THE WORK OF STREAMS 



129 



which derived their material more or less directly from the glaciers 

 as well as from the rapid erosion of new gorges. The depression of 

 the land in many places, as in the Connecticut valley, reduced the 

 velocity of the streams, and occasionally ice jams of long duration 

 also caused deposition. The deposits thus built in valleys have 

 since been partly removed, thus causing the formation of terraces. 

 Discontinuity of Terraces. — The terraces on the two sides of a 

 valley do not necessarily agree in height. This is due to the fact 

 that, in swinging to \ 



and fro across its 

 valley, a stream not 

 only cuts laterally 

 but also at the same 

 time degrades its bed 

 (Fig. 119 A, B), 

 the flood plain often 

 being higher on one 

 side than on the 

 other. In the Brat- 

 tleboro, Vermont, 

 region, for example, 

 the stream appears 

 to deepen its valley 

 about 12 feet in 

 each swing. (E. H. 

 Fisher.) If a stream 

 meanders entirely 

 across its valley, it 

 will destroy its flood 

 plain, but if it fails 



toZ«^ 



3 



Fig. 119. — Block diagrams showing the origin of stream 

 terraces defended by rock ledges. The terraces H, K, A, 

 B, C, D, E, F, G, owe their preservation to the presence of 

 rock ledges which prevented the stream from cutting them 

 away as the valley was deepened. The relation of rock 

 to alluvium on the right of the block diagram is also 

 shown in figure B. (Modified after W. H. Davis.) 



to make a complete swing, a fragment will remain as a terrace. 

 When in its meanderings a stream encounters a rock ledge (Fig. 

 119) in its valley floor, the lateral cutting may be retarded to 

 such a degree that it will begin to swing to the opposite side of its 

 valley before completing its usual lateral movement. In this way a 

 portion of the flood plain will be preserved as a terrace. When other 

 rock ledges are encountered in its further swings across the valley 

 more terraces will be left, and the " meander belt " will be narrowed. 

 The theory of defending rock ledges affords a better explanation 

 than any other for many of the terraces of the New England valleys. 



