LARGER RELATIONS OF DELTAS o83 



under temporary delta conditions. A basin is a structural depression 

 surrounded by uplands, and if rivers maintain a land surface, notwith- 

 standing subsidence, then deltas do not result. ]f, however, a rapid sub- 

 sidence gives temporary lacustrine conditions the rivers woik to reclaim 

 the land, partly by supplying sediment to the lake bottoms, more largely 

 by building out deltas into the lake, but on the margins of the basin the 

 deposits differ in no respect from those accumulated under purely fiuvia- 

 tile conditions. If subsidence is progressive the rivers build upward, but 

 deltas exist only in so far as the rivers keep flowing into bodies of per- 

 manent water. 



From the shore front the river-built plains may extend landward many 

 hundred miles, as seen at present on the great delta plain of China. Li 

 the other direction may extend a still wider reach of sea. The determi- 

 nation of ancient delta conditions from the study of the strata requires, 

 therefore, the demonstration of evidence that both subaerial and sub- 

 aqueous sediments were deposited. The nature of each part may not, 

 however, be different from that of formations which were wliolly suIj- 

 aerial on the one hand or subaqueous on the other, so that a broad study 

 of the formations will commonly be necessary to prove the synchronous 

 and contiguovis development of both kinds of strata and hence the exist- 

 ence of deltas. 



Deltas indicate a somewhat balanced contest between the constructive 

 rivers and the opposing lake or sea, but the relative aspects are con- 

 tinually changing; deltas if now building Tapidly outward show by that 

 fact that but recently subsidence brought the sea inland. The problem 

 of the shifting conditions through time as recorded by successive strata 

 is therefore much more complicated than the study of the surface of the 

 existing stage. 



The factors in delta construction and destruction. — "Rivers excavate 

 tlM'ii- chauuels and undei-niine Ihcii- bniiks during times oT flood, nnd 

 allhough in this manner they act locally as degrading agents and con- 

 tribute to the sea rather than to the land, yet the delivery of this material 

 to the offshore sea bottom builds it up and weakens the effect of the 

 waves on the coast. Indirectly, therefore, erosive work of the channel 

 waters advances the work of delta-building. The overflow waters, on the 

 other hand, deposit a large proportion of their waste on the floodplain 

 and directly build it upward and outward. With the draining away of 

 the flood waters the work of aggradation is shifted to the channel, the 

 excess of sediment diminishing its cross-section and maintaining the 

 velocity of the current until the latter is able to carry through the balance 

 of its much diminished load. 



