pression. 



The rate at which marsh deposits accumulate has been measured in sev- 

 eral places. Marshes with a close vegetation and which are frequently covered 

 by the tide, rise in level most rapidly. The higher a marsh becomes, the less 

 the number of flooding tides that cover it, and the rate of sedimentation con- 

 sequently falls off. At low levels the rate of accumulation is variable: in 

 sheltered places it may be rapid, but in more exposed places the effect of the 

 tides and waves may bring about deposition one week and erosion the next. 

 Thus, upward growth will be spasmodic until a fairly dense spread of plants 

 has been established. 



The overall physiognomy of a marsh depends upon a variety of factors, 

 including the nature of the deposits, whether mainly silt, silty sand, or peat; 

 the vegetation; the height of the surface; and the local effects of wind and 

 wave. There is a striking difference between the mud marshes, with their con- 

 siderable variety of plants, in Norfolk, and the rather sandy marshes of Cardi- 

 gan Bay (Wales). Both of these stand in contrast to the Spartina marshes of 

 parts of the Channel coast of England. 



Marshes are characterized by an intricate drainage pattern of tidal creeks. 

 The vegetation spreads from sheltered spots and patches which first formed on 

 the original unvegetated surface. Their upward and lateral growth in the course 

 of a long period of years, concentrates the flow of water into definite channels, 

 the bottoms of which mark approximately the level of the original surface. 



Creeks formed in this way may sometimes lengthen by a process of head- 

 ward erosion as a result of tidal water draining into them at the ebb. This is 

 beautifully demonstrated in certain Norfolk marshes where only the lower parts 

 of an original sand flat carry a mud and plant cover which is still spreading in- 

 wards, but has not yet covered all the higher parts of a flat. Consequently, on 

 the ebb of big tides, when the water falls in level, some of that on the upper flat, 

 which cannot get away until the lower levels are drained, falls into the heads of 

 creeks and so cuts them backwards. The rate of this recession has been mea- 

 sured in some creeks. 



Dams often form in creeks and cause permanent blocks, so that the part 

 above the dam may become an elongated lake or pan. Oftentimes several dams 

 are made, partly by undercutting as a result of erosion, partly perhaps by man 

 or animals causing part of a creek bank to slide in. Thus, a series of pans are 

 formed, elongate in form. Other pans are sometimes more or less circular, 

 but, in fact, of any shape. They owe their origin to the irregular spread of 

 plants on the original flat. The plants begin their growth in favourable places, 

 irregularly scattered, so that it often happens that patches completely devoid 

 of plants are enclosed, and form pans. They may occur on the main marsh 

 surface or on what is sometimes called secondary marsh produced partly by 

 the erosion of the main marsh by wave action. This process usually gives an 

 irregular surface in front of the main marsh. Plants may colonize it and pans 

 form on it. The erosion of the main marsh leads to the formation of a small 



