SEA-COAST SWAMP BEPOSITS 325 



of it may be again carried seaward by the bottom current or 

 undertow as the wave recedes. One who has stood upon a high 

 rock on the sea-shore and watched the waves come tumbling at his 

 feet and then go creeping oeeanward once more cannot have 

 failed to notice the continual seething sound due to the drag of 

 the rock fragments as they are impelled inward and outward by 

 the alternating currents. A considerable part of the mud thus 

 formed is taken out to sea by the undertow, which always sets 

 from a storm-beaten beach along the bottom, but another part is 

 urged by the movement of the water caused by the waves and the 

 tidal flow into the fjords, where it falls to the bottom. In this 

 process the mud is generally conveyed along the shores and most 

 commonly deposited in the parts of the inlets near the shore 

 line. "Wherever there is a bay within which the tidal current 

 is deadened and where the waves have little play, the sediment 

 is most rapidly laid down. If the process of deposition begins 

 on a pebbly bottom, it is at first aided by the irregularities be- 

 tween the stones and the friction of the water among the sea- 

 weeds, which attach themselves to the stones. As soon as a 

 sheet of mud is established, it commonly becomes occupied by 

 a dense growth of eel-grass, which greatly favors the deposition of 

 sediment. The stems of the grass are set very closely together, 

 the interspaces generally not exceeding 1 or 2 inches. A 

 tidal current of 2 miles an hour, swift enough to carry much 

 sediment, is almost entirely deadened in this tangle of plants. 



At half tide on the New England coast these eel-grass fields 

 are generally covered with water to the depth of several feet; 

 at this stage the tidal currents are commonly strongest. The 

 water above the level of the grass has its usual freedom of 

 motion and brings much sedimentary matter above the level of 

 the foliage. As the tide falls, a part of this waste is entangled 

 and held until it gradually sinks to the bottom, so that each 

 run of the tide gives a certain contribution of sedimentary 

 matter, which goes to shallow the water. The process is easily 

 observed from a boat floating over a field of these plants. The 

 deadening of the current when the lowered tide brings the tops 

 of the plants near the surface is very noticeable. The mass of 

 floating matter — mud, fronds of sea-weed (often with shells or 

 small pebbles attached to their bases), dead fish, and other refuse, 

 — ^is seen to collect in the mesh of foliage and sink. The dead 

 stems of the eel-grass and the bodies of many small crustaceans 



