LARGER RELATIONS OF DELTAS 397 



The river is assumed to begin its work on a tilted continental surface, 

 which slopes gently beneath the sealevel and gives a wide area of shallow 

 bottom. This is an initial attitude, which has resulted frequently from 

 periodic diastrophism through geologic time, and the majority of forma- 

 tions open to observation have been deposited near the shores or on the 

 bottoms of interior shallow seas, where waves and currents were conse- 

 quently less effective than on the outer slopes of the continent. The 

 delta cycle may then be divided into two phases : first, the normal cycle 

 aependent on deposition with a stationary crust, and, second, the modifi- 

 cation imposed by vertical movement of the bottom — normally subsidence, 

 abnormally elevation. The subject of the relations of deltas to sedimen- 

 tation has been previously discussed by the writer,^^ so that a condensed 

 formulation will here suffice, bringing out more particularly the ])riii(i|)lo 

 of the cvcle. 



Delta cycle with stationary crust (see figure 3). — In the stage of youth 

 before the drainage system has become well developed the detritus de- 

 livered at the river mouth is somewhat smaller in amount but coarser in 

 texture. The subaqueous wave-cut profile is also undeveloped, the bot- 

 tom still inheriting its original slope. If this initial slope is gentler than 

 the subaqueous profile of equilibrium^^ the waves have at first less power 

 of erosion at the coast line. If the initial slope is steeper they will pos- 

 sess an initially greater power. Assuming, however, that the river is 

 dominant over the sea, the delta is rapidly built outward, and on account 

 of the coarse waste, the steeper river grades, and shallow bottom near 

 shore, the initial proportion of subaerial topset beds is relatively high. 

 During maturity the quantity of waste is larger, as all parts of the 

 drainage system now supply sediment, but as the river is graded and its 

 gradient is also flattened the waste is finer in texture. The delta is 

 extended outward and the greater deposit is on the oulei- ])ortioiis. It 

 grows inland also for a time, but owing to the flattening grade the beds 

 in this direction show decreasing thickness. The maximum rale of out- 

 ward growth is reached early because of the increasing surface area, whicli 

 requires a greater volume of sediment to give a unit thickness, and the 

 increasing depth of the water, which involves a continually deeper fill. 

 Furthermore, the increasing shoreline and greater exposure to the waves 

 increase the power of the latter to carry away the waste, which with the 

 progress of the cycle becomes finer in texture and more readily removed 



" Relative geological importance of continental, littoral, and marine sedimentation. 

 Journal of Geology, vol. xiv. 1906. pp. .3.36-3.54. 



^- X. M. Fenneman : Development of the profile of ociuililtrium of the subaqueous shore 

 terrace. .Tournal of Geologj-. vol. x, 10(»2. pp. 1.32, 



