LARGER RELATIONS Ol-' DELTAS 



aoo 



of those geologic pcM-iods with their iiiarine ijitercalatioiis and offsliore 

 equivalents. 



Result of a niorement of suhsidmce. — Movements of subsidence, per- 

 haps attended farther inland by elevation, rather commonly, however, 

 rejuvenate the delta cycle before it has passed beyond the stage of ma- 

 turity, causing new beds to be built on top of the old and burying the 

 older part below the reach of surface agencies. Where the delta surface 

 is very large and the movements of subsidence are intermittent but pi-o- 

 gressive, each downward movement will bring in the sea, each pause will 

 witness it crowded back. The result on the whole will be an upbuilding 

 rather than an outbuilding of the delta. Figure 4 shows in such a case 

 how dominant the topset beds become, constituting the greater part of 

 the delta volume, whereas in outward growth with stationary water Icxcl 

 the foreset beds because of their steeper slope are volumetrically of more 

 importance, sharply distinguishing the two modes of delta-building. 

 The great depth of fresh-water beds and old soils in the greater deltas 



ly 5ubaerial topset beds 



XVol Subaqueous topset beds 



3 Foreset beds 

 3 Bottomset beds 



Vertical scale magnified several hundred times 



Figure 4. — Relations 'between Mode of Delta-building and Subsidence 

 First stage. — Delta built out into water of constant level ; basin deeper than wave 



base. No subsidence. Dominance of foreset beds. Increasing importance of topset beds. 



shallowing of basin, and decrensing importance of foreset beds. 



Second stage. — Intermittent subsidence balanced hy dei)osili()n. l)(<lta built iip\\;nd, 



not outward. Dominance of topset beds. 



Third stage. — Subsidence at a faster rale, maintaining a larger i-atio of subiujueon.s 



topset beds. 



of the present period, extending in many cases hundreds of feet l)el(>\v 

 sealevel, shows that this is a common feature of delta growth. It is 

 natural, furthermore, that such should be the case, since the great rivers 

 tend to drain toward subsiding areas and their sediment ])romotes fur- 

 ther subsidence. It is a mode of delta growth wliich is whollv lacki no- 

 where small deltas are built into lakes of stationary level. This feature 

 of the larger modern deltas indicates how the great depth of certain 

 fresh-water deposits of the older geological periods may be interpreted as 

 delta formations, shading off into the contemporary marine deposits of 

 epicontinental seas, 



