22 NEW YORK STATE MUSEUM" 



grass areas, (2) mud banks, (3a) grass marshes and (3b) man- 

 grove marshes. 



These marine marshes are in most cases formed in bays where 

 the wave action is slight. Their history in general is as follows : 

 The waves wear away large quantities of material from rocky 

 headlands and beds of glacial drift. The rock fragments are car- 

 ried to adjoining beaches, where the pebbles are slowly ground to 

 a fine mud, which may be carried by even a slight current to a 

 great distance. Clay from the drift is reduced to the same condi- 

 tion. The mud is not deposited to any great extent where the 

 waters are disturbed ; but, when it is driven into protected bays, it 

 slowly settles and forms a bed on which a crop of eel grass 

 rapidly springs up. These eel grass fields are usually covered 

 with 3 or 4 feet of water at half tide, when the tidal current is 

 greatest. On account of the habit of the plant, the tidal current 

 is practically stopped where the grass is growing, though the 

 water above the grass is usually more or less laden with fine 

 mud, which slowly settles to the quiet water below and, becoming 

 entangled by the stems of the grass, gradually increases the thick- 

 ness of the deposit in this place. The dying portions of the eel 

 grass and the bodies of many Crustacea and Mollusca are depos- 

 ited with this mud and rapidly increase the thickness of the 

 deposit. When the deposit reaches such a thickness that it is 

 dry at low tide, the eel grass ceases to grow, and the increase in 

 the deposit comes entirely from the sediment borne in by the tide. 

 On the highest part of the mud flat thus formed, grasses and othei^ 

 forms of vegetation begin to grow and gradually form a covering 

 which raises the level of the marsh so that it is only overflowed 

 by the highest tides. Sometimes the mud flat between the eel 

 grass area and the grass marsh is as much as a mile wide, but 

 this is exceptional. The growth of the deposit is more rapid in 

 the grass marsh than in the eel grass area, because the plant, 

 leaves and stems a-re larger, and for this reason gather sediment 

 more effectively. The grassy marshes are more carbonaceous 

 than the eel grass areas, but they rarely contain 50^ of carbon 

 and usually do not furnish a supply of peat suitable for fueL 



