348 STUDIES FOR STUDENTS 



is shown by the' section (Fig. i) plotted from the bathymetric con- 

 tours shown in Andree's Hand atlas. 



Turning to deUas which show a greater or less frontal submergence, 

 the submerged portions are observed to lie either within or without 

 the outer limit of the general coast-line, as determined by the off- 

 shore reefs. Such important shallow submarine delta platforms lie 

 in front of and completely beyond the limits of the land in the case of 

 the great Chinese rivers and the Amazon. In these instances the 

 development of the submerged delta surface may be due in part to 

 the fineness of the river deposit, allowing the sediment to be largely 

 flooded out to sea in the fresh river water, as it spreads over the salt 

 water below. On the other hand, the waves, tides, and currents are 

 important and aid in cutting back the land surface by marine plana- 

 tion, the land material being swept by the undertow to seaward into 

 the gradually deepening water. Thus in any particular case a cer- 

 tain equilibrium tends to be maintained between the continental and 

 marine portions of the delta surface — an equilibrium which is espe- 

 cially liable, however, to be temporarily destroyed by irregular move- 

 ments of subsidence or upheaval, since a small vertical movement 

 will transfer the beach-line a long horizontal distance across the 

 almost level surface. 



Ejject of Variable Point of Discharge. — In the above instances 

 the delta surface is sharply divided into a land and water portion, 

 and the littoral zone is at a minimum. 



In another class of cases, iUustrated by the Mississippi delta and 

 the combined deltas forming the Netherlands, the deltas inclose within 

 their outer land limits considerable bodies of shallow water or brackish 

 lagoons, as the Mississippi and Chandeleur Sounds, and the Zuider 

 Zee. In these cases it is observed that a large amount of sediment 

 is discharged at one or two separated points over an extended delta 

 coast-line, building out the delta at those places. The waves, drift- 

 ing the material laterally, throw up barrier beaches, and shut off more 

 or less completely large bodies of water, which thus lie within the front 

 limits of the land delta. Usually it is only a matter of a few centuries 

 at the most until the river abandons its built-out mouths and turns 

 into the intermediate lagoons, as the Mississippi has been known to 

 do near New Orleans, breaking through its levees in times of flood 



