CLIMATE AND TERRESTRIAL DEPOSITS 261 



A number of conditions besides tlie climate will be found, however 

 to affect the ratio over the marginal portion of the delta, of swamp 

 to the well drained areas. These may be enumerated as follows : 



First, a slowly rising water level tends to flood the lower portion of 

 a delta and bring large tracts into the condition of permanent swamps. 

 As such movements of water level are variable and intermittent the 

 extent and ratio of the seaward paludal deposits seen in cross-section 

 in ancient deltas will vary through the section. Such, however, will 

 be practically absent from the region of the apex of the delta, but will 

 be marginal to, and most commonly underlie, marine strata marking 

 invasions of the sea. 



Second, the contest of two or more rivers in building up a common 

 flood plain or delta results in the damming back of the weaker members 

 of the system. The paludal regions tend to migrate away from the 

 greater sources of sediment. 



Third, the possession of a wide flood plain, as in the case of the 

 lower Mississippi Valley, is liable to result in a considerable area of 

 back swamp, there being less infilling from the sides, and the river 

 with its natural levees occupying a lesser portion of the whole. 



As factors tending on the contrary toward good drainage of the 

 lower river plains may be mentioned : first, a stationary or even slowly 

 subsiding water level; second, the possession of a flood plain by a 

 single river such as the Nile as contrasted with the Euphrates-Tigris 

 system of Mesopotamia; third, aggradation within a confined valley, 

 exemplified by the Great Valley of California, where side wash is 

 present to such an extent that the ground slopes gently but continu- 

 ously from the hillsides to the trough of the valley. 



ELIMINATION OF LOCAL GEOGRAPHIC FACTORS 



The preceding discussion has dealt with the upper or lower flood 

 plains of the river system as a whole and it has been seen that the 

 character of the sediments deposited must be largely influenced by 

 the physical conditions existing in the region of deposition, whether 

 near the mountains as piedmont slopes, or at a distance as deltas 

 built into shallow seas. Within the confines of the terminal deltas, 

 however, there exists greater local diversity of conditions. 



In the study of ancient deposits now exposed to view in scattered 



