GEOLOGICAL IMPORTANCE OF SEDIMENT A TION 349 



and pouring into Lake Pontchartrain. Such a course would possibly 

 before this have become permanent, had the river not been restrained 

 by artificial means. Even in these cases, however, of land-protected 

 sounds and lagoons, it is to be noted that the area of mud-flats exposed 

 between tides is much less than the areas permanently covered by 

 lagoon waters. 



A river possessing a broad land delta, and shifting across it periodi- 

 cally, will thus build up a delta formation whose seaward portions 

 will embrace an alternation of marine or lacustrine and fluviatile 

 deposits. Farther inland the deposits will be entirely fluviatile. 



Reject of Variable Water-Level. — Another condition which must 

 normally modify the relations of land and sea in delta-building 

 is that of changing level, resulting either from those variations in the 

 water-level to which inland seas are peculiarly liable, or from irregular 

 vertical movements, usually of subsidence, such as are known to be 

 characteristic of regions undergoing sedimentation. 



Interior seas. — The case of inland seas is illustrated by the delta 

 of the Volga, shown in cross-section in Fig. 4, and which may be 



$cA(e lo Kilometers. 

 VerHcal scale multiplied by 20. 



Fig. 4. — Profile. Delta of the Volga in the Caspian Sea. , 



contrasted with Fig. 3. A good map shows that the entire northern 

 portion of the Caspian Sea, lying chiefly east of the line of the section, 

 is but from i to 1 1 meters deep. Such an extended shelf to an inland 

 sea facing two large rivers cannot reasonably be ascribed to wave 

 planation of the river deltas, nor, on comparing this shelf with other 

 deltas, can it be supposed that waves in the absence of tides and cur- 

 rents could have transported the land- waste to such a distance from 

 the river mouths. 



It seems certain, then, that the present submerged shelf was 



