LARGER RELATIONS OF DELTAS 401 



merely shift the topset beds farther seaward and, owing to tlie rapid 

 movement of unconsolidated material, produce possibly a lens of terres- 

 trial deposit when previously the sedimentation of that zone had been 

 wholly marine. Oscillation, therefore, will shift the zone of maxinuiin 

 deposition back and forth. Slow subsidence favors great dei)t]is and 

 volume of topset beds, a considerable percentage of which is, however, 

 marine, although the absolute amount of terrestrial deposits is at the 

 same time greatly increased. Small uplifts favor great volumes of fore- 

 set beds and tend to shift farther seaward the zone of terrestrial sedi- 

 mentation. 



Imperfect application to present conditions — Effects of recent crustal 

 movements. — A crustal uplift which interrupts an erosion cycle and 

 initiates a new baselevel brings about a destruction of the former base- 

 level forms, but only in a time interval comparable to that which devel- 

 oped them. A long cycle, for example, reduces hard rocks to a peneplain, 

 remnants of which persist for another long period of time. This per- 

 sistence of topographic forms permits the study of the physiographic 

 history of the upper parts of the river systems backward through several 

 erosion cycles of increasing time intervals. It is not true to the same 

 degree with delta growth. Such structures are built against the sea and 

 feel immediately the effects of crust movements. They consist of uncon- 

 solidated materials and rapidly suffer from erosion or burial under 

 changed conditions due to crust movements. The subcycles which inter- 

 rupt the orderly progress of surface processes are thus peculiarly niagni- 

 fied in the case of deltas and the sequence of successive stages can best 

 be studied in the upturned and partly eroded older formations rather 

 than in the delta which still lies beneath our feet. 



At the present time, as a result of the great Pleistocene uplifts, fol- 

 lowed by the TJecent oscillations and ]iartial subsidence wljich mark the 

 later Cenozoic as a [xM'iod of world-wide ciMisial unrest, I he rivers are out 

 of adjustment with the present sealevel. The streams are intrenched in 

 inner rock gorges, most piedmont alluvial slopes have become dissected, 

 river valleys have been turned into estuaries, lakes have arisen in in- 

 land basins, many deltas have been partly drowned, and, as seen espe- 

 cially around the shores of Asia, the rivers are just beginning to again 

 reclaim their subaerial plains from the last submergence. Certain ones, 

 however, like those of the Ganges, Indus, and Nile, have maintained their 

 fronts at the limits set by the deep water of the ocean. Since the early 

 Tertiary, furthermore, the interior shallow seas which in the earlier geo- 

 logical ages were the favored regions for delta growth have been very 

 largely excluded from the warped and uplifted continents. For all these 



