FLOOD PLAIN DEPOSITS 589 



Flood plains of streams of less velocity are made up of fine 

 materials left by the river when it overflows its banks. As most of 

 the deposit is formed near the river the banks on either side are 

 higher than the surface of the plain away from it, there being thus 

 a gentle slope away from the river. In this way the natural levees 

 of the Mississippi are built, which rise considerably above the level 

 of the "back swamps" on either side. Here the slope away from 

 the river east or west is 5 or 10 feet to the mile, while the general 

 southward slope of the entire flood plain is under half a foot to 

 the mile. 



A striking example of a flood plain is afforded by that of the 

 Nile, which flows from a well-watered region through a desert 

 country without receiving a tributary for a thousand miles, except 

 a few small wet weather streams. Entrenched beneath the desert 

 uplands this flood plain holds its own for a length of 500 miles, 

 and maintains a width of from 5 to 15 miles, broadening on the 

 delta to over 100 miles. The annual inundation of the flopd plain 

 is caused by the northward movement of the belt of equatorial rains 

 in summer. The flood Ijegins in June and usually rises 25 feet or 

 more at Cairo in the late summer or early autumn. The annual 

 addition of the river silt causes a slow rising of the entire flood 

 plain estimated to amount to 4^ inches a century. 



This region furnishes an instructive example of widely varying 

 contemporaneous deposits within the same general area. On the 

 one hand occur the drifting, cross-bedded, well rounded and pure 

 quartz sands of the desert, and^ on the other, the extremely fine, 

 well-stratified muds of the river flood plain. Both enclose the re- 

 mains of organisms or of structures built by man, but there is an es- 

 sential difference between the remains found in each deposit. Aside 

 from ruins of human habitations and other works of man, only 

 occasional remains of animals which were adapted to a dry climate 

 are preserved in the desert sands. These, on the whole, will be 

 rare because burial is often a slow process, and the bones of the 

 dead creatures will crumble unless quickly covered. On the flood 

 plain, on the other hand, aquatic organisms abound, giving a 

 totally different faunal as well as floral association. According to 

 the rash doctrine of the persistence of lithologic units, recently 

 advocated by some geologists, such contemporaneous deposits would 

 be interpreted as of different ages. 



From the nature of deposits on river flood plains, perfect and 

 often very fine stratification is to be expected. This may be con- 

 sidered as characteristic of typical flood plains. "The plains of the 

 Po arid of the Ganges and the great fan of the Huang-ho are very 



