130 



PHYSICAL GEOLOGY 



Characteristics of River Deposits. — A cross section through a 

 river deposit does not show a homogeneous deposit of stratified sedi- 

 ment, but rather lens-shaped masses of coarse sands and gravels at 

 different levels, buried in stratified sands and clays (Fig. 107). When 

 traced up or down the valley, these deposits are found to lie in long 

 and comparatively narrow belts. They represent the former channels 

 of the aggrading river, where the current was strong enough to remove 

 all but the coarser material of its load. The finer deposits — the 

 mud and fine sand — were laid down in the more sluggish water on 

 either side of the channel and on the flood plain. Beds of muck, 

 marking the sites of shallow lakes and swamps, are also common. 



Deltas 



Deltas are formed where streams enter either lakes or seas. If 

 the body of water into which the river flows is large, all of the sedi- 

 ment carried in by the stream is dropped, and the bottom is gradually 



built up at the river's mouth. 

 Since sediment settles much 

 more quickly in salt than in 

 fresh water, it is dropped 

 more quickly in the ocean. 

 Because of the low gradient, 

 a river often splits into sev- 

 eral channels (Fig. 120) as it 

 enters its delta, the branches 

 being known as distributaries. 

 The shape of a delta, as 

 the name implies, is usually 

 that of the Greek letter of 

 that name, with one angle of the triangle pointing upstream. 



Growth of Deltas. — The rate of growth of a delta depends upon 

 (1) the amount of sediment carried by the river, (2) the depth of the 

 sea or lake, (3) the strength of the waves or currents, and (4) the 

 stability of the bottom of the sea or lake where the deposition is tak- 

 ing place. Deltas are apt to be largest in seas in which the tide is 

 weak, since under such conditions practically all of the sediment is 

 dropped soon after it reaches still water. When the ocean bottom at 

 the mouths of rivers is subsiding, the upbuilding of the bottom may 

 be insufficient to compensate for the subsidence. The Mississippi 



Fig. 120. — The delta of the Mississippi River. 



