GEOLOGICAL IMPORTANCE OF SEDIMENTATION 435 



Along river valleys, as north of Nevi^ark Bay, they extend some 3 miles from 

 the open water, passing into^ the fresh-water marshes. Tidal channels fairly 

 well developed upon marshes. 



TIDE 7 FEET (SAVANNAH, GA., ENTRANCE TO SAVANNAH RIVER) 



Mud-flats o . 33 to o . 66 mile wide. More commonly present than in previous 

 examples. 



Salt marshes filling up a former estuary, 4 to 5 miles wide, with a well-developed 

 network of tidal channels ramifying through them. 



TIDE 10 FEET (BOSTON HARBOR AND DELTA OF THE INDUS RIVER) 



Mud-flats extensive. At Boston they average from 0.5 to i . o mile wide, 

 but are cut up by tidal scour into smaller separated areas. 



Salt marshes. — At Boston, filling up protected depressions, they average o . 25 

 to o . 75 mile wide. On the Indus delta a well-developed network of tidal channels, 

 2 to 4 fathoms deep, and distinct from the distributaries of the river, extends 17 

 miles from the coast, and this may be taken as the limit at which the salt-water 

 tidal marsh gives place to the fresh -water swamp. Tide reaches 11 feet in 

 spring tides. 



TIDE 16 FEET (dELTA OF THE GANGES AND BliAHMAPOOTRA) 



Mud-flats. — Extent not mentioned. 



Salt marshes. — Extent indicated on the map by means of the tidal channels 

 (see reference later). The fresh -water swamps of the delta are protected from 

 the sea by a chain of sandy islands, separated from each other by tidal channels 

 and known as the Sunderhuns, the name evidently signifying severed mounds. 

 This chain of islands averages 58 miles wide, but only the outer half is markedly 

 cut up by tidal channels. Therefore it seems probable that the present tidal 

 flooding extends some 30 miles inland, and the inner portion may have been 

 built at an earlier date. Tidal effects on rivers are of course felt much farther 

 but do not flood wide stretches of their banks. 



TIDE 40 TO 70 FEET (bAY OF FUNDY, BASIN OF MINAS) 



Mud-flats. — Widest in protected heads of bays. In the basin of Minas, esti- 

 mated by J. A. Bancroft to average o . 75 mile wide. Along sides of the bay 

 the current of from 6 to 8 miles per hour keeps the channel deep and open, and 

 the sides scoured clean. 



Salt marshes. — Shaler speaks of the dominance of the mud-flats over the upper 

 marshes, and the rapidity with which the latter are built up by sediment thrown 

 down at flood-tide when obstructions are built across the tidal marshes. Lyell 

 speaks of thousands of acres having been reclaimed in this way. 



REFERENCES 



United States Coast: 



United States Coast and Geodetic Survey Charts. 



