492 PRINCIPLES OF STRATIGRAPHY 



Considerable variation from this normal section is to be ex- 

 pected, for salt marsh building may be interrupted at any stage by 

 blowing sands, and the whole may be greatly complicated by sub- 

 sidence or elevation of the coast. In some cases studied, as on the 

 Massachusetts coast, the structure of the marsh does not conform 

 to this succession in some of its major features, showing that the 

 history here has been quite different from that outlined. "In many 

 cases . . . the deposits below the surface, instead of contain- 

 ing eel grass and salt thatch remains, was entirely composed of the 

 very different and definitely recognizable sort of vegetation that 

 never grows where salt water reaches, i. e., the deposits were- 

 largely of fresh water origin." These deposits often show consid- 

 erable accumulation of woody material, including stumps of large 

 pine trees, near the surface and for a few feet below. "The struc- 

 ture of these marshes also often showed salt water intrusion in 

 varying degree in different parts of the same area, the deposit of 

 the salt marsh material being progressively thicker as the ocean was 

 approached." (Davis-15.) In the case of these marshes, the salt 

 water deposits were made up of the species of plants which grow 

 at or near high-tide level, while eel grasses and species which can 

 stand partial submergence were rare or absent. Even where no 

 fresh water peat was found underlying the salt peats the species 

 making up the latter were those growing at present at high tide 

 level or above, although the peat formed by these was often below 

 present low- water level. This, if not due to compacting, indicates 

 subsidence of the coast during or since the formation of the peaty 

 deposits. Oscillation of the coast, or at any rate of the tidal range, 

 is indicated by the intercalation in some cases of fresh water peat 

 between layers of salt marsh peat. Salt marshes of this type must 

 be interpreted as follows : On a flat coastal plain, above the reach 

 of tide, owing to the moist climatic conditions and probably to the 

 growth of trees preventing free drainage, are formed swampy areas 

 in which an accumulation of fresh water peat goes on to an in- 

 definite extent. By a slight but constant progressive subsidence 

 of this coast the sea enters in, kills the fresh water peat, and be- 

 gins to cover more or less permanently a part of the coastal strip of 

 this plain. Since the growth of marsh grasses on the open exposed 

 coast is ordinarily an impossibility, owing to the violent wave work, 

 it follows that the first step in the conversion of the fresh to the 

 salt marsh is the building of the barrier beach. If the barrier 

 beach is built far enough from the shore, owing to the gradual 

 shoaling of the water, a broad lagoon of salt water will be left be- 

 tween it and the shore, and in this the salt peat may gradually form, 



