192 GEOLOGY 



of our continent during the glacial period, when the drainage from 

 the ice coursed through them, have subsequently been partially 

 destroyed by erosion, and their remnants have become terraces. 

 (3) A notable increase in the volume of a stream, without corre- 

 sponding increase in load, as when one stream captures another, 

 may occasion the development of terraces by allowing the enlarged 

 stream to deepen its channel. (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, may give rise 

 to terraces. 



Apart from the terraces due to the more or less Occasional 

 causes mentioned above, terraces are developed in the normal 

 course of every stream's history. The causes are as follows: 



(1) The head of the valley-plain where the first notable depo- 

 sition 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. 



(2) Again, so long as a stream is actively eroding at its head, 

 there is likely to be some aggradation below. At a later stage in 

 the stream's history, when erosion at the head has become less Be- 

 cause of the reduction of the surface, less material will be carried 

 from the upper part of the valley, and the stream on the flood- 

 plain below, formerly loaded with material from up the valley, is 

 now free to take up and carry away material temporarily left on 

 the flood-plain. The result is a deepening of the channel. 



(3) 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 is available. For a relatively small anumi 

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

 taken up, both because fine sediment takes less of a stream's 

 energy, pound for pound, than coarse does, and because mon 



a stream's energy may be utilized in carrying fine material. Other 

 things being equal, it follows that when a stream drops co 

 material to take fine, its channel is degraded unless there is at the 

 same time a great reduction in the stream's energy. Such reduction 



