DELTAS, AND CRUSTAL MOVEMENTS 619 



Effect of elevation. Slight elevation of the delta area results 

 in the destruction by erosion of the top-set series in the upper parts 

 of the delta, and the carrying forward of the material to iDe added 

 to the subaqueous fore-set series. The zone over which continental 

 deposition takes place will also advance seaward, and thus terres- 

 trial sediments will come to rest on marine deposits. Where the in- 

 clination of the fore-set beds is very gentle these terrestrial beds may 

 come to rest upon them with practically no change in dip. Cessation 

 of elevation or reversal of movement and the resumption of normal 

 delta building vv^ill result in the deposition of a new terrestrial 

 series on top of the erosion plane, and a new marine series on top 

 of the terrestrial series nearer the sea. A hiatus and discon- 

 formable relationship will thus appear between the two members of 

 the terrestrial part of the delta, and this disconformity will be re- 



Submanne delho, deposit-^. 



ConHnenfol delhc\ deposih-s" ,1?' . v:^,,,— ,^ 



^^=:...Channel silt-sand Subaerial clous. ~^^^r:^^ 



Fig. 127. Diagrammatic section of a seashore delta, showing the relation 

 of continental and submarine deposits in a region of fluctuating 

 sea-level. (After Barrell.) 



placed seaward by a terrestrial layer interpolated between two 

 marine series. If the delta front is comparatively steep it may, 

 of course, happen that the uplift carries the erosion to the top of 

 the delta front, beyond which deposition will be submarine. In 

 a very large delta slow rising will result in the slow seaward migra- 

 tion of the zones of erosion and the zone of terrestrial deposition, 

 the lowering of the upper part of the delta by erosion progressing 

 steadily, while at the same time the zone of terrestrial deposition 

 extends farther and farther outward. (Barrell-6.) 



Deltas Merging into Desert Deposits. A remarkable combina- 

 tion of a river delta with desert deposits, on the one hand, and 

 marine deposits, on the other, is shown by the Colorado on the 

 Arizona-California and Lower California boundary. This delta was 

 originally built into the Gulf of California, the head of which was 

 by it completely severed from the main part of this funnel sea. 

 The cut-off portion dried out completely under the influence of 



