THE LIFE HISTORIES OF RIVERS 167 



ture of money and no small degree of engineering skill and ex- 

 perience to construct. So important to the life of the nation is 

 the proper management of its dikes, that in the past history of 

 China each weak administration has been marked by the develop- 

 ment of graft in this important department and by floods which 

 have destroyed the lives of hundreds of thousands of people. 



Wherever there has been a markedly rapid sinking upon a 

 delta region, and depressions are common in delta territory, no 

 doubt as a result of the loading down 

 of the crust, the river may present the 

 paradoxical condition of flowing at a 

 higher level than the surrounding coun- 

 try. Between the levees of neighboring 

 distributaries there are peculiar saucer- 

 shaped depressions of the country which 

 easily become filled with water. At the 

 extremity of the delta the levee may be 

 the only land which shows above the FlG - 1 73. "Bird-foot" delta 



* i ,-, i of the Mississippi River. 



ocean surface, and so present the pecul- 

 iar " bird-foot " outline which is characteristic of the extremity 

 of the Mississippi delta, though other processes than the mere 

 sinking of the deposits may contribute to this result (Fig. 173). 



The sections of delta deposits. If now we leave the plan of 

 the delta to consider the section of its deposits, we find them so 

 characteristic as to be easily recognized. Considered broadly, 

 the delta advances seaward after the manner of a railroad embank- 

 ment which is being carried across a lake. Though the greater 

 portion of the deposit is unloaded upon a steep slope at the front, 

 a smaller amount of material is dropped along the way, and a 

 layer of extremely fine material settles in advance as the water 

 clears of its finely suspended particles (Fig. 174). Simultaneous 

 deposits within a delta thus comprise a nearly horizontal layer 

 of coarser materials, the so-called top-set bed; the bulk of the 

 deposit in a forward sloping layer, the so-called fore-set bed; 

 and a thin film of clay which is extended far in advance, the 

 bottom-set bed (Fig. 174, 2). If at any point a vertical section is 

 made through the deposits, beds deposited in different periods 

 are encountered; the oldest at the bottom in a horizontal posi- 

 tion, the next younger above them and with forward dip, and the 



