is likely to meander on its second 

 and later flood-plains, as on its first and highest one. Wherever the 

 meanders on its second flood-plain reach the borders of the first flood- 

 plain, the terrace at that point disappears, and since the meanders 

 are continually migrating, terraces are continually disappearing. The 

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

 The removal of portions of a terrace by the sweep of meanders is likely 

 to leave the remnants cuspate toward the stream.^ Again, tributary 

 streams, in bringing their channels into topographic adjustment with 

 their mains, cut through the terraces of the latter. New gullies develop 

 on the faces of the terraces and their heads work back across them, 

 dissecting them stiU further. At the same time, sheet erosion and 

 other phases of slope wash tend to drive the scarps of the terraces back 

 1 Davis, Bull. Mus. Comp. Zool., Geol. Ser., Vol. V. 



