TERRACES 1 39 



of the flood plain, and thus the surface of the plain slopes away 

 from the river. Along the lower Mississippi this slope is as much 

 as seven feet for the first mile. The vegetation, grass, bushes, and 

 trees, which grows over the flood plain, acts as a sieve and catches 

 the sediment, so that little escapes. Lands thus annually renewed 

 by a deposit of river mud are of wonderful fertility, and, like Egypt, 

 for example, remain so after thousands of years of cultivation. In 

 this manner the flood plain is slowly raised, while, at the same time, 

 the channel is deepened, and by the river's double activity of 

 scour and deposition, certain very characteristic features result. 



River Terraces and Old Gravels. — The lower courses of many 

 rivers, including most of those in the northern United States, and 

 some in the southern, are bordered by a succession of terraces that 

 rise symmetrically on the two sides of the stream. Sometimes, as 

 in many English rivers, the terraces are at different levels on oppo- 

 site sides. The formation of these terraces is due to the twofold 

 activity of the river already described. The combined deepening 

 of the channel and building up of the flood plain at length make the 

 trough of the river so deep that floods no longer fill it, especially if 

 the velocity of the current be maintained or increased by an ele- 

 vation of the region drained by the river. Then the energy of the 

 current is partly employed in widening the channel and forming a 

 new flood plain, cutting back the edges of the old flood plain, which 

 it can no longer overflow, thus converting it into a terrace, which is 

 the remnant of an old flood plain. The process may be repeated 

 many times, and thus successive terraces rise, one above another, 

 as we recede from the river. 



It necessarily follows from this account that the highest terrace 

 is the oldest, and the lowest is the last formed. This seems to be 

 a violation of the rule that, in any series of sedimentary deposits, 

 the oldest must be at the bottom and the newest at the top ; but 

 the violation is only apparent, not real. Were the river to flow at 

 a constant level, no terraces could be formed, and the deposits 

 would follow the rule, just as they do now in each successive flood 

 plain and terrace. Because, however, the stream flows at suc- 

 cessively lower levels, the lower flood plain is made up of the 



