206 GEOLOGY. 



The amount which a river channel must be deepened in order to 

 change the remnants of its flood-plain to terraces cannot be definitely 

 stated. When a channel is so deep that the remnants of a former 

 flood-plain are no longer flooded, they would be called terraces, espe- 

 cially if a lower flood-plain has been developed. Even though not 

 above the reach of floods, they are often called terraces if they are notably 

 above the channel and separated from it by a lower plain. Thus the> 



Fig. 194. — Diagram illustrating the beginning of the development of a terrace from 



a flood-plain. 



flat at h, Fig. 193, would be called a terrace, even though covered by 

 water in exceptional floods; but the flat at c, but slightly above the 

 channel, would hardly be called a terrace. 



The question now arises why a stream, having once developed a 

 flood-plain, should sink its channel to a lower level, leaving parts of 

 the old flood-plain as terraces. This may be brought about by the 

 operation of various causes. 



(1) In the first place, the head of the vafley-plain where the first 

 notable deposition takes place normally advances up-stream. After 

 the advance has been considerable, the descending stream may, on 

 reaching the head of its valley-plain, lose so much of its load as to be 

 able to sink its channel into the flood-plain farther down the valley 

 (Fig. 194). 



(2) Ordinarily a stream does not drop all its load at the head of 

 its plain, but only its excess; but it will always drop coarse sediment 

 to take fine, if fine be available. For a relatively small amount of 

 coarse material dropped, a relatively large amount of fine may be taken 

 up (p. 179). Other things being equal, it follows that when a stream 



