ALLUVIAL TERRACES 



119 



Most river terraces are remnants of former Hood-plains, below 

 \\hirh tin- streams which made them have cut their channels, hut 

 tin- (It-tails of tlu-ir history are various. 



Normal alluvial terraces. Alluvial terraces are developed in the 

 normal course of every stream's history, because the first graded 

 plain which a stream develops in its valley is above the level to which 

 the stream can cut at a later time. After the stream has sunk its 

 channel well below the former flood-plain, such parts of the latter 

 as -till remain are alluvial terraces. Where a stream's deepened 

 channel is in the middle of its flood-plain, there is a terrace on either 





Fig. 1 24. Terraces on the Eraser River at Lilloet, B. C. (Photo, by Calvin.) 



side; but wherever the deepened channel is at one margin of its 

 flood-plain, a terrace remains on the other side only. In some 

 valleys there are several alluvial terraces at different levels. The 

 second terrace (regarding the highest as the first) is developed in 

 tin- sann- way as the first, for after the stream has developed a second 

 flood-plain, below the level of the first, it may cut its channel still 

 lowt-r, leaving the remnants of the second flood-plain as terraces. 

 This process may continue until several sets of terraces have 

 been developed. Alluvial terraces developed by the normal activi- 

 ties of a stream are always low, and ordinarily would not be 

 conspicuous. They are not very long-lived, for all processes of sub- 

 aerial erosion conspire to destroy them. A stream is likely to mean- 



