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NEW YORK STATE MUSEUM 



either side. During periods of flood these lower lying parts of 

 the terrace may become covered with water which remains after 

 the river subsides. Additional water may drain into the de- 

 pression from the sides of the valley, and, if the terrace is under- 

 lain by a clay layer, tjae drainage of the area will proceed very 

 elowly or be suspended, thus maintaining a pond in which water 

 and moisture-loving plants spring up, resulting in the develop- 

 ment of a swamp. Swamps of this type are less numerous than 

 the other two. 



Delta swamps are common in many regions where streams 

 heavily laden with sediment enter a lake, for, the speed of the 

 river current being checked at this point, much of the sediment 

 is deposited around its mouth, forming a delta. As the sedi- 

 ment accumulates year after year, the level of the delta is built 

 up, till the water over it is sufficiently shallow to permit the 

 growth of water plants. These serve to catch more sediment, 

 so that the level of the delta is raised high enough to form first 

 a swampy tract and later dry land, which may be overflowed 

 only in springtime, when it receives an additional layer of sedi- 

 ment, but later none at all. 



The result of both these kinds of swamp development and 

 filling is to form a rich, black soil, often of considerable depth, 

 and made up of a mixture of decaying plant fragments and 

 mineral matter. 



In some cases the filling up of a lake and formation of a 

 swamp may be due almost entirely to the accumulation of 

 vegetable matter. In such cases the streams flowing into 

 the lake carry little or no sediment, as the water supplying the 

 pond may soak into it through the soil from the surrounding 

 slopes, as in the case of Wilmurt lake in the Adirondacks. 1 



Many ponds and lakes in north temperate regions are sur- 

 rounded by a growth of water-loving plants, and particularly 

 with mosses. As these multiply, they not only send their reots- 

 downward into the shallow waters around the edge of the pond, 



^Srnyth, 0. H. jr. Lake Filling in the Adirondacks. Am. Geologist. 

 1893. 11:85. 



