FRKSII-WATKR SWAMPS 



497 



outset be so shallow as to quickly assume the character of a swamp 

 or bog, without passing through all the stages described. Further- 

 more, it has been shown that lakes may actually be produced by the 

 growth of vegetation, as in an upland moor, this forming either a 

 barrier or an enclosing rim. In such a case the bog precedes the 

 lake, which may subsequently be filled by the development and en- 

 croachment of the swamp vegetation. 



River and Estuarine Szvamps. These are produced where the 

 borders of the rivers with broad flood plains are raised through the 

 building of natural levees by the overflowing stream. The vegeta- 

 tion growing on the borders of a river will tend to arrest the sedi- 

 ment carried by the rising water in times of flood, when the flood 

 plains on either side are inundated. These higher marginal ridges 

 will therefore separate the lower flood plain from the river, and 

 thus the waters resulting from the overflow of the river will find 



Fig. 112. Diagram showing how plants fill depressions from the sides and 

 top. (i) zone of Chara and floating aquatics; (2) zone of pond- 

 weeds or potamogetons ; (3) zone of water-lilies; (4) floating 

 sedge mass; (5) advance plants of conifers and shrubs; (6) 

 shrub and sphagnum zone; (7) zone of tamarack and spruce; 

 (8) marginal fosse. (After* Davis.) 



it impossible to return to the river on its subsidence. Thus exten- 

 sive swampy lands are formed over the inundated parts of the 

 flood plain, which may also enclose an extensive series of shallow 

 lakes. The back Swamps of the Mississippi are examples of this 

 sort of occurrence. 



Delta surfaces may be covered with swamps in the same manner 

 that lakes are formed on deltas, through irregular deposition and 

 obstruction of drainage. At the heads of estuaries, fresh-water 

 swamps may likewise form which further out may merge into or 

 pass beneath 'grass marshes due to encroachment -of the sea. 



From a consideration of the succession of plant associations dur- 

 ing the gradual filling of a lake, it must be apparent that in the re- 

 sulting deposits there will be noticeable a stratification, due to the 

 superposition of successive types of ^ ,ad vegetal matter. In gen- 

 eral, these may be grouped according to their decreasing hygro- 

 phylous characters into the following divisions: i, limnic; 2, tel- 



