1913] JOHNSON—COASTAL SUBSIDENCE 461 
maximum, and has begun to fall, the surface of the bay will still 
remain much lower. When it is low tide in the sea, the water in 
the bay will remain at a higher level, because this water cannot 
escape fast enough to maintain equality of levels between the 
two water bodies. Hence, high tide level in the bay is lower than 
high tide level in the sea. This is shown in fig. 7, which represents 
a cross-section of such a bay as fig. 6 in the direction AB. It is 
evident that around the shores of the bay, trees and other fresh 
water vegetation will grow down to the level of the high tide of 
the bay, and thus below the high tide level of the adjacent sea. 
Salt marshes in the bays will likewise grow up to the high tide 
level of the bay, farmers will build dikes to reclaim their marshes 




f th t; as long as the barrier 

Fic. ¢— . aN Pau 
beach (D) nearly closes the mouth of the bay, high tide (H7) in the bay is lower than 
high tide in the open sea; trees grow down to this lower level (AB) along the shores 
of the bay; when the barrier beach is broken through or removed, high tide in the 
bay rises as high (CD) as the open sea, and all the trees between the levels AB and 
CD are killed by the salt water; if the bay narrows going inland, the tide is forced 
to rise even above the level it attains in the open sea, or to the position ED, and at 
the head of the bay all the trees between A and E are killed; in addition to these 
submerged forests, other fictitious indications of subsidence are thus produced. 
at this same level, and in other ways the level will be so marked 
as to render readily perceptible any increase in the height of the 
tides. 
Now let us consider the consequences which must follow if 
storm waves make a large breach in the barrier beach. With 
free access to the bay through the larger opening, the tidal waters 
will at once rise as high in the bay as in the open sea (CD, fig. 7)- 
All trees whose bases are below the line CD will be washed by the 
tides and killed. The standing forests of dead trees will later be 
represented by submerged stumps. Dikes raised by the farmers 
will be overflowed by the tides. The surface of the salt marsh 
will build up to the new high tide level, enveloping both stumps 
