182 



PHYSICAL GEOLOGY 



The materials and structures of flood plains. - As already 

 implied, the materials of flood plains range from coarse gravel 

 to finest mud. The coarser material deposited by a river is 



confined in general to the 

 vicinity of the channel, 

 where the velocity of the 

 overflow is checked first 

 and most. This grades 

 more or less irregularly 

 into the fine muds which 

 gather in the quiet back- 

 waters. When the river 

 changes its position on 

 its valley floor, the coarser 

 deposits along the new 

 channel cover finer de- 

 posits made at a distance 

 from the old channel, 

 whose coarser material is 

 in turn buried with fine. 

 Frequent changes in the 

 position of the aggrading 



FIG. 190. Map showing changes in the 

 course of a portion of the Missouri 

 River. 



river result in many ver- 

 tical alternations in 

 coarseness among its sed- 

 iments. Minor variations 

 may be brought about by the unequal strength of the 

 overflow, capable of moving particles of varying size to a 

 given place at different times. Further complexity in the 

 distribution of the materials of a flood plain is introduced by 

 irregular contributions made by wash from the bluffs and 

 by tributary streams. Figure 185 shows the general structure 

 of stream-laid beds. 



The structure described above has made it possible to 

 determine that certain ancient formations were laid down 

 on the land by rivers, and not in lakes or the sea. 



