208 GEOLOGY. 



plain. As the meander belt widens, the depression which it develops 

 becomes more and more capacious. Presently it may attain such 

 dimensions as to hold the water of ordinary floods. At this stage, or 

 even before, such parts of the earher flood-plain as remain, are terraces. 



These several tendencies conspire to partially destroy river flood- 

 plains, and to transform such parts as remain into terraces in the nor- 

 mal course of a river's history. They appear first in the lower part of 

 the valley, and migrate headward, following the course of nearly every 

 other phase of activity in a stream's history. The heads of the ter- 

 races follow, at a respectful distance, the head of the flood-plain, just 

 as the head of the flood-plain follows at a distance the head of the val- 

 ley. The second and subsequent flood-plains and the terraces to which 

 they give origin follow the same course. 



Terraces developed by the normal activities of a stream are always 

 low, and it is improbable that they would ordinarily be conspicuous. 

 The vertical distance between the first (highest) and second is greater 

 than that between the second and third. The principles developed 

 on page 65 et seq., in connection with the erosion of the hypothetical 

 island, are applicable here. 



Flood-plain terraces due to other causes. — Certain other causes, 

 accidental rather than normal to a stream, result in the development 

 of terraces from flood-plains. (1) If there be uplift in a region where 

 the rivers have flats, the streams are rejuvenated, and the remnants 

 of their former flood-plains become terraces. (2) If an alluvial flood- 

 plain has been built as the result of an excessive supply of sediment 

 (p. 186), the exhaustion or withdrawal of the excessive supply would 

 leave the stream again relatively clear, and free to erode where it had 

 been depositing. It would forthwith set to work to carry away the 

 material which it had temporarily unloaded on the plain. The plains 

 built up in many valleys in the northern part 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. A notable reduction in the 

 amount of available sediment, even when the earher supply was not 

 excessive, produces a similar result. (3) A notable increase in the 

 volume of a stream, without corresponding increase in load, as when 

 one stream captures another, may occasion the development of ter- 

 races by allowing the stream to deepen its channel. (4) Above any 



