490 PRINCIPLES OF STRATIGRAPHY 



the waves and shore ice. They often accumulate along the high- 

 water line or are carried out to sea. 



This plant, which is thus submerged for about half the time, 

 is especially abundant in the banks of the tidal creeks. The deposits 

 which it forms are rich in mineral matter, they themselves rarely 

 forming accumulations approximating in purity those due to the 

 species growing at a higher level. Above the salt thatch zone is 

 the peat zone proper, which is submerged by salt water only from 

 I to 4 hours each day. Many species grow here, sedges (Carex) 

 as well as grasses, but only two are common on the northern coast, 

 Spartina patens Muhl. and Distichlis spicata (L.) Greene. These 

 salt marsh grasses are both short, from 6 to 12 or 14 inches tall, 

 with rather slender, tough, wiry stems, and dull grayish-green, slen- 

 der, involute leaves. Their root stocks are slender, tough and numer- 

 ous, their roots long, fibrous, and branching. The peat formed by 

 these plants cannot be mistaken for that of any other form of vege- 

 table deposit with which it is likely to be associated. "It differs 

 from the turf formed by sedges in the persistence with which the 

 underground stems retain their form and individuality instead of 

 collapsing and flattening, in the lack of the remains of leaves and 

 aerial branches, in color, in the absence of definite lamination, in 

 the amount of silt generally contained, and, more than all else, in 

 the presence of the white or light-colored finely branching roots, 

 which penetrate the mass in every direction and make up the great 

 bulk of the material." (Davis-15 :(5ji'.) 



The leaves and stems of these two salt marsh grasses are more 

 persistent than those of the salt thatch, but they also are largely 

 removed during the winter by ice, wind, and tides. A fresh water 

 species, S. cynosuroides Willd. with a culm 2 to 6 feet high and 

 narrow leaves 2 to 4 feet long and a half inch or less in width 

 below and tapering to a slender point, inhabits the banks of rivers 

 and lakes, or occurs in rich soil from the Atlantic to the Pacific. 



Sedges, Carex salina and Care.v maritima, are also characteristic 

 of the salt marshes, and often add a considerable part to the peat 

 formed. 



The tidal marshes are dissected by meandering channels through 

 which the salt water ebbs and flows twice a day. These channels 

 are generally narrow and deep, the width being determined by 

 various factors, chief among which is the scouring force of the 

 tidal current, and the resisting force and growing power of the 

 marsh vegetation. 



During the slow conversion of the lagoon into a marsh the 

 sand dunes from the beach commonly advance over the growing 



