GEOLOGICAL IMPORTANCE OF SEDIMENTATION 443 



be as great as the depth of the sea in front. The advancing sea will 

 therefore tend to cut away and destroy whatever littoral deposits 

 may be made in advance of it, as illustrated in Fig. 8^. Obser- 

 vations confirming this statement may be occasionally made along a 

 subsiding coast, as that of New England. To illustrate, Boston is 

 found to be sinking at the rate of a foot a century and New York 

 City, according to the most recent estimates, at the rate of i . 5 feet 

 per century.^ 



That the coast between the two cities participates in this move- 

 ment is indicated by the sharp boundaries of the salt-water marshes. 



Fig. 8. — A, diagram illustrating progressive destruction of littoral deposits by 

 marine planation over a subsiding land. B, cross-section on line of an estuary, illus- 

 trating part preserval of estuarine deposits during the transgression of the sea over 

 a subsiding land. 



These extend sharply to old alluvial slopes which rise at gentle angles 

 from beneath them without any belt of fresh-water meadow being 

 found between. If the land had been stationary for many centuries, 

 the wash from the land, aided by the sediment from the highest 

 tides, would have raised the border somewhat above the salt-marsh 

 level. The opposite condition of affairs indicates therefore continual 

 subsidence, but at a rate so slow that the organic and inorganic 

 detritus, held by the roots of the marsh grass, is able to accumulate 

 with suihcient rapidity to keep the surface at about the level of mean 

 high tide. 



A particular instance where the beach may be observed cutting 



I G. W. Tuttle, American Journal of Science, Vol. XVII (1904), pp. 333-46. 



