184 



PHYSICAL GEOLOGY 



turn swing, a terrace would result, as at E-F-A. Many 

 terraces at successively lower levels might result from a 

 continuation of this process. These terraces might extend 



a considerable dis- 

 tance along the val- 

 ley, or only a short 

 distance, and their 

 width might vary 

 notably. It is evi- 

 dent that when 

 formed in this way, 

 terraces upon oppo- 

 site sides of a valley 

 will not correspond in 

 elevation. Small ter- 

 races are common 

 even in young val- 

 leys, where they are 

 due in many cases to 

 the fact that, as the 

 streams degraded, 

 they also shifted their 

 positions laterally. 



Terraces may be destroyed wholly or in part by the widen- 

 ing of the flood plain at a lower level. Indeed, since the 

 goal of stream-borne waste is the sea, the depositional fea- 

 tures discussed in the preceding 

 paragraphs may all be regarded 

 as composed of material which 

 has been dropped only tempo- 

 rarily by overloaded streams, 

 and which sooner or later will 

 resume its journey to the ocean. 

 Many cities are located partly 

 or wholly upon the terraces of 

 great rivers. Peoria, Illinois 



FIG. 193. Delta of the Mississippi River. 



FIG. 194. Delta of the Nile 

 River. The dotted area is 

 desert. 



